Seakeeping Performance of a New Coastal Patrol Ship for the Croatian Navy

: This paper presents seakeeping test results for a coastal patrol ship (CPS) in the Croatian Navy (CN). The full-scale tests were conducted on a CPS prototype that was accepted by the CN. The seakeeping numerical prediction and model tests were done during preliminary project design. However, these results are not fully comparable with the prototype tests since the ship was lengthened in the last phases of the project. Key numerical calculations are presented. The CPS project aims to renew a part of the Croatian Coast Guard with ﬁve ships. After successful prototype acceptance trials, the Croatian Ministry of Defence (MoD) will continue building the ﬁrst ship in the series in early 2020. Full-scale prototype seakeeping test results could be valuable in the design of similar CPS projects. The main aim of this paper is to publish parts of the sea trial results related to the seakeeping performance of the CPS. Coast guards around the world have numerous challenges related to peacetime tasks such as preventing human and drug tra ﬃ cking, ﬁghting terrorism, controlling immigration, and protecting the marine environmental. They must have reliable platforms with good seakeeping characteristics that are important for overall ship operations. The scientiﬁc purpose of this paper is to contribute to the design process of similar CPS projects in terms of the development of seakeeping requirements and their level of fulﬁllment on an actual


Literature Review
Full-scale ship sea trials are the most important test of a ship's structure, equipment, and crew in order to prove its security, reliability, and operational capability. In principle, a ship's trials should provide the final check of the adequacy of theoretical and experimental predictions of ship behavior [1]. Ship trials are carried out for a variety of reasons, including to: Confirm that the ship meets her design intention as regards performance; 2.
Predict performance during service; 3.
Prove that equipment can function properly in the shipboard environment; 4.
Provide data on which future ship designs can be based; and 5.
Determine the effect on human performance.
Ship seakeeping design predictions are based on numerical analyses and model tests that are conducted in model tanks. Validation, using full-scale experimental data, is essential to the development of a ship's motion prediction code [2]. Reference [2] presents validations of ship motion predictions using model tank tests and full-scale sea trials for a Canadian naval destroyer. Full-scale trials are considered crucial because they include physical phenomena lacking in model tests due to oversight or scaling effects.
The results of full-scale seakeeping trials on several ships, including the Dutch naval destroyer "HM Groningen" are described in [3]. The main goal of these tests was to compare full-scale test results with numerical predictions that had been developed. It was noted that suitable conditions at sea are hard to find, or that the measuring or analyzing methods were not sufficient for correlation purposes.
A new experimental methodology to accurately predict wave-induced motions and load responses of ships was proposed in [4]. It is based on self-propelled large-scale model measurements that were conducted in natural environmental conditions. Onboard systems, operated by the crew, were used to measure and record sea waves and the responses of a model. A post-voyage analysis of the measurements, both of the sea waves and the model's responses, was conducted to predict the ship's motion and short term load responses to a corresponding sea state.
The results of extensive full-scale seakeeping trials of an all-weather lifeboat, conducted by the Royal National Lifeboat Institution in collaboration with Newcastle University and Lloyd´s Register, are shown in Reference [5]. The trials investigated the seakeeping behavior of the craft in real operational conditions. Patrol vessels must have good performance criteria for seaworthiness, and an analysis of the hydrodynamic aspects of a ship's design is one of the designer's primary tasks [6]. This analysis describes ship motion and ship resistance. The seakeeping tests conducted two variations of a ship loading condition and involved two sea states, namely the World Meteorological Organization (WMO) sea states 3 and 4.
In [7], the stern boat deployment system was investigated to evaluate the capability of launching and recovering a rigid hull inflatable boat (RHIB) via the stern ramp. The seakeeping characteristics required for the successful operation of a mother ship and inflatable boat were analyzed.

