Review Reports - Shaft-Power-Based Load Reconstruction for Operating-Point Alignment During Sea Trials of a CPP-Equipped Two-Stroke Marine Diesel Engine

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article presents an analysis of data from a sea trial of a vessel equipped with a CPP propeller, where engine operating loads are determined from measured shaft power and compared with data from the engine control unit.

The article is written extremely clearly and systematically. The information about the vessel, engine, fuel used and the method of conducting the measurements is complete. The English is fine and does not require any improvement.

There are only few minor details that could be improved.

In the line 161, the sentence that begins with "The specified fuel efficiency at SMCR is 173.0 g/kWh ..." should be changed to "The specific fuel oil consumption at SMCR is 173.0 g/kWh ..."

The diagram on Figure 6. represents ISO-corrected SFOC and fuel flow across operating points. The SFOC values shown in the diagram are significantly higher than those achieved on the test bench. Also, the values show a significant jump at the 50% load level which is not typical for the FPP propeller propulsion. It would be necessary for the authors to add an explanation or comment to the text of the article as to why the SFOC values have such values and trend, which would also contribute to a clearer understanding for the potential reader.

The discussion of the results and conclusions were explained in detail and are in accordance with the obtained results, and there are no objections to the conclusions presented.

Author Response

To Reviewer 1

We sincerely thank the reviewer for the careful reading of our manuscript and for the highly positive and constructive comments. We greatly appreciate the reviewer’s recognition that the manuscript is clearly and systematically written, that the information on the vessel, engine, fuel, and measurement procedure is complete, and that the discussion and conclusions are well supported by the obtained results. We have carefully revised the manuscript in accordance with the reviewer’s suggestions. Our point-by-point responses are provided below.

Comment 1

In the line 161, the sentence that begins with “The specified fuel efficiency at SMCR is 173.0 g/kWh ...” should be changed to “The specific fuel oil consumption at SMCR is 173.0 g/kWh ...”

Response

We sincerely thank the reviewer for this precise comment. We agree that “fuel efficiency” was not the appropriate technical term in this context. The sentence has been revised accordingly.

Revised text:
“The specific fuel oil consumption at SMCR is 173.0 g/kWh ...”

Comment 2

The diagram on Figure 6 represents ISO-corrected SFOC and fuel flow across operating points. The SFOC values shown in the diagram are significantly higher than those achieved on the test bench. Also, the values show a significant jump at the 50% load level which is not typical for the FPP propeller propulsion. It would be necessary for the authors to add an explanation or comment to the text of the article as to why the SFOC values have such values and trend, which would also contribute to a clearer understanding for the potential reader.

Response

We thank the reviewer for this valuable suggestion. We agree that additional explanation improves the interpretation of Figure 6 and enhances the clarity of the fuel-performance trend for readers.

In the revised manuscript, we added explanatory text in Section 3.4 to clarify that the elevated ISO-corrected SFOC values observed during the sea trial, particularly around the nominal 50% operating region, may be understood in light of the difference between controlled shop-test conditions and actual ship operating conditions. We clarified that the maker’s shop-test SFOC represents a controlled reference condition, whereas the sea-trial values reflect the combined effects of hull resistance, propulsion-system interaction, auxiliary demand, and off-design operation. We also added that, because the present vessel employs CPP propulsion, the intermediate-load operating region may additionally be influenced by pitch-related absorbed-load characteristics. On this basis, the local SFOC increase near the 50% region was explained as a combined effect of real sea-trial operating conditions and CPP-related propulsion characteristics.

Added text in the revised manuscript:
“The elevated ISO-corrected SFOC values observed during the sea trial, particularly around the nominal 50% operating region, may be understood in light of the difference between controlled shop-test conditions and actual ship operating conditions. Whereas the maker’s shop-test SFOC represents a controlled reference condition, the sea-trial values reflect the combined effects of hull resistance, propulsion-system interaction, auxiliary demand, and off-design operation. In addition, because the present vessel employs CPP propulsion, the operating point in the intermediate load range may also be influenced by pitch-related absorbed-load characteristics. Accordingly, the observed SFOC increase near the 50% region may reflect the combined effect of real sea-trial operating conditions and CPP-related propulsion characteristics.”

We once again sincerely thank the reviewer for the thoughtful and encouraging comments. These suggestions were very helpful in improving the precision of the terminology and the interpretation of the fuel-performance characteristics discussed in the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study uses full-scale sea-trial data to reconstruct engine load based on measured shaft power in a CPP-equipped marine diesel system. It shows that shaft-power-based load estimation aligns well with engine performance trends and provides a realistic representation of operating conditions. The results highlight differences between speed-trial and endurance conditions, indicating they should not be directly compared as a single load curve. Overall, the work demonstrates a practical experimental framework but stops short of developing a generalized predictive correlation model.

Comments :

The study would benefit from deriving an explicit empirical correlation between shaft power and fuel consumption.
Consider formulating a regression-based model linking reconstructed load with key performance parameters.
A generalized correlation equation could enhance applicability beyond the tested vessel.
Quantifying relationships between torque, RPM, and fuel flow would strengthen predictive capability.
Incorporating multivariable analysis could reveal deeper dependencies among operating variables.
Developing a normalized, dimensionless correlation would improve cross-system applicability.
Statistical validation (R², error metrics) should be included for any proposed correlations.
A comparison with existing propulsion performance models would contextualize the findings.
Extending the dataset could support building a more robust and transferable correlation model.
The study is well-positioned to evolve from trend analysis into a predictive correlation framework.

Author Response

To Reviewer 2

We sincerely thank Reviewer 2 for the thoughtful, constructive, and encouraging comments. We greatly appreciate the reviewer’s positive assessment of the practical value of our full-scale sea-trial framework, particularly the recognition that shaft-power-based load reconstruction provides a more realistic interpretation of operating conditions in a CPP-equipped low-speed two-stroke marine diesel engine, and that speed-trial and endurance conditions should not be treated as a single continuous load curve.

In response to the reviewer’s suggestions, we revised the manuscript while maintaining the main scope of the study, namely, shaft-power-based operating-point alignment and load reinterpretation under sea-trial conditions. In the revised manuscript, we strengthened the quantitative part of the study by adding regression-based relationships between measured shaft power and major engine indicators, together with statistical descriptors such as Pearson’s r, R², and p-values. At the same time, we carefully limited the interpretation of these added relationships to descriptive, dataset-specific empirical correlations, rather than universal predictive laws. For suggestions that could not be fully implemented within the present dataset and manuscript scope, we respectfully acknowledge their importance and identify them as valuable directions for future work.

Below, we provide our point-by-point responses.

Comment 1

The study would benefit from deriving an explicit empirical correlation between shaft power and fuel consumption.

Response:
Thank you for this helpful suggestion. In response, we added an explicit quantitative relationship between measured shaft power and fuel flow in the revised manuscript. Specifically, Section 3.6 and Table 8 now present a linear regression relationship between shaft power and fuel flow, together with the associated correlation coefficient and goodness-of-fit indicators. This addition was intended to show more clearly that the shaft-power-based operating-point framework is quantitatively consistent with the fuel-demand trend observed in the sea-trial data.

At the same time, we respectfully note that the aim of the present study is not to propose a universal fuel-consumption prediction law applicable to all vessels and propulsion systems. Rather, our purpose is to demonstrate that, when operating points are reinterpreted on the basis of measured shaft power, fuel-related behavior can be analyzed in a physically consistent manner. Therefore, the added relationship is presented as an empirical and descriptive correlation within the present dataset only.

Comment 2

Consider formulating a regression-based model linking reconstructed load with key performance parameters.

Response:
We appreciate this valuable suggestion. In the revised manuscript, we added regression-based quantitative relationships using measured shaft power as the reference variable within the shaft-power-based operating-point framework. Accordingly, linear regression relationships were introduced for fuel flow, Pmax, and IMEP, while a power-law-type descriptive fit was additionally presented for the shaft speed–shaft power relationship to reflect the nonlinear trend visible in the present dataset.

We respectfully clarify that the main objective of this study is not the construction of a generalized predictive model itself, but rather the validation of the physical consistency and interpretive usefulness of shaft-power-based load reinterpretation. For this reason, the added regression relationships are presented as quantitative supporting tools that complement the core contribution of the study.

Comment 3

A generalized correlation equation could enhance applicability beyond the tested vessel.

Response:
Thank you for this important observation. We agree that a more generalized correlation equation could enhance applicability in future studies. However, the present work is based on a single full-scale vessel, a single engine-propulsion system, and a limited number of representative sea-trial operating points. Under such conditions, introducing a generalized correlation equation at this stage could risk overgeneralizing trends that remain case-specific.

Accordingly, in the revised manuscript we chose to present the added quantitative relationships cautiously as descriptive empirical correlations within the present dataset, rather than as broadly transferable predictive laws. We fully agree that a generalized correlation framework would be a meaningful extension, and we have therefore identified this as an important direction for future work based on expanded multi-vessel and multi-condition datasets.

Comment 4

Quantifying relationships between torque, RPM, and fuel flow would strengthen predictive capability.

Response:
We thank the reviewer for this constructive comment. In the revised manuscript, we clarified more explicitly the physical basis of the study, namely that shaft power represents the shaft-side mechanical output determined by torque and rotational speed, and that this quantity can serve as a physically meaningful reference for integrating shaft-side and engine-side information.

In addition, by presenting quantitative relationships between measured shaft power and both fuel flow and major combustion indicators, we strengthened the evidence that shaft-side quantities capture the actual operating state in a consistent manner. We respectfully note, however, that the development of a more direct and comprehensive predictive formulation simultaneously including torque, RPM, and fuel flow would require a broader dataset and a denser operating-point distribution than are available in the present study. We therefore regard this as a very worthwhile topic for future work and would like to pursue it carefully in subsequent research with expanded data coverage.

Comment 5

Incorporating multivariable analysis could reveal deeper dependencies among operating variables.

Response:
We are grateful for this insightful suggestion. We agree that multivariable analysis can be highly valuable for revealing deeper dependencies among operating variables. However, the present sea-trial dataset contains a limited number of representative operating points, and several variables such as shaft power, torque, and shaft speed are physically and mathematically closely linked. Under these conditions, introducing a more complex multivariable predictive model in the present revision would require caution in terms of robustness and interpretability.

Therefore, in this revised manuscript we focused first on strengthening the study through single-variable regression and correlation analysis referenced to measured shaft power, which is the central quantity in the proposed framework. We respectfully acknowledge the reviewer’s suggestion as highly meaningful, and we have identified broader multivariable modeling as an important subject for future work once a sufficiently expanded dataset becomes available.

Comment 6

Developing a normalized, dimensionless correlation would improve cross-system applicability.

Response:
Thank you for this important recommendation. In the present study, the shaft-load reconstruction itself was defined relative to the 100% reference shaft-power condition, so a normalized concept was already inherent in the load reinterpretation procedure. However, we respectfully acknowledge that the broader normalized or dimensionless correlation framework suggested by the reviewer was not fully developed in the present revision.

Given the scope of the present manuscript and the limitations of the available dataset, we chose to focus first on clarifying the physical and quantitative validity of the shaft-power-based alignment framework itself. We fully agree that a more general dimensionless correlation structure could enhance cross-system applicability, and we would like to pursue this carefully in future work using broader multi-vessel and multi-condition datasets.

Comment 7

Statistical validation (R², error metrics) should be included for any proposed correlations.

Response:
We appreciate this important point. In response, we added Pearson’s r, R², and p-values for the newly introduced regression and correlation relationships in the revised manuscript. These additions were intended to provide a clearer statistical basis for assessing the consistency and descriptive adequacy of the added empirical relationships.

At the same time, we respectfully note that the purpose of the present revision was to strengthen the quantitative support for the shaft-power-based operating-point framework, rather than to perform a fully developed predictive-model validation exercise. For this reason, the present revision focuses on the basic correlation and fit indicators most directly relevant to the scope of the paper. We agree that broader error-metric-based evaluation would be valuable, and we would be pleased to pursue such expanded validation in future work when more extensive datasets are available.

Comment 8

A comparison with existing propulsion performance models would contextualize the findings.

Response:
Thank you for this useful suggestion. In the revised Discussion, we clarified more explicitly how the present approach differs from interpretations based mainly on nominal load labels. In particular, under CPP operating conditions, nominal operating labels do not necessarily represent the actual shaft-side mechanical output in a physically consistent manner. For this reason, we emphasized that interpreting operating states using measured shaft power, shaft torque, and actual shaft speed provides a more direct and physically meaningful reference basis.

We respectfully note that the present study does not aim to replace existing propulsion performance models with a new universal model. Rather, it proposes a practical framework for sea-trial data interpretation, in which measured shaft power can serve as a more consistent reference quantity for operating-point alignment. We agree that broader and more direct comparisons with existing propulsion models would be valuable, and we would like to consider this carefully in future work.

Comment 9

Extending the dataset could support building a more robust and transferable correlation model.

Response:
We fully agree with the reviewer on this point. A more robust and transferable correlation model would indeed require a broader dataset covering different vessel types, engine configurations, propulsion arrangements, and operating environments. However, the primary purpose of the present study is to demonstrate the validity and practical usefulness of measured-shaft-power-based operating-point realignment and load reinterpretation through a single full-scale sea-trial case.

Accordingly, in this revision we performed quantitative strengthening only within the range supported by the currently available representative operating-point data. We agree that developing a more transferable model is a very valuable next step, and we have indicated this respectfully as an important direction for future research based on expanded datasets.

Comment 10

The study is well-positioned to evolve from trend analysis into a predictive correlation framework.

Response:
We sincerely thank the reviewer for this encouraging and constructive observation. We also believe that the present framework has the potential to evolve toward a more advanced predictive correlation framework in future research. In the current revision, however, we chose to maintain the original scope of the manuscript and focus primarily on demonstrating the physical validity and practical interpretive value of shaft-power-based operating-point alignment using full-scale sea-trial data.

Accordingly, the revised manuscript now includes additional quantitative relationships between measured shaft power and major engine indicators, thereby improving the quantitative completeness of the study. At the same time, we explicitly state that these relationships should be interpreted as descriptive correlations limited to the present dataset, rather than generalized predictive laws. We respectfully consider the reviewer’s suggested broader predictive development to be a highly meaningful future direction, and we hope to pursue it in subsequent work with richer and more extensive datasets.

Closing Remark

Once again, we sincerely thank Reviewer 2 for the careful, constructive, and encouraging review. The reviewer’s suggestions were highly valuable in helping us improve the clarity and quantitative strength of the manuscript. While maintaining the core scope of the study—shaft-power-based load reconstruction and operating-point harmonization under sea-trial conditions—we revised the manuscript to strengthen the quantitative relationships between measured shaft power and major engine indicators, and to clarify more explicitly both the meaning and the limitations of these added correlations. For the broader aspects that could not be fully implemented in the present revision, including generalized correlation models, expanded multivariable analysis, and broader dimensionless frameworks, we respectfully acknowledge their importance and would be pleased to pursue them carefully in future work.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study focuses on the core issue of operating-point alignment during sea trials of low-speed two-stroke marine diesel engines equipped with controllable pitch propellers (CPPs). It reconstructs the operating load based on full-scale measured shaft power and clarifies the independent operating characteristics of speed trial points and endurance rating points, with the research direction aligning with the engineering requirements for performance evaluation of marine propulsion systems. However, the paper has room for further improvement in several aspects, and specific suggestions are as follows:

The description of the test procedure in the abstract is somewhat verbose, and the conclusion section fails to specify the scope of application of the research findings. It is recommended to simplify the description of the test procedure in the abstract and supplement the scope of application of the research findings in the conclusion.
Shaft power during full-scale sea trials is significantly affected by external factors such as sea conditions, hull status, and loading conditions, yet the paper does not mention specific information, including sea state class and hull cleanliness, during the sea trials. It is recommended to add a table of sea trial environmental parameters.
This study only analyzes four speed trial points (25%, 50%, 80%, 100%) and two endurance rating points (NCR, MCR). It is recommended to supplement sea trial data for low and medium loads, or clarify the representativeness and applicable scope of the existing data for the model.
The paper mentions that the low/medium load conditions adopt the ecoEGR (Tier II) mode while the endurance rating points use the EGR (Tier III) mode. It is recommended to supplement a comparative analysis of indicators under different EGR modes to enhance the rigor of the conclusions.
The paper only qualitatively describes that shaft power and indicators such as fuel flow, Pmax, and IMEP show a "physically consistent trend". It is recommended to add correlation analysis and fitting models and provide specific correlation formulas.

Author Response

To Reviewer 3

We sincerely thank the reviewer for the careful reading of our manuscript and for the constructive and technically insightful comments. We greatly appreciate the reviewer’s positive assessment that this study addresses an important engineering issue in the operating-point alignment of CPP-equipped low-speed two-stroke marine diesel engines during sea trials. We have carefully revised the manuscript in response to all comments. Our point-by-point responses are provided below.

Comment 1

The description of the test procedure in the abstract is somewhat verbose, and the conclusion section fails to specify the scope of application of the research findings. It is recommended to simplify the description of the test procedure in the abstract and supplement the scope of application of the research findings in the conclusion.

Response

Thank you for this helpful comment. We agree that the original Abstract contained excessive procedural detail relative to the main scientific message. Accordingly, the Abstract has been revised to reduce the description of the test procedure and to place greater emphasis on the research objective, the shaft-power-based operating-point framework, the key quantitative findings, and the engineering significance of the study.

In addition, the Conclusion has been revised to clarify the scope of application of the present findings. We now explicitly state that the proposed shaft-power-based operating-point alignment framework is most directly applicable to full-scale sea-trial analyses of CPP-equipped low-speed two-stroke marine diesel engines under comparable measurement and trial conditions. We also clarify that the present findings are based on one vessel-engine configuration and a limited set of representative operating points, and therefore should be interpreted as a physically consistent alignment framework rather than as a universal predictive model across all operating conditions.

Revision made in manuscript:

The Abstract was shortened and refocused on the objective, major findings, and engineering implications.
The Conclusion was expanded to define the applicability, limitations, and future scope of the present framework.

Comment 2

Shaft power during full-scale sea trials is significantly affected by external factors such as sea conditions, hull status, and loading conditions, yet the paper does not mention specific information, including sea state class and hull cleanliness, during the sea trials. It is recommended to add a table of sea trial environmental parameters.

Response

We appreciate this important comment and agree that full-scale shaft power measured during sea trials can be influenced by environmental and vessel-condition factors in addition to engine output. In response, we added tables summarizing the available sea-trial environmental information and general official-trial condition data to provide clearer context for the full-scale shaft-power measurements used in this study.

At the same time, some vessel-condition items such as hull cleanliness, draft, trim, and loading condition were not explicitly documented in the archived source records. Rather than inferring unavailable values, we now acknowledge these record limitations explicitly in the manuscript. We also clarified that reciprocal-run averaging and stabilized-point extraction were used to reduce the influence of transient or asymmetric external disturbance to the extent possible within the available dataset.

Accordingly, the revised manuscript now more clearly distinguishes between:
(1) the shaft-power-based operating-point alignment itself, and
(2) the environmental and vessel-condition factors that may affect the absolute propulsion demand under full-scale sea-trial conditions.

Revision made in manuscript:

Added tables summarizing the available sea-trial environmental descriptors and general official-trial condition information.
Added explanatory text in the Methods and Discussion regarding the influence of environmental factors and the role of reciprocal-run averaging and stabilized-point extraction.
Explicitly acknowledged the limitations of archived vessel-condition information where such details were not recorded.

Comment 3

This study only analyzes four speed trial points (25%, 50%, 80%, 100%) and two endurance rating points (NCR, MCR). It is recommended to supplement sea trial data for low and medium loads, or clarify the representativeness and applicable scope of the existing data for the model.

Response

We thank the reviewer for raising this important point. We agree that the limited number of available operating points should be clearly framed in terms of representativeness and scope.

In the revised manuscript, we clarified that the purpose of this study is not to establish a universal continuous predictive model across the entire operating range, but rather to develop and demonstrate a physically consistent operating-point harmonization framework using representative full-scale sea-trial points defined by the acceptance-test protocol. The analyzed points correspond to standard speed-trial and endurance-test conditions conventionally used for propulsion performance verification. Therefore, while the present dataset is not sufficient to represent a dense full-range operating map, it is appropriate for the stated objective of operating-point alignment and comparison across sea-trial records.

We also explicitly acknowledge in the revised manuscript that additional low- and medium-load points would improve the extensibility of the framework and would be beneficial for future studies aimed at denser operating maps or broader predictive applications.

Revision made in manuscript:

Clarified that the present study is a representative operating-point alignment study rather than a full-range predictive modeling study.
Added statements in the Discussion and Conclusion regarding the representativeness, applicability, and limitations of the six-point dataset.
Added future-work statements concerning the need for denser low- and medium-load datasets.

Comment 4

The paper mentions that the low/medium load conditions adopt the ecoEGR (Tier II) mode while the endurance rating points use the EGR (Tier III) mode. It is recommended to supplement a comparative analysis of indicators under different EGR modes to enhance the rigor of the conclusions.

Response

We appreciate this insightful comment. We agree that the distinction between ecoEGR-associated operation and Tier III EGR operation should be addressed more explicitly in order to avoid possible overinterpretation.

In response, we revised the manuscript to clarify that the primary objective of the present study is not to compare emission-control modes themselves, but to establish a common propulsion-side operating-point basis using measured shaft power. We now explicitly state that differences in EGR operating mode may influence combustion- and fuel-related absolute indicator levels, including Pmax, IMEP, fuel flow, and SFOC, and should therefore be taken into account when interpreting engine-side variables across the full operating-point set.

At the same time, the revised manuscript clarifies that the shaft-power-based reconstruction method itself remains valid as a common propulsion-side reference because it is grounded in measured transmitted shaft output rather than nominal control-side load labels alone. We also identify matched-condition comparison between ecoEGR-associated operation and Tier III EGR operation as a meaningful subject for future work.

Revision made in manuscript:

Added clarification in the Discussion that EGR mode differences may influence the absolute levels of fuel- and combustion-related indicators.
Clarified that the methodological contribution of the paper is shaft-power-based operating-point alignment, not direct mode-to-mode EGR performance comparison.
Added a future-work statement recommending matched-condition comparison between ecoEGR and Tier III EGR operation.

Comment 5

The paper only qualitatively describes that shaft power and indicators such as fuel flow, Pmax, and IMEP show a “physically consistent trend”. It is recommended to add correlation analysis and fitting models and provide specific correlation formulas.

Response

Thank you for this valuable suggestion. We agree that the original manuscript relied too heavily on qualitative trend description and that a more quantitative treatment strengthens the rigor of the analysis.

In response, we added a dedicated quantitative analysis of the relationships between measured shaft power and the principal engine indicators. The revised manuscript now includes a new Results subsection presenting correlation and regression analyses, together with a summary table of the fitted relationships and associated statistical measures. We also revised the Discussion to clarify that these fitted relationships are intended as descriptive trends within the present representative sea-trial operating-point dataset, rather than as universal predictive models applicable to other vessels or operating environments.

Revision made in manuscript:

Added a new Results subsection on the quantitative relationship between shaft power and major engine indicators.
Added a summary table presenting correlation and fitting results.
Revised the Discussion to define the interpretive scope of these fitted relationships as dataset-specific descriptive trends.

Closing Statement

We sincerely thank the reviewer again for the careful and constructive comments. These suggestions significantly improved the clarity, rigor, and boundary definition of the manuscript. In particular, they helped us strengthen the manuscript in terms of Abstract and Conclusion structure, environmental-context reporting, dataset representativeness, interpretation of EGR-mode differences, and quantitative analysis of shaft-power-related performance trends. We hope that the revised manuscript now addresses the reviewer’s concerns more clearly and effectively.