Navigational Risk from the Desynchronization of Safety Information Across Chart Systems on SOLAS and Non-SOLAS Vessels

Abstract

The regulatory disparity in nautical chart carriage requirements between SOLAS and non-SOLAS vessels poses a systemic risk for maritime safety. This study analyzes maritime traffic and grounding accidents in the East Adriatic Sea to examine the empirical patterns associated with this risk and to quantify this risk. Analysis of 2,699,930 vessel arrivals in Croatian ports (2012–2020) reveals that non-SOLAS vessels dominate traffic, outnumbering SOLAS vessels by factors of 3.44 (cargo) and 13.92 (passenger). Correspondingly, over 92% of 170 recorded groundings (2017–2019) and 90.6% of 202 groundings (2020–2025) involved non-SOLAS ships, predominantly during favourable weather conditions. The core problem is identified as the desynchronization in disseminating Maritime Safety Information (MSI) across official and unofficial chart systems, leading to inconsistent situational awareness. This risk is formalized through a conceptual objective function modelling the time delay in MSI updates across different chart production chains. These quantitative metrics demonstrate that the current regulatory gap exposes the majority of maritime users to a reduced safety standard, directly affecting risk management at the operational and regulatory levels. To minimize this delay and enhance navigational safety, the paper proposes a dual-pathway amendment: extending the ECDIS/ENC mandate to all SOLAS ships and establishing a new framework mandating the use of official ENC data within Electronic Chart Systems (ECS) for non-SOLAS vessels. This study is intended as a system-level analysis of regulatory and informational asymmetry rather than as a full accident-causation investigation. The findings should therefore be interpreted as exploratory and policy-relevant, with caution regarding direct causal inference for individual incidents. The proposed measures are essential to synchronize critical maritime safety information across all maritime users.

1. Introduction

1.1. The Evolution of Nautical Charts and the ECDIS Mandate

Nautical charts, the fundamental aid for maritime navigation, have evolved from paper to digital databases. While paper chart development has stagnated, electronic chart technology continues to advance [1]. The International Maritime Organization (IMO) sets carriage requirements and performance standards for navigational systems, while the International Hydrographic Organization (IHO) provides the technical data standards and chart specifications [2,3].

A key development in this evolution is ECDIS. Under the International Convention for the Safety of Life at Sea (SOLAS), carriage of ECDIS is mandatory for specific vessel types on international voyages: passenger ships of 500 gross tonnage (GT) and above, and cargo ships, including tankers, of 3000 GT and above. The implementation deadline for these SOLAS ships was 1 July 2018 [3,4]. When operated with updated, official ENCs, ECDIS can be used as the primary means of navigation, fulfilling the chart carriage requirement [3].

1.2. SOLAS vs. Non-SOLAS Vessels

The SOLAS Convention’s provisions create a fundamental regulatory divide. SOLAS vessels not meeting the specific size and voyage criteria and all non-SOLAS vessels have no obligation to carry ECDIS or use official ENCs. This category encompasses a vast fleet, including small cargo and passenger vessels, fishing boats, pleasure craft, and warships. Consequently, a dual market for electronic navigation has emerged. SOLAS vessels use standardized, official ENCs displayed on type-approved ECDIS equipment. Non-SOLAS vessels often employ ECS, which are governed by different standards, such as ISO 19379:2003, and can utilize data compiled from both official and private sources [5,6]. Hydrographic offices explicitly disclaim responsibility for the accuracy of these unofficial products [6].

1.3. Evaluation of Updating Procedures for Different Types of Navigational Charts

All paper and electronic navigational charts must be updated on a regular basis. Compared with the correction of paper nautical charts, the updating of ENCs is significantly more advanced, faster, and easier. Printed editions of Notices to Mariners (NtM) may sometimes reach vessels too late for paper charts to be kept in a fully up-to-date condition. Furthermore, the manual correction of paper charts is time-consuming and may result in additional costs, including penalties arising from the use of outdated navigational charts [7]. The existing ENC update system, developed in accordance with IHO standards, offers significant advantages over the correction of paper nautical charts. Hydrographic offices continuously receive information concerning important changes affecting navigation, including seabed alterations, modifications to the type or characteristics of buoys, and other MSI [8].

ENC updates are generally produced on a weekly basis and incorporated into the ENC database. These updates typically correspond to the weekly or monthly corrections issued for paper charts [8]. One of the major advantages of ECDIS is its capability for semi-automatic and fully automatic ENC updating [7].

Today, many ENC service providers offer update services through online platforms, including internet servers, e-mail distribution, and satellite communications. These services can be configured for automatic updating and provide considerable savings compared with the labour costs associated with manually correcting paper charts [9].

The maintenance of unofficial electronic navigational charts is generally governed by ISO standards. A wide range of unofficial electronic charts is currently available on the market and used in ECS. However, the principal challenges associated with updating unofficial electronic charts arise from the selective application of updating systems and the variety of update approaches employed, which differ substantially from those used for official ENCs. Since ECS installations are widely used on numerous vessels, it is important to highlight the differences between official and unofficial electronic charts. Regarding timeliness, ENCs are produced by hydrographic offices using official source data. This is not always the case for commercial producers of unofficial electronic charts, who frequently compile data from publicly available paper charts [7] as well as from their own unofficial sources, including publicly available bathymetric datasets and crowdsourced bathymetry [10].

In terms of accuracy, ENCs are generated directly from the original datasets maintained by hydrographic offices. These datasets include both graphical and tabular information derived from authoritative databases with a high degree of positional and attribute accuracy. Commercial chart producers, by contrast, often rely on paper charts as their primary source material. The limitations of such data become increasingly apparent when digital chart products are enlarged beyond the scale for which the original chart data were intended [7].

Finally, regarding accountability, hydrographic offices assume responsibility for the accuracy and reliability of the ENC products they publish. They are accountable for any errors identified within official ENC data, whereas the level of responsibility associated with unofficial electronic chart products may vary depending on the producer and the applicable legal framework [7].

Consequently, some ECS may not always provide users with the same level of data reliability, accuracy, and update assurance as ENCs maintained in accordance with IHO standards.

1.4. Chart-System Disparity and MSI Synchronization Risk

All mariners, regardless of their vessel’s status, require accurate and timely MSI—navigational warnings, meteorological forecasts, and other safety messages [11]. SOLAS contracting governments are responsible for promulgating MSI and keeping charts up to date [3]. For official ENCs used in ECDIS, a standardized and rigorous update mechanism exists.