Paper Content and Main Particulars of the Coastal Partrol Ship
This paper presents the seakeeping requirements for a coastal patrol ship (CPS) for the Croatian Navy (CN) and the results of the full-scale seakeeping performance of the ship. The seakeeping requirements are based on Reference [8], which presents the seakeeping criteria for the reference ship, a frigate-size vessel. Because the CPS is a significantly smaller ship, the requirements are downsized accordingly, and presented in Section 2. The seakeeping requirements are composed of: the motion criteria of the ship (roll, pitch); vertical and lateral accelerations at significant positions on the ship; propeller emergence; deck wetness; slamming; and relative vertical motions of the edge of the ship stern. Full-scale trials were conducted during the acceptance of the ship, and the corresponding above-mentioned criteria were applied. After the ship was accepted by the CN, the prototype sea trials were conducted. The purpose of these tests was to check the utmost capabilities of the ship on higher sea states and see how those conditions affect the ship's hull, machinery, other equipment, and the crew. The results of full-scale trials are presented in Section 4. The main CPS particulars are shown in Table 1.
The seakeeping numerical prediction and model tests were done during preliminary project design, but these results are not comparable with the full-scale tests since the ship was significantly changed in the last phases of the project (e.g., lengthened).
The general arrangement and body plan of the CPS are shown in Figures 1 and 2. The main tasks for the new coastal patrol ship are to conduct:

1.
Peacetime tasks: • Low enforcement at sea; • Protection of fishing; • Control and prevention of possible ecology incidents; • Combat against terrorism; • Trafficking of people and narcotics;

2.
Miscellaneous tasks such as search and rescue and support of the local population in crises; 3.

Seakeeping Requirements
Seakeeping requirements are based on Reference [8], and are reduced to suit the CPS size. Following Reference [9], the CPS should be fully operational at mid sea state 4 (H1/3 = 1.8 m) following the WMO scale, but not including the launch and recovery of the RHIB. Launch and recovery of the

Seakeeping Requirements
Seakeeping requirements are based on Reference [8], and are reduced to suit the CPS size. Following Reference [9], the CPS should be fully operational at mid sea state 4 (H 1/3 = 1.8 m) following the WMO scale, but not including the launch and recovery of the RHIB. Launch and recovery of the RHIB should be feasible at sea state 3 (H 1/3 = 1.25 m-WMO) and with the CPS sailing at least 5 knots with heading waves. The CPS should be partially operational up to sea state 5 (H 1/3 = 3.1 m-WMO) including sailing in a manner suitable for surveillance, monitoring the operational situation, and reporting. The requirements also include survivability of the CPS on the highest sea state observed on the Adriatic Sea. Other specific seakeeping requirements are shown in Table 2.

Test Conditions and Measuring Equipment
The trials were conducted with half-full fuel and water tanks, in addition to stocked supplies, at departure. A record sheet captured additional information: sea state; wind speed; wind direction; ship's heading; GPS coordinates; engine RPM; speed over ground; speed through water; draft and displacement of ship at departure; and trial times and duration. The sea states were estimated by three crew members with extensive nautical experience. They were based on sea state descriptions of the Adriatic Sea set by Prof. Tonko Tabain according to experiments conducted during the 1970s [10]. Prof. Tabain also set the relationship between the Adriatic sea state scale and the WMO scale. Sea state estimates were also compared with the sea states provided by the Croatian Meteorological Institute (CMI) to validate the estimates provided by the crew members. The CMI uses a network of buoys for wave measurement and providing related waves statistics. It was difficult to find the required sea state conditions and the trials therefore lasted longer than expected. To record parameters for all seakeeping criteria described in Section 2, the CPS sailed for 20 minutes for each set of criteria. For measuring rolling and pitching angles, a fibro optic gyro (FOG) onboard the CPS was used. The FOG provided raw motion data that were recorded on a laptop and processed later. A typical sequence of maritime courses for seakeeping tests was used as shown in Figure 3. Following Reference [11], root mean square (rms) values of signals were calculated using the expression: (1) where N represents the number of samples, δ is the measured signal value, and δ is the mean value of the signal. The highest expected measured frequency was less than 0.5 Hz and the frequency of FOG data was 4.0 Hz, which assured consistent and well-sampled data. Accelerometers were used to measure vertical and lateral accelerations of specific points on the ship (i.e., the wheelhouse, the control cabin near the engine room, and the stern of the ship). Vertical displacement of the stern was measured using a liquid level sensor that was attached to the stern of the ship. Deck wetness, slamming, and propeller emergence were determined by counting their occurrences during one sailing course of 20 min, and then recalculating at the number of occurrences per hour. required sea state conditions and the trials therefore lasted longer than expected. To record parameters for all seakeeping criteria described in Section 2, the CPS sailed for 20 minutes for each set of criteria. For measuring rolling and pitching angles, a fibro optic gyro (FOG) onboard the CPS was used. The FOG provided raw motion data that were recorded on a laptop and processed later. A typical sequence of maritime courses for seakeeping tests was used as shown in Figure 3. Following Reference [11], root mean square (rms) values of signals were calculated using the expression: where N represents the number of samples, δ is the measured signal value, and ̅ is the mean value of the signal. The highest expected measured frequency was less than 0.5 Hz and the frequency of FOG data was 4.0 Hz, which assured consistent and well-sampled data. Accelerometers were used to measure vertical and lateral accelerations of specific points on the ship (i.e., the wheelhouse, the control cabin near the engine room, and the stern of the ship). Vertical displacement of the stern was measured using a liquid level sensor that was attached to the stern of the ship. Deck wetness, slamming, and propeller emergence were determined by counting their occurrences during one sailing course of 20 minutes, and then recalculating at the number of occurrences per hour.

Seakeeping Performance Results
Seakeeping performance results are presented in Tables 3-5, and were taken from the report by the company hired by the Croatian Ministry of Defence (MoD) to conduct the trials [13]. Table 3 presents the results of the trial at sea state 3-4 (WMO), and at speeds of 0 and 5 knots. The speed of 5 knots was one of the required operational speeds designed for launch and recovery of the RHIB. One of the main goals, in addition to measuring all seakeeping parameters, was to check vertical displacement at the aft perpendicular (AP). This parameter is important for assessing the safe launch and recovery of the RHIB. For the operational sea state 3-4 (WMO), this parameter was 0.6 m, a result below the required limits and signaling the success of this crucial operation. This was born out in practice, with the RHIB successfully being launched and recovered during sea trials. Later on, when the crew had become more experienced, they launched and recovered the RHIB even at sea state 4-5 (WMO). All other parameters were within the required limits. Table 4 presents the results of the trial at sea state 3-4 (WMO), and at a speed of 15 knots, representing the cruising speed of the CPS. As can be seen, all parameters were within required limits, except the angle of pitch (1.9 • rms) at the heading angle of 90 • , and the number of slammings that were slightly above the allowable limits. Table 5 shows the results of the trial at sea state 3-4 (WMO), and at a speed of 27 knots (i.e., maximum continuous speed). The parameters that exceeded the required thresholds were the angle of pitching at the heading of 45 • and 135 • , deck wetness (33), and slamming (70). It was expected that the parameters for maximum continuous speed would be exceeded, although there were no requirements to measure these parameters at this speed.    Tables 6 and 7 show the results of numerical calculations, based on strip theory and Jonswap spectra, conducted during the preliminary phase of ship design.
Measuring signal examples for the angles of pitching and rolling at v = 15 kn and wave heading = 45 • are given on Figures 4 and 5 respectively.
In further trials, the prototype of the vessel was tested at the higher sea states 4-5 (WMO), and the results obtained were more than satisfactory. Even in these sea state conditions, the RHIB could be successfully launched and recovered, and there was negligible loss of speed. Generally, the vessel demonstrated very good seakeeping characteristics.  Measuring signal examples for the angles of pitching and rolling at v = 15 kn and wave heading = 45° are given on Figures 4 and 5 respectively.  In further trials, the prototype of the vessel was tested at the higher sea states 4-5 (WMO), and the results obtained were more than satisfactory. Even in these sea state conditions, the RHIB could be successfully launched and recovered, and there was negligible loss of speed. Generally, the vessel demonstrated very good seakeeping characteristics.

Discussion
The presented seakeeping results and performances of the CPS show very good seaworthiness capabilities of the ship. The ship is fully operational at the mid sea state 4 (WMO), which was one of the main seakeeping criteria. When the crew had enough experience manipulating the RHIB, the RHIB could be launched and recovered even at the upper limit of sea state 4. During tests on higher sea states 5 (WMO), the ship showed reasonable seakeeping characteristics, a high level of reliability of all ship subsystems, and a low level of speed loss on waves. Considering some specific criteria that were exceeded (e.g., slamming at maximum continuous speed in a head sea), this was expected and does not mean that ship has unsatisfactory seakeeping performance. There were also some deviations   In further trials, the prototype of the vessel was tested at the higher sea states 4-5 (WMO), and the results obtained were more than satisfactory. Even in these sea state conditions, the RHIB could be successfully launched and recovered, and there was negligible loss of speed. Generally, the vessel demonstrated very good seakeeping characteristics.

Discussion
The presented seakeeping results and performances of the CPS show very good seaworthiness capabilities of the ship. The ship is fully operational at the mid sea state 4 (WMO), which was one of the main seakeeping criteria. When the crew had enough experience manipulating the RHIB, the RHIB could be launched and recovered even at the upper limit of sea state 4. During tests on higher sea states 5 (WMO), the ship showed reasonable seakeeping characteristics, a high level of reliability of all ship subsystems, and a low level of speed loss on waves. Considering some specific criteria that were exceeded (e.g., slamming at maximum continuous speed in a head sea), this was expected and does not mean that ship has unsatisfactory seakeeping performance. There were also some deviations in the angle of pitching at a wave direction of 0 and 45 degrees that were unsurprising due to the size

Discussion
The presented seakeeping results and performances of the CPS show very good seaworthiness capabilities of the ship. The ship is fully operational at the mid sea state 4 (WMO), which was one of the main seakeeping criteria. When the crew had enough experience manipulating the RHIB, the RHIB could be launched and recovered even at the upper limit of sea state 4. During tests on higher sea states 5 (WMO), the ship showed reasonable seakeeping characteristics, a high level of reliability of all ship subsystems, and a low level of speed loss on waves. Considering some specific criteria that were exceeded (e.g., slamming at maximum continuous speed in a head sea), this was expected and does not mean that ship has unsatisfactory seakeeping performance. There were also some deviations in the angle of pitching at a wave direction of 0 and 45 degrees that were unsurprising due to the size of the ship.
Upon increasing ship speed from 5 to 15 knots (cruising speed), the seakeeping parameters changed as follows: • Roll angle became lower for the wave headings 0 • and 180 • due to the hydrodynamic stabilization of ship, but it became higher for beam seas (90 • ).

•
Pitch angle was generally higher, except for a wave heading of 0 • . • Vertical accelerations were higher for all wave headings.

•
Lateral accelerations had a similar pattern to the rolling of the ship.
Upon raising the ship speed to maximum continuous, it can be observed that all seakeeping parameters rose significantly, and the highest rising rate was related to the vertical acceleration, slamming, and deck wetness, which was expected especially from the heading seas.
Comparing numerical calculations and full-scale test results, the following conclusions can be drawn: • Numerical calculations showed much higher roll and lateral acceleration responses because the numerical model could not take into account the influence of viscous forces on ship roll by ship appendages such as bilge keels, stern trim plate, etc.

•
Numerical prediction of pitch angle and vertical accelerations showed reasonable agreement.
Seakeeping is one of the most important characteristics of a ship, and there is a need to continuously and rigorously consider it throughout all phases of ship design. This was the case with the CPS; seakeeping was tested through numerical analyses, model tests, and finally, full-scale tests on the ship. Setting seakeeping requirements at an early phase of the project is crucial for successful ship design, and the results presented herein indicate that it is reasonable and useful to set these seakeeping requirements for patrol ships of the CPS' size, as it helps designers to satisfy them.
Author Contributions: A.L. contributed to the paper by establishing the concept of paper and definitions of research goals and aims. He participated in conducting the research, control of experiments, and data collection and integration. He wrote an initial paper draft including data and graphics design and presentation. V.S. contributed to the paper through its review, validation, and its final preparation for publication. He participated in the analysis of references dealing with this research area. He also did management and coordination of the research planning. He is responsible for the presentation and interpretation of the work. All authors have read and agree to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.