Retrofitting a Pre-Propeller Duct on a Motor Yacht: A Full-Scale CFD Validation Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study focuses on the demand for improving energy efficiency in the luxury yacht sector, taking a 45-meter motor yacht (M/Y Incal) as the research object. By establishing a full-scale self-propulsion Computational Fluid Dynamics (CFD) model and verifying it with full-scale sea trial data, a pre-propeller duct-type Energy Saving Device (ESD) is designed and optimized. The core conclusions are clear and have strong engineering application value. However, the paper still needs to be improved in the following aspects:
1. The current analysis mainly focuses on the cruising speed (12.3 knots) and its adjacent speed points (11.5 knots, 12.8 knots). Nevertheless, motor yachts often operate under various working conditions, such as low speeds (e.g., 5–7 knots) and high speeds (e.g., 13–15 knots) in actual operation. It is recommended to supplement the performance evaluation of the low-speed segment (especially speeds below 7.7 knots) and the high-speed segment to fully reflect the energy-saving effect of the device under different sailing speeds.
2. The study does not involve the performance of the device under non-calm sea conditions. It is suggested to introduce simplified sea condition parameters (such as regular wave loads) into the numerical simulation, evaluate the adaptability of the device in environments like slight waves and moderate waves, and clarify whether there is efficiency degradation or potential structural risks under off-design conditions. 
3. If the energy-saving device causes significant cavitation, it will still be difficult to meet the engineering practical requirements. Can the risk of cavitation be appropriately considered?
4. The pre-propulsion diversion structure may affect the maneuverability of the ship. It is suggested to further conduct simulation analysis on key maneuverability indicators such as turning diameter and course stability to ensure that the ship still has good maneuvering performance after the device is installed.

Author Response

1. The current analysis mainly focuses on the cruising speed (12.3 knots) and its adjacent speed points (11.5 knots, 12.8 knots). Nevertheless, motor yachts often operate under various working conditions, such as low speeds (e.g., 5–7 knots) and high speeds (e.g., 13–15 knots) in actual operation. It is recommended to supplement the performance evaluation of the low-speed segment (especially speeds below 7.7 knots) and the high-speed segment to fully reflect the energy-saving effect of the device under different sailing speeds.

**We have substantially expanded the performance analysis to cover the full operational profile, including low-speed segments (5.0 and 6.0 knots) and high-speed segments (14.0 knots). The discussion now explicitly addresses the trade-offs and penalties observed at low speeds.**

>2. The study does not involve the performance of the device under non-calm sea conditions. It is suggested to introduce simplified sea condition parameters (such as regular wave loads) into the numerical simulation, evaluate the adaptability of the device in environments like slight waves and moderate waves, and clarify whether there is efficiency degradation or potential structural risks under off-design conditions.   

**We agree this is a critical limitation for engineering practicality. We have added a dedicated section to the Discussion/Future Work and established a clear path to address it: validating high-fidelity VOF simulations with pending field data from the yacht's operational trials.**

>3. If the energy-saving device causes significant cavitation, it will still be difficult to meet the engineering practical requirements. Can the risk of cavitation be appropriately considered?  

**The critical importance of cavitation has been incorporated into the future research directions. We note that the conditioned, more uniform wake profile is expected to be beneficial, but a full cavitation analysis is necessary and planned.**

>4. The pre-propulsion diversion structure may affect the maneuverability of the ship. It is suggested to further conduct simulation analysis on key maneuverability indicators such as turning diameter and course stability to ensure that the ship still has good maneuvering performance after the device is installed.

**This structural and hydrodynamic concern is now explicitly identified as a limitation and a key direction for future research, ensuring the scope of the current work is clearly defined while acknowledging the engineering requirement.**

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents an overview of CFD modelling of a propeller retrofitted on a Yaght. The paper is very well written, the focus is sound but it reads more like a demonstrator/technical report. For the paper to be accepted the following major corrections should be implemented :