The critical safety risk, however, lies in the update process for other chart types. The timelines for incorporating the same MSI into official paper charts, raster navigational charts (RNCs), and various unofficial electronic formats are not synchronized. This creates a potential time lag, or desynchronization, in the flow of critical maritime safety information to the mariner. A mariner using an ECS may therefore receive critical MSI with a significant delay compared to a mariner on a SOLAS vessel using ECDIS. This delay degrades situational awareness—the accurate and timely perception of the navigational environment and its risks [12].

This paper argues that this desynchronization in MSI distribution across disparate chart systems creates a major, system-wide risk to navigation.

1.5. Identification of the Research Gap

While previous studies have extensively analyzed ECDIS-related accidents [13,14,15,16,17,18], post-incident human factors [15,16], the static quality of chart data [7,8,9,10], and ship-grounding risk assessment and prediction models [19], they have predominantly focused on the operational and technical aspects of individual navigation systems. However, the temporal dimension of Maritime Safety Information (MSI) dissemination remains critically under-examined. Specifically, no study has addressed the systematic desynchronization in incorporating identical safety-critical updates across official ENCs, paper charts, and various unofficial electronic formats.

The existing literature has not quantified the scale of vessels affected by these asynchronous update regimes. It has also not formalized the resulting time lag as a distinct, proactive systemic risk factor. Furthermore, the operational consequences of this informational asymmetry for shared situational awareness among mariners in the same navigational area remain inadequately addressed.

Therefore, this study explicitly fills this research gap. It combines quantitative traffic and accident analyses with a novel mathematical formulation of the MSI update delay, offering a proactive, systemic perspective on navigational safety.

1.6. Research Objectives and Paper Structure

This study aims to investigate the practical impact of regulatory and technological differences in the chart update systems that affected navigational safety. The primary objectives are to:

Quantify the share of SOLAS versus non-SOLAS traffic in a dense coastal sailing area;

Analyze grounding accidents in relation to the type of vessels and chart system in use;

Conduct comparative analyses of regulatory and technical frameworks governing the chart systems used by SOLAS and non-SOLAS vessels;

Develop a model to quantify the MSI synchronization problem, leading to practical recommendations.

The data presented in this study support a plausible systemic risk mechanism associated with regulatory and technical asymmetry; however, they do not demonstrate direct causality for individual grounding incidents. Accordingly, the results should be interpreted as indicating a broader safety environment in which chart-system disparity and MSI desynchronization may contribute to risk, rather than as proof that they caused each event.

The paper is structured as follows. Section 2 reviews the relevant standards, regulations, and previous research. Section 3 details the methodology, centered on traffic and grounding analysis in the East Adriatic Sea. Section 4 presents and discusses the results, introducing a conceptual objective function to model the MSI update delay problem. Section 5 provides concluding remarks, proposing regulatory amendments and recommendations for enhancing safety across all vessel types.

1.7. Main Contributions of This Study

This study provides several original contributions to the fields of maritime safety, navigational risk management, and regulatory policy:

• Empirical quantification of the regulatory exposure. This study provides the first comprehensive, multi-year statistical analysis of SOLAS vs. non-SOLAS traffic in the Eastern Adriatic Sea, quantifying the overwhelming dominance of non-SOLAS vessels (3.44:1 for cargo and 13.92:1 for passenger ships). This contribution offers concrete evidence to maritime administrations and IMO regarding the scale of the fleet currently operating outside the ECDIS/ENC framework.
• Formalization of the MSI synchronization risk. The study introduces a novel conceptual objective function that models the time delay in incorporating identical MSI across different chart production chains (p₁, p₂, p₃). This mathematical formalization transforms a previously qualitative safety concern into a quantifiable risk metric, allowing for future empirical validation and comparison.
• Proactive, systemic perspective on navigational safety. Unlike previous studies that focused on post-incident human factors or static chart quality, this research shifts the focus to the temporal dimension of information flow. By identifying the desynchronization of MSI updates as a latent, systemic risk factor, the study provides a new conceptual framework for understanding grounding accidents in shared waterways.
• Actionable dual-pathway regulatory proposal. This study translates empirical evidence into a clear, two-pronged strategy for regulatory reform: (a) lowering the SOLAS tonnage threshold to mandate ECDIS and official ENCs for all cargo and passenger ships, and (b) requiring that all ECS platforms on non-SOLAS vessels operate exclusively with official ENC data. These proposals are designed to be actionable by IMO and national maritime authorities, offering a realistic starting point for closing the current safety gap.

2. Literature Review and Regulatory Framework

2.1. IHO Standards for Official Charts (S-57, S-101)

The International Hydrographic Organization (IHO) provides the fundamental technical framework for official digital nautical charts. The current operative standard for ENC data is S-57, which defines the object-oriented data model, structure, and product specifications to ensure global uniformity [20]. Complementary standards govern critical aspects of the ENC: S-58 governs critical aspects of ENC mandates validation checks, S-63 specifies data encryption and security, and S-52 standardizes their visual display on ECDIS screens [21,22]. For RNCs, which are digital facsimiles of paper charts, the IHO standard S-61 applies [23].

Looking forward, the new IHO S-100 Universal Hydrographic Data Model provides a geospatial framework for a broader range of marine data products. Its implementation, particularly the next-generation ENC standard S-101, marks a significant technological leap that also poses major challenges for hydrographic offices [24,25]. From the beginning of 2026, the installation of ECDIS devices supporting the new S-101 cartographic standard has been available and possible, while their installation on new ships will become mandatory from 1 January 2029 [26]. This ongoing transition underscores the dynamic nature of chart standardization, yet it does not, in itself, resolve the core disparity in chart carriage and update obligations between vessel categories.

2.2. IMO Performance Standards for ECDIS and ECS

The IMO sets mandatory performance standards for bridge navigation systems. For ECDIS, the foundational standard is IMO Resolution A.817(19) [27], which was significantly enhanced by the revised performance standards in Resolution MSC.232(82) [27,28] (1998; 2006). The revised ECDIS Performance Standards were formally adopted by the IMO Maritime Safety Committee (MSC) at its 106th session in November 2022, through resolution MSC.530(106) [29]. This adoption followed the consideration and recommendation of the proposed amendments by the IMO’s Sub-Committee on Navigation, Communications and Search and Rescue (NCSR). The evolution of these performance standards, detailed in

Table 1. Evolution of IMO performance standards for ECDIS (authors’ own view considering [4,27,28,29].

Feature/Aspect	General Supported Options IMO MSC.86(70)—1998	Additional Performance Requirements IMO MSC.232(82)—2006	S-100 Integration and Advanced Data IMO MSC.530(106)—2022
Core display and navigation functions	• Radar image display • Information about supporting objects on Automatic identification system (AIS) • Other data that may help in route planning • Determination of geographical position from a known point with azimuth and distance	• Manually fixed position (by using Lines of Positions—LOP) • Indication of the difference between LOP fix and primary source position (e.g., GNSS) • Ability to measure the actual distance and azimuth between two positions • Geodetic distance (loxodrome and orthodrome) calculations	• Formalized requirements for displaying s-100 product specifications. • Enhanced management of multiple, standardized data layers.
Chart standard and data products	Not applicable	Not applicable	• S-101 ENC becomes the new primary chart standard. • Enables use of S-100-based data products (e.g., S-102 bathymetric surface, S-111 surface currents).
Implementation timeline	Not applicable	Not applicable	• Defines the “S-100 ECDIS” and establishes an implementation timeline: • Voluntary use from 1 January 2026 • Mandatory for new installations from 1 January 2029

, shows a progression from core display functions (1998) and enhanced navigation tools (2006) to the key transition mandated by the 2022 amendments. These amendments establish the S-100 framework, introduce the S-101 ENC as the new primary standard, and set a clear timeline for the implementation of S-100 ECDIS.

These standards also mandate ECDIS’s integration with other sensors, stipulate backup arrangements, and crucially require the use of official, up-to-date ENCs. Compliance is verified through technical test specifications developed by the International Electrotechnical Commission (IEC) [4,30].

In contrast, for ECS used predominantly on non-SOLAS vessels, IMO has not issued mandatory performance standards. Guidance is found in non-mandatory instruments like the ISO 19379 standard, which permits the use of data from various sources without guaranteeing compliance with IHO specifications [5]. This regulatory asymmetry is reflected in divergent national approaches—for example, the United States Coast Guard provides guidelines for ECS use on domestic vessels [31], while Italy permits certain vessels to use ECS in lieu of paper charts [32]. This fragmentation of standards results in a wide variance in system capability and data quality for a large part of the maritime fleet.

2.3. Studies on Chart/ECDIS-Related Accidents and the Role of Training

Academic and institutional research consistently identifies human factors and training as critical elements in the safe use of navigation systems. Studies of accidents involving ECDIS-equipped vessels frequently cite causes such as over-reliance on automation, incorrect system settings, inadequate alarm management, and the use of inappropriate chart scales [13]. The IHO Data Quality Working Group (2014) [14], analyzing a global sample of groundings, concluded that improving mariner training on chart data limitations was as crucial as improving the data itself.

The importance of tailored, continuous education is emphasized by numerous scholars [15,16]. A joint study by the UK and Danish accident investigation branches (MAIB & DMAIB) highlighted that differences in navigation modes—between ships using ECDIS as the primary means and those using a combination of ECDIS and paper charts—complicate the establishment of single best practices [17]. These findings point to a complex safety environment where technology, regulation, and human competency are deeply intertwined. To provide a broader context on grounding causes related to chart use, a sample of international investigation reports from the UK Marine Accident Investigation Branch [18] was analyzed.

The review of 41 MAIB grounding reports provided deeper insight into causation related to navigational means. The distribution of causes is summarized in Figure 1.

Analysis of the data reveals a consistent pattern where most incidents remain associated with electronic navigation systems. For SOLAS vessels equipped with ECDIS, only 26.7% of incidents (4 out of 15) can be attributed to purely external, non-system-related factors. Similarly, among vessels using ECS, only 30.0% of incidents (3 out of 10) arise from external influences. This high prevalence of system-related incidents becomes particularly significant when compared to traditional paper charts, where only 50% of incidents show a direct systemic connection. The consistent pattern observed in both ECDIS and ECS categories points to a common underlying challenge. This challenge extends beyond the technical features of any single navigation system. In both cases, only about a quarter to a third of incidents stem from purely external causes. This points to the underlying complexity of integrating electronic navigation into routine maritime practice.

According to statistical data on Search and Rescue (SAR) operations within the area of responsibility of the Croatian Maritime Rescue Coordination Center (MRCC) Rijeka, a considerable number of maritime accidents and incidents were recorded along the Eastern Adriatic coast. Vessel groundings represented the most frequent category of recorded maritime accidents and incidents [33]. The occurrence of grounding accidents should also be examined in the context of deficiencies in voyage planning and execution, including the maintenance, updating, and effective use of nautical charts and navigational publications, whether carried in paper or electronic form. Failure to ensure that navigational information is accurate and up to date may significantly increase the risk of navigational errors and subsequent grounding incidents. This finding suggests that, in similar maritime contexts, issues related to chart systems and their use are a prevalent contributing factor to grounding accidents.

2.4. MSI Synchronization Gap as a Systemic Risk

The reviewed literature and regulations reveal a comprehensive yet fragmented system. Robust, mandatory standards exist for SOLAS vessels using ECDIS/ENC, while a more permissive, variable regime applies to others. Research has effectively analyzed accidents post-factum and stressed the importance of improved training. However, a proactive, systemic analysis of the information flow as a risk factor remains under-examined.

The critical gap is the lack of focus on the temporal dimension of MSI distribution across different charts. While SOLAS Regulation V/2 obliges Contracting Governments to issue MSI and IMO performance standards require ECDIS to use “the latest” information, no mechanism ensures synchronized updating across the parallel channels of official ENCs, official paper charts, and various unofficial electronic navigational charts. This creates a latent condition for inconsistent situational awareness among vessels sharing a waterway, a risk factor that precedes and potentially precipitates human error. This paper addresses this gap by investigating the scale of the affected vessels, analyzing related accident patterns, and proposing a model to formalize this synchronization problem.

3. Methodology for Assessing Traffic and Safety in the Adriatic Context

3.1. Study Area and Data Sources

This research focuses on the Eastern Adriatic Sea, a region characterized by dense maritime traffic, a complex coastline with numerous islands, and a high proportion of non-SOLAS vessels, including pleasure craft, fishing vessels, and numerous small boats (often generically recorded as ‘other maritime objects’). To investigate the impact of navigational chart regulations, a multi-source methodological approach was designed. The methodological flow, encompassing three distinct analytical processes, is summarized in Figure 2.

The analysis integrates quantitative traffic data and qualitative accident reports. The flowchart illustrates the integration of traffic data (categorized by GT thresholds) and grounding accident data (local and international) for the period 2012–2020. The analysis proceeds through a comparative regulatory review, leading to two primary study outputs: (1) a conceptual mathematical framework modelling MSI update time delays, and (2) a dual-pathway regulatory framework proposing amendments for SOLAS and non-SOLAS vessels.

The primary database for this study consists of two distinct datasets:

• Traffic database. This dataset comprises 2,699,930 records of vessel arrivals at Croatian seaports for the period 2012–2020. The data were extracted from the official annual statistical reports of the Croatian Bureau of Statistics (DZS) [34] and include vessel type (cargo/passenger) and gross tonnage (GT) categories.
• Grounding database. This dataset consists of 372 recorded grounding incidents in the Eastern Adriatic Sea, divided into two observation periods: 170 incidents (2017–2019) and 202 incidents (2020–2025). The data were obtained from the official information receipt logs of the Directorate for Safety of Navigation, Ministry of Sea, Transport and Infrastructure (MSTI) of the Republic of Croatia [34,35].

Additionally, to provide a broader international context, a supplementary sample of 41 grounding investigation reports (2012–2020) was sourced from the UK Marine Accident Investigation Branch (MAIB) [31] for qualitative analysis of chart-related causation.

Data were obtained from the official publications of the Croatian Bureau of Statistics [34]. Data on maritime accidents and SAR operations in Croatian waters were sourced from the annual reports of the Maritime Safety Directorate within the Ministry of Sea, Transport and Infrastructure [36].

Additionally, a survey was conducted to determine which navigational charts and display systems are used on recreational non-SOLAS vessels. The survey aimed to identify installed chart display systems, types of nautical charts (paper and electronic) carried on board, and the frequency of chart updates. Data were collected using a structured questionnaire consisting of an introductory section (with objectives’ description and instructions) and two question groups. The first group focused on the characteristics of the participating charter agencies and their fleets, including the number of managed vessels, the number of electronic chart display systems from different manufacturers, contracts for regular chart update services, and chart update intervals. Both closed-ended and open-ended questions were included. The second group assessed the perceived importance and urgency of voyage planning tasks using a five-point Likert scale (1 = lowest, 5 = highest). The responses were used to examine the relationship between the answers. The questionnaire was pilot-tested by experienced maritime experts, revised accordingly, and distributed online to Croatian charter agencies. Follow-up communication by email and telephone was conducted during and after the survey to clarify any ambiguous responses.

3.2. Analysis of Maritime Traffic (SOLAS vs. Non-SOLAS Share)

To quantify the scale of the vessels affected by differing chart standards, the total traffic of vessels arriving at Croatian seaports was categorized according to ECDIS carriage mandates. Cargo vessels were divided into two groups: those under 3000 GT (no ECDIS mandate) and those of 3000 GT and above (ECDIS mandatory). Passenger ships were similarly split: those under 500 GT and those of 500 GT and above. The gross tonnage (GT) thresholds were strictly applied based on SOLAS Chapter V, Regulation 19. This categorization allowed for a direct comparison of the ratio of vessels operating under mandatory, high-standard digital charting rules versus those that are not.

3.3. Analysis of Grounding Accidents

As defined in Section 3.1, the grounding database comprises a total of 372 recorded incidents from the Eastern Adriatic Sea, divided into two observation periods: 170 incidents (2017–2019) and 202 incidents (2020–2025). The investigation of grounding accidents employed a two-tiered analysis. First, all groundings in Croatian waters between 2017 and 2019, as recorded in official records of the Directorate for Safety of Navigation, Ministry of the Sea, Transport and Infrastructure (MSTI) of the Republic of Croatia [36], were analyzed. For each case, the type of vessel (SOLAS/non-SOLAS) and the time of year were noted. It is important to acknowledge a key limitation: detailed official investigations into groundings without casualties or pollution are often not conducted in the study area, which restricts available data on root causes.

The MAIB reports were retrieved from the official online repository using a specific set of criteria: merchant vessels of 100 gross tonnage or over, with a report type limited to investigation reports, and with dates of occurrence from 1 January 2012 to 1 January 2021. These criteria ensured that the sample included all relevant accident investigations for deep-sea and coastal merchant vessels over the defined period. A random sample of 41 MAIB grounding reports from 2012 to 2020 was reviewed. Each report was analyzed to determine whether the cause was linked to the use, misuse, or status (e.g., outdated) of nautical charts or their display system (ECDIS, ECS, or paper). This provided a qualitative link between accident causation and the type of navigational means employed.

3.4. An Analytical Framework Tracing Chart Disparity to Situational Awareness

The core analytical framework of this study connects the empirical data (traffic volume and accident cases) to the theoretical safety risk. The underlying premise is that the regulatory divide between SOLAS and non-SOLAS vessels creates disparities in the acquisition and updating of navigational information. Mariners operating in the same navigational area may use different chart products that are updated at different time intervals. This informational asymmetry may result in differing perceptions of navigational hazards and varying levels of situational awareness.

We argue that the most critical factor contributing to this disparity is the timing of MSI incorporation. Therefore, the traffic analysis establishes the scale of the problem (how many vessels are in the lower-standard category), the accident analysis seeks indicators of the problem, and the regulatory review identifies the mechanism (the unsynchronized MSI update processes). This framework sets the stage for the subsequent formulation of an objective function in Chapter 4 (Output 1) and the development of a dual-pathway regulatory response (Output 2), both aimed at mitigating the systemic risk.

4. Results and Discussion

4.1. Traffic Results Reveal the Overwhelming Dominance of Non-SOLAS Vessels

The analysis of ship arrivals in Croatian seaports from 2012 to 2020 reveals a striking imbalance in the application of high-standard digital charting rules. As presented in

Table 2. Total traffic of ships arriving in Croatian seaports from 2012 to 2020 categorized according to ECDIS carrying requirements (authors’ own view considering Croatian Bureau of Statistics data 2012–2020).

Year	Type of Ships
	Cargo Ships < 3000 GT	Cargo Ships ≥ 3000 GT	Passenger Ships ≥ 500 GT	Passenger Ships < 500 GT	Total Traffic of Arriving Ships in the Ports
2020	83,774	34,062	12,160	119,016	249,012
2019	103,726	36,022	16,939	202,536	359,223
2018	105,147	37,092	14,889	196,592	353,720
2017	107,866	35,755	16,145	178,837	338,603
2016	119,459	24,903	13,666	174,019	332,047
2015	103,721	35,225	13,623	161,576	314,145
2014	100,404	24,424	6952	126,890	258,670
2013	103,579	23,137	3595	116,628	246,939
2012	106,388	21,271	2150	117,762	247,571
Total traffic of ships by type
	934,064	271,891	100,119	1,393,856	2,699,930

, representing total traffic of ships arriving in Croatian seaports categorized according to ECDIS carrying requirements, cargo ships under 3000 GT (no ECDIS mandate) accounted for 934,064 arrivals, compared to 271,891 arrivals for mandated cargo ships of 3000 GT and above—a ratio of 3.44 to 1.

The disparity is even more pronounced for passenger vessels. Passenger ships under 500 GT (non-SOLAS in this context) recorded 1,393,856 arrivals, vastly outnumbering the 100,119 arrivals of larger, ECDIS-mandated passenger ships by a factor of 13.92. These data, visualized in Figure 3, demonstrate unequivocally that the vast majority of vessel movements in the studied area are conducted by ships not required to use ECDIS and official ENCs.

The traffic analysis revealed that vessels not mandated to use ECDIS dominate maritime movements, outnumbering SOLAS vessels by a factor of 3.44 for cargo ships and 13.92 for passenger ships. Consequently, the potential safety deficit associated with alternative chart systems affects the predominant user of the waterways.

The predominance of non-SOLAS vessels in the traffic and grounding data should not be interpreted as evidence that vessel category alone caused the analyzed incidents. Vessel category is relevant because it reflects the regulatory and informational environment in which navigation takes place, but the available data do not permit attribution of individual groundings to vessel class, chart type, or update status. Accordingly, the observed distribution of incidents should be treated as a contextual pattern rather than as proof of causation. The results should be interpreted as indicating a broader safety environment in which chart-system disparity and MSI desynchronization may contribute to risk, rather than as proof that they caused each grounding event.

The traffic data presented in this study are descriptive and are intended to illustrate the scale of vessel exposure to the regulatory divide. They do not constitute a fully normalized risk estimate, as consistent exposure metrics such as vessel-hours, voyage frequency, and operator-specific variables were not available. Accordingly, the present analysis should be interpreted as descriptive rather than inferential with respect to risk. Exposure-normalized risk assessment could therefore not be undertaken with the available data.

4.2. Grounding Accident Patterns by Vessel and Chart-System Type

The analysis of 170 grounding incidents in the Eastern Adriatic from 2017 to 2019, shown in Figure 4, found that 92.35% involved recreational craft, with the remainder being small cargo, passenger, fishing, and public vessels—all non-SOLAS. Only one involved a SOLAS ship obliged to carry ECDIS, and that grounding was reportedly unrelated to chart information [37].

As illustrated in Figure 5, the majority of groundings occurred in the second and third quarters, coinciding with peak seasonal traffic and predominantly favourable weather, suggesting causal factors other than poor weather conditions.

To better characterize the operational (chart system) environment of recreational vessels in Croatia, a survey was conducted among 18 charter agencies managing 848 vessels. The results indicate that none of the vessels were equipped with a system capable of displaying official ENCs; instead, the installed systems supported ECS functionality. Furthermore, charter agencies managing 93% of the vessels did not have a contracted regular update service for unofficial electronic charts, but updated them once per year by replacing them with newer editions. At the same time, 100% of the vessels were equipped with official paper nautical charts, in accordance with the applicable regulations. These findings describe the navigational equipment environment of the recreational fleet, but they do not permit attribution of individual grounding incidents to a specific onboard system or update practice. The absence of a matched comparison group means that the study can describe the prevailing chart-system environment in the recreational fleet, but it cannot determine whether different configurations are associated with different grounding rates. Accordingly, the survey results should be interpreted as contextual evidence rather than as a causal test of chart-system effectiveness.

To improve the safety of navigation, the Croatian Ministry of the Sea, Transport and Infrastructure issued a circular letter on 10 March 2020, emphasizing the mandatory use of official nautical charts and publications on all non-SOLAS vessels. According to the amended regulations, yachts are required to carry up-to-date official nautical charts and publications appropriate to their area of navigation. When electronic chart systems, such as ECDIS or ECS, are used, the electronic navigational charts must be official and regularly updated. The same requirement applies to recreational boats. Private and recreational boats, as well as commercial and public-service vessels, are required to carry official nautical charts and publications. Electronic navigational charts may be used as an alternative to paper charts, if they are official and kept up to date. In conclusion, vessel owners, operators, manufacturers, and distributors of electronic navigation products are required to use only official and updated nautical charts and publications. Failure to comply with these requirements, including violations of copyright regulations, may result in legal penalties and sanctions [35]. In 2026, the authors obtained an updated dataset on vessel grounding incidents from the MSTI for the eastern Adriatic coast. The objective was to examine recent trends and further evaluate the occurrence of grounding accidents in the region. The processed data are presented in Figure 6, Figure 7 and Figure 8.

The analysis of 202 grounding incidents in the Eastern Adriatic from 2020 to 2025, shown in Figure 6, found that 90.6% involved recreational craft, with the remaining being cargo, passenger, fishing, and high-speed craft [38]. The results indicate that recreational vessels maintained a disproportionately high share of the total number of grounding incidents in the research area, even after the introduction of regulations prohibiting the use of unofficial nautical charts. As shown in Figure 7, most grounding incidents occurred during the second and third quarters, corresponding to periods of intensified seasonal maritime traffic and predominantly favourable weather conditions. This pattern is consistent with the findings for the 2017–2019 period, indicating that vessel groundings are more likely associated with human, operational, or navigational factors than with adverse weather conditions.

Furthermore, the geographical distribution of grounding incidents along the eastern Adriatic coast, shown in Figure 8, reveals that most groundings occurred in highly demanding navigational areas characterized by numerous islands, shoals, and restricted waterways.

An analysis of grounding incidents across two observation periods on the eastern Adriatic coast reveals a consistent pattern of a high number of recreational vessel groundings occurring during periods of favourable weather conditions and in navigationally demanding areas. Safe navigation in such environments requires appropriate maritime knowledge, navigational competence, and, importantly, the use of up-to-date official nautical charts and publications for effective voyage planning.

Human factors remain a central determinant of grounding risk, particularly in the recreational sector. Poor voyage planning, inadequate position monitoring, excessive speed in confined waters, inappropriate chart-scale selection, insufficient knowledge of local maritime traffic and weather conditions, and limited navigational competence may all contribute to the occurrence of groundings. The seasonal concentration of incidents during periods of favourable weather and increased leisure traffic further indicates that operational behaviour and human decision-making are likely to influence grounding risk. Accordingly, the present study should not be read as attributing groundings to chart-system factors alone, but rather as showing that such factors operate within a broader risk environment in which human and operational elements play a decisive role.

4.3. Synthesizing the Evidence on the MSI Synchronization Problem

The convergence of traffic and accident data highlights a fundamental systemic risk. In the observed waterway, the majority of vessels (non-SOLAS) rely on chart data of varying quality and update status, while a smaller proportion of vessels (SOLAS) use standardized and regularly updated official chart products. Accident records, both local and international, consistently identify the use of non-official or inadequately updated charts as a recurring contributing factor in grounding incidents.

This operational reality directly affects situational awareness. Mariners on different vessels in the same area may be looking at chart displays containing different information, particularly regarding the latest MSI such as new wrecks or changed aids to navigation. Consequently, mariners may develop different perceptions of the same navigational environment despite operating in identical physical conditions. This state of informational asymmetry is visually represented in Figure 9, which depicts the incomplete overlap in situational awareness between mariners using different chart systems.

The critical factor underlying this asymmetry is the delay and inconsistency in the incorporation of MSI into different chart production and distribution chains. The extent to which such differences arise, however, is largely determined by the regulatory and technical frameworks governing the chart systems used by SOLAS and non-SOLAS vessels. Therefore, an examination of these frameworks is necessary to understand the root causes of the observed disparity in navigational awareness.

Table 3. Comparative analysis of regulatory and technical frameworks for SOLAS and non-SOLAS vessels.

Aspect	SOLAS Vessels (ECDIS/Official ENCs)	Non-SOLAS Vessels (ECS/Unofficial Charts)
Mandatory carriage of ECDIS	Required for passenger ships ≥ 500 GT and cargo ships ≥ 3000 GT on international voyages	Not required
Mandatory use of official ENCs	Yes, for vessels subject to ECDIS carriage requirements	No, In Croatian currently obligatory on boats and yachts by Circular letter from MSTI
Chart data source	Official ENCs produced by national hydrographic offices	Official ENCs (if used) or unofficial data from commercial producers, paper charts, crowdsourced bathymetry
Update procedure	Weekly updates via automated/semi-automated systems; fully integrated with MSI	Varies by producer; often manual, irregular, or derived from heterogeneous sources
Quality assurance & validation	IHO standards (S-57, S-58, S-63); formal validation and data protection	No formally prescribed QA or validation procedures; ISO 19379 does not require IHO compliance
Accountability	Hydrographic offices are responsible for accuracy and reliability	Varies by producer; no clear legal liability framework
Interoperability	Standardized format (S-57/S-101) across all type-approved ECDIS	Multiple proprietary formats; compatibility issues between different manufacturers

provides a comparative overview of the key regulatory and technical differences between the two vessel categories.

An analysis of the regulations and standards governing the carriage and use of ENCs and ECDIS on SOLAS vessels reveals several important strengths.

As shown in Table 3, the comparison demonstrates that the informational asymmetry identified in Figure 9 is not merely a consequence of individual operational practices, but rather the result of two fundamentally different regulatory and technical approaches to electronic navigation. While the SOLAS framework promotes the use of standardized, quality-controlled and regularly updated hydrographic information, the non-SOLAS sector allows a considerably broader range of chart products, update procedures and data sources. As a result, vessels operating in the same navigational area may have access to different levels of navigational information at any given time.

This disparity is particularly significant in the context of MSI dissemination. The dynamic element that most critically governs this asymmetry is the time delay in incorporating critical MSI updates across different chart production and distribution chains. Delays or inconsistencies in the incorporation of safety-critical information into unofficial chart products can reduce situational awareness and increase the likelihood of navigational errors. The accident data presented earlier suggests that such differences are not merely theoretical but have practical safety implications. Consequently, the identified regulatory gap between SOLAS and non-SOLAS vessels represents a systemic navigational safety issue that warrants further consideration in future regulatory and technological developments.

These differences highlight a substantial disparity between the regulatory frameworks applicable to SOLAS and non-SOLAS vessels. While the SOLAS framework promotes standardization, data integrity, and the use of official hydrographic information, the non-SOLAS sector remains characterized by a diversity of chart formats, varying update procedures, and the widespread use of unofficial navigational products.

4.4. Conceptual Modelling of the Risk with an Objective Function for Update Time Delay

The current legal and technical framework represents a significant advancement in the use of electronic navigational charts and navigation systems. Nevertheless, a considerable number of vessels remain exempt from mandatory ECDIS carriage requirements. The differences in response times for incorporating identical MSI into official and unofficial navigational charts can be expressed through a conceptual objective function. Time, denoted by t, is taken as the independent variable and expressed in days.

In system management problems involving multiple organizations responsible for the production and updating of official and unofficial charts, additional independent variables must also be considered. Accordingly, chart update processes are denoted by (p_n) and represented as functions of the independent time variable (t) and a set of additional independent variables (y_n).

$$p_n=f_n\{t,y_n\}$$

(1)

where (p_n) denotes the maintenance process of a particular navigational chart system, (t) represents time, and (y_n) represents other factors influencing the chart updating process, such as organizational procedures, technological capabilities, regulatory requirements, data distribution methods, and human factors.

We define distinct processes for the inclusion of the same MSI into different chart production chains:

p₁(t, y₁): inclusion in official ENCs by hydrographic offices.

p₂(t, y₂): inclusion in official paper/RNC products.

p₃(t, y₃): inclusion in unofficial electronic charts by private producers.

Here, y_n represents the technical and organizational capabilities of each producer. The time-dependent change, or speed of each update process, is given by its partial derivative with respect to time:

$${\displaystyle \frac{{\partial p}_n}{\partial t}}=\underset{∆\mathrm{t}\to 0}{\lim }{\displaystyle \frac{f_n(t+∆t,y_n)-f_n(t,y_n)}{∆t}}$$

(2)

The goal is to minimize the difference between these update speeds, reducing the time window during which mariners have conflicting chart information.

In an ideal, zero-risk state, ${\displaystyle \frac{{\partial p}_1}{\partial t}}={\displaystyle \frac{{\partial p}_2}{\partial t}}={\displaystyle \frac{{\partial p}_3}{\partial t}}$, meaning all update processes are synchronized. Real-world warnings, such as those from NOAA [39] regarding update prioritization and monthly delays for some products, confirm that p₂ and p₃ often lag significantly behind p₁, thereby increasing the risk function.

Given that not all published MSIs are of equal significance to navigational safety or have the same duration, a classification model for nautical chart-update entities is proposed. A nautical chart-update entity is defined as any MSI that affects navigational safety and whose changes can be quantified and represented on charts used on SOLAS and non-SOLAS vessels.

From a navigational safety perspective, nautical charts, as well as the information displayed on them, do not possess equal value. Certain MSI may represent a significant hazard to navigation, whereas others may have only a limited or negligible impact on navigational safety. It is therefore assumed that each represents, to some extent, a potential threat to navigational safety.

The primary classification criterion adopted in this study is the duration of the entity. Based on this criterion, chart-update entities may be classified into:

• Entities that cause temporary changes in the environment; and
• Entities that cause permanent changes in the environment.

If a nautical chart-update entity is considered as a system, it may be represented by the model shown in Figure 10.

Entities causing temporary and permanent changes constitute the fundamental components of the nautical chart update system. The system consists of two subsystems and interacts with its environment, defined as the navigational area—it comprises the seabed, the water column above the seabed, and the coastal zone.

Each of the proposed subsystems may be further subdivided and classified. In this study, it is proposed that the entities identified in IHO Publication SP-53 be used as the basis for a more detailed classification, as illustrated in Figure 11.

The proposed classification enables the differentiation of entities according to the way their changes are reflected in nautical charts and incorporated into chart updating processes. Entities that result in permanent environmental changes are generally represented through the regular chart updating system, and their depiction is internationally standardized through established procedures for the collection, processing, and dissemination of hydrographic information.

In contrast, entities that result in temporary changes—particularly those promulgated through Temporary (T) and Preliminary (P) NtMs—are not displayed in a uniform manner on electronic nautical charts worldwide [40]. This lack of uniformity represents one of the underlying causes of the informational asymmetry between users of different navigational systems discussed in the previous section. Although national hydrographic offices provide web-based platforms that effectively support the near real-time dissemination of MSI [41], information remains separated from the primary navigational chart display used by mariners. Consequently, access to and interpretation of such information may vary among users. It is therefore argued that only the direct integration of MSI into electronic and paper-update processes can minimize informational asymmetry and ensure that all mariners operating in the same navigational environment base their decisions on the same up-to-date safety information, thereby enhancing situational awareness and navigational safety.

From a navigational safety perspective, certain temporary changes should be displayed on electronic nautical charts in near real time. Examples include military exercises involving live firing, mine clearance operations, diving activities, underwater works, and other events that may constitute a significant hazard to navigation within a specific spatial and temporal context.

The present study should be interpreted as an exploratory analysis of regulatory and informational asymmetry. The observed patterns indicate a systemic vulnerability arising from parallel chart systems and uneven MSI dissemination, but they do not establish direct causal relationships for individual grounding incidents. The conceptual objective function introduced in this study formalizes the temporal asymmetry in MSI propagation across different chart-system chains. It is not presented as a validated predictive model. Although this study proposes a classification of entities according to the duration of their effects, the level of navigational risk associated with individual entities is not examined in detail. The development of a model for assessing and ranking the navigational risk posed by different entities represents a potential area for future research.

5. Conclusions and Recommendations

The data presented in this study support only a descriptive conclusion regarding the dominant share of recreational and non-SOLAS vessels among the analyzed grounding cases. They do not demonstrate that these incidents were caused by chart-system disparity or delayed MSI updates. The findings should therefore be interpreted as indicating a systemic vulnerability rather than proving a direct causal mechanism. This study examined the navigational safety implications of the regulatory divide between SOLAS and non-SOLAS vessels regarding nautical chart carriage requirements in the Eastern Adriatic Sea. The traffic analysis demonstrated that vessels not subject to mandatory ECDIS carriage requirements dominate maritime traffic, while the grounding analysis showed that more than 90% of recorded grounding incidents involved non-SOLAS vessels, predominantly during favourable weather conditions and high-traffic seasons. International accident investigations further corroborate that the use of non-official and inadequately maintained chart systems is a significant contributory factor in grounding accidents [18,36,38]. These findings confirm that vessels operating under different navigational information standards routinely share the same waterways, creating conditions in which mariners may rely on different sources and versions of safety-critical information.

The results suggest that the safety challenge extends beyond the distinction between official and unofficial electronic navigational charts. While chart quality and data integrity remain important considerations, the findings indicate that the synchronization of MSI updates constitutes a critical systemic factor influencing navigational safety. As shown in the comparative analysis of regulatory frameworks (Table 3) and the model of update time delays (Section 4.4), differences in the mechanisms used to distribute, update, and present navigational information can result in mariners operating within the same geographical area making decisions based on different versions of MSI. The central argument in this paper is therefore that beyond the static differences in chart quality, the dynamic element of MSI update synchronization constitutes a critical, systemic risk.

The formalization of this risk through a conceptual objective function—modelling the time delays ${\displaystyle \frac{{\partial p}_n}{\partial t}}$ in MSI incorporation across different chart production chains—provides a conceptual tool to understand and address the hazard. The ultimate safety goal is to minimize the divergence between these update processes, thereby ensuring that all mariners, regardless of their vessel’s status or equipment, base their decisions on the same core safety information.

The findings further indicate that a simple prohibition of unofficial electronic navigational charts is unlikely to achieve the desired safety improvements. Existing ECS equipment remains widely used throughout the non-SOLAS sector and, in many cases, is not technically capable of displaying official ENCs compliant with current IHO standards. Moreover, the continued installation of ECS equipment on newly built vessels demonstrates that practical, operational, and economic considerations must be taken into account when designing future regulatory measures. In practice, many crews continue to navigate using paper nautical charts supplemented by mandatory NtM. Consequently, a substantial proportion of the non-SOLAS fleet operates without fully benefiting from the operational advantages offered by official ENCs and approved electronic chart-display systems. This situation illustrates the complexity of integrating modern electronic navigation technologies into everyday maritime operations. A prohibition on unofficial charts does not address the core risk—the time lag between the updating of official ENCs (p₁) and unofficial products (p₃)—unless accompanied by a mandatory transition to official ENC data across all electronic chart systems. The regulatory recommendations proposed in this paper should be interpreted as long-term policy directions rather than immediately implementable measures. Their practical implementation would require a phased and coordinated transition involving regulatory harmonization, technical readiness assessment, training provision, enforcement capacity, and economic evaluation at both national and international levels. In particular, any extension of mandatory ECDIS/ENC carriage, or any requirement for official ENC data within ECS platforms, would need to be accompanied by a realistic transition strategy that accounts for vessel categories, existing equipment, operator competence, and the operational and financial constraints of the affected fleets. The present study therefore identifies a regulatory and informational asymmetry that warrants policy attention, but it does not provide a complete implementation roadmap or feasibility framework for immediate adoption.

To mitigate the identified risk and promote synchronized MSI dissemination, a two-pronged regulatory strategy is proposed, as summarized in

Table 4. List of proposed amendments to the existing regulations.

Application Standards for SOLAS Ships	Application Standards for Non-SOLAS Ships
Extension of the carrying requirements to apply the ECDIS to the following SOLAS ships: cargo ships of less than 3000 GT tankers of less than 3000 GT passenger ships of less than 500 GT	Adoption of mandatory IMO guidelines for use of the ECS with a standardized format for ENC according to the IHO standards S-57/S-101
Following the adoption of amendments to extend the obligation to carry ECDIS and ENC on board SOLAS ships, the necessary changes to the rules on the holding of certificates of competency for seafarers should be harmonized and re-regulated in the national regulations	Adoption of regulations (with the appropriate transition period) on the mandatory and exclusive application of ENC on ECS
	During transition period, encourage hydrographic organizations to enable and expand print-on-demand paper charts production and distribution system for all SOLAS and non-SOLAS vessels without ECDIS as primary system
Compulsory examination of seafarers for the use of ECDIS or ECS with an issue of a time-limited (5 years) certificate.

.

The proposed amendments are detailed below for each vessel category. For SOLAS vessels, the study supports extending mandatory ECDIS and official ENC carriage requirements to all cargo and passenger vessels irrespective of tonnage. This measure would standardize the primary navigational information system across the entire regulated commercial fleet, ensuring that the high-integrity update process p₁ becomes the universal standard for professional maritime traffic and directly elevating the baseline of situational awareness within this sector. For non-SOLAS vessels, the results suggest the need for a harmonized regulatory framework that would require the use of official ENC data within ECS platforms and establish standardized mechanisms for MSI integration and updating. The objective is to legally and technically align the private data update process p₃ with the official process p₁, directly targeting the minimization of the time-delay objective function for the dominant user group and reducing informational asymmetry.

These core regulatory proposals require two supporting measures. First, mandatory, standardized training on system use and MSI interpretation is essential to ensure that seafarers can effectively utilize synchronized information. Without adequate training and practical navigational skills, the introduction of advanced electronic navigation systems on non-SOLAS vessels is unlikely to achieve the intended safety benefits. Such a transition would require vessel operators and navigational personnel to undergo appropriate ECDIS training, including both generic and type-specific courses, in accordance with the applicable requirements of the International Convention on Standards of Training, Certification, and Watchkeeping for Seafarers (STCW). Only through the combined implementation of official electronic charts, approved display systems, and adequate training can the full safety benefits of modern electronic navigation be realized. Second, the expansion of reliable print-on-demand services for official paper charts is necessary. This service provides a crucial bridge during any transition, ensuring the update channel for traditional paper charts (p₂) remains viable and current, thereby supporting vessels that may not immediately adopt electronic systems.

The transition towards the IHO S-100 framework presents a significant opportunity to address the challenges identified in this study. Future regulatory and technical developments should prioritize the synchronization of MSI delivery across all vessel categories, ensuring that mariners, regardless of vessel type or equipment carried, have access to the same authoritative and up-to-date MSI. Achieving this objective would represent an important step toward reducing navigational risk and enhancing maritime safety across the entire spectrum of vessels operating in shared waterways.

5.1. Significance and Implications for Maritime Stakeholders

The findings of this study hold significant implications for key stakeholders in the maritime domain, offering both empirical evidence and a conceptual framework for enhancing navigational safety.

For the International Maritime Organization (IMO), this study provides a data-driven argument to extend ECDIS and ENC carriage requirements to all SOLAS vessels, regardless of tonnage. It also supports a mandatory framework for official ENC use within ECS on non-SOLAS vessels. The mathematical model of MSI update delays offers a novel tool that could assist future regulatory impact assessments and cost–benefit analyses.

National maritime administrations and safety bureaus can use these results to reinforce the enforcement of existing regulations, such as the Croatian Circular letter from 2020. The study demonstrates that simply prohibiting unofficial charts is insufficient unless combined with a mandatory transition to official ENC data and standardized training. It also highlights the urgent need for systematic investigations of non-SOLAS vessel groundings, since current data limitations often obscure true root causes.

For hydrographic offices and chart makers, the findings emphasize the importance of timely MSI dissemination. The proposed conceptual objective function can serve as a benchmark for comparing update delays across different production chains (p₁, p₂, p₃). This may encourage hydrographic offices to improve the speed and integration of their update services, particularly for paper and RNC products.

Mariners and vessel operators are reminded of the critical role that up-to-date, authoritative information plays in situational awareness. The study provides a clear rationale for investing in official ENC-compatible equipment and relevant training. Better synchronization of MSI directly contributes to reducing the risk of grounding incidents.

Ultimately, this study contributes to a broader shift in maritime safety thinking—from reactive, post-incident analysis to proactive, systemic risk management based on the synchronization of critical information flows across all vessel categories.

5.2. Limitations and Future Work

A matched control group of comparable vessels with verified chart-system configurations was not available in the official data sources used in this study. This limitation constrains causal inference and prevents a direct comparison between vessels operating with different chart-system arrangements. The survey of charter agencies provides useful background on the prevailing chart-system configuration in recreational crafts, but it does not substitute for a matched comparative design. The traffic and grounding analyses were geographically limited to the Eastern Adriatic Sea, while the availability of detailed causal information for non-SOLAS grounding incidents was constrained. This highlights the need for systematic and comprehensive investigations of non-SOLAS vessel accidents in the Eastern Adriatic. The collection of detailed accident-causation data would provide a stronger empirical basis for validating the risk factors identified in this study and for developing targeted regulatory, technical, and operational measures to reduce the likelihood of similar accidents occurring in the future.

The proposed objective function is a conceptual model; its parameters (e.g., precise time delays for p₂ and p₃) require further empirical validation through collaboration with hydrographic offices and chart producers. Future research should focus on quantifying the real-world time delays in different MSI update chains and modelling the correlation between these delays and accident probability. The ongoing transition to the IHO S-100 framework presents a pivotal opportunity in this regard: empirical data collected during this transition could provide an evidence-based foundation for the development of future international standards and regulatory frameworks. Such efforts would directly support the broader objective of closing the data synchronization gap between SOLAS and non-SOLAS vessels, thereby contributing to enhanced navigational safety for all.

The present study has important limitations that constrain the strength of causal inference. Detailed vessel-level information on navigational equipment, chart usage, update status, operator behaviour, and bridge procedures was not available in the Croatian official records. Likewise, exposure-normalized data and matched control groups were not accessible in a consistent form. For this reason, the study should be interpreted as an exploratory system-level analysis of regulatory and informational asymmetry, rather than as a full accident-causation investigation. Future research should build on these findings with richer incident data, vessel-level observations, and comparative risk modelling.