1. Computational and boundary conditions should be well demonstrated in tabulated format and through discussion and in schematic. The authors should explain on what basis the conditions were chosen 
2. You should explain more details on the grid system and its verification or validation according to ITTC methods 
3. I do nto see any graphs confirming the comparison of thrust coeffriceint, torque coefficient and open water efficiency between EFD and CFD or between the parent ship and the retrofitted ship
4. In the results sections I woudl like to see clear comparisons of the hydrodynamic forces coefficients and their efficiency clearly demonstrating by comparison the benefits of retrofitting. This should nclude also the net thrust values. I would like to see the a demonstration of the instanateneous vector of the thrust velocity field in the xy plane and different angles of attack. What are the contours of the pressure coefficient, axial velocity, free stream velocity in the longitudinal plane? 
5. Theliterature survey should include a table highlighting key papers in this area and the engineering science value of the paper should be highlighted in the abtract and the introduction

Author Response

1. Computational and boundary conditions should be well demonstrated in tabulated format and through discussion and in schematic. The authors should explain on what basis the conditions were chosen 

**We have significantly enhanced Section 4 to provide comprehensive details on the methodology, including new tables and extensive discussion on domain sizing and boundary conditions, and the physical basis for their selection (e.g., Froude number dependence).**

>2. You should explain more details on the grid system and its verification or validation according to ITTC methods 

**We have included a detailed ITTC-compliant grid convergence study. The results, showing a GCI of $0.8\%$, confirm the accuracy and reliability of the chosen mesh size. Mesh quality metrics are also provided.**

>3. I do nto see any graphs confirming the comparison of thrust coeffriceint, torque coefficient and open water efficiency between EFD and CFD or between the parent ship and the retrofitted ship

**We included new figures that validate the body-force propeller model. The figures show the CFD operating points (for both baseline and ducted) plotted directly onto the industry-standard Wageningen B-Series curves (EFD data), confirming correct propeller loading and efficiency predictions.**

>4. In the results sections I woudl like to see clear comparisons of the hydrodynamic forces coefficients and their efficiency clearly demonstrating by comparison the benefits of retrofitting. This should nclude also the net thrust values. I would like to see the a demonstration of the instanateneous vector of the thrust velocity field in the xy plane and different angles of attack. What are the contours of the pressure coefficient, axial velocity, free stream velocity in the longitudinal plane? 

**We added a comprehensive table detailing the force balance and new contour plots. We show RANS streamlines/contours, which are appropriate for time-averaged simulations, instead of instantaneous vectors.**

>5. Theliterature survey should include a table highlighting key papers in this area and the engineering science value of the paper should be highlighted in the abtract and the introduction

**A new contextual literature table has been added to place our work among others, clearly demonstrating our unique contribution. We have also added a dedicated paragraph to the Introduction to highlight the engineering science value of the full-scale CFD validation.**

Reviewer 3 Report

Comments and Suggestions for Authors

In this manuscript, the authors presented a comprehensive hydrodynamic assessment of a 45-meter motor yacht, M/Y Incal, using a full-scale self-propulsion CFD model validated directly against sea trial data. 
The manuscript is interesting and well structured. The introduction chapter provides a sufficient detailed information about similar research. The methods and results presented in the paper are adequately described.
The topic of the manuscript may actually be interesting for the research community working in the same field.
Conclusions are supported by results and mostly illustrated with relevant graphs. 
The manuscript contains no information about cavitation, especially vortex cavitation, which can occur on propeller blades and on the NACA 0006 foil profile .
My comments are not critical flaws, but rather suggestions for completing the manuscript.
Generally, the manuscript "Retrofitting a Pre-Propeller Duct on a Motor Yacht: A Full-Scale CFD Validation Study" is good and can be published in the Journal of Marine Science and Engineering.
The manuscript be checked against misprints and grammatical mistakes.

Author Response

Thank you  for their positive feedback and the excellent suggestion about cavitation analysis. We agree this is a key point and have addressed it in revision 2.

We've now explicitly discussed cavitation in the Introduction (Lines 27-35), noting that the original propellers were suffering from cavitation erosion. We also expanded the Discussion (Lines 606-611) to state that future work should definitely analyze how the more uniform wake could benefit propeller cavitation also including tip vortex cavitation and underwater noise. To support this, we've included two relevant references on cavitation optimization ([33], [34]). We should note that a detailed cavitation analysis was unfortunately beyond the scope of this current study, as our main goal was to validate the CFD methodology and quantify power savings. The body-force propeller model we used is good at predicting thrust and power, but it doesn't resolve the actual blade geometry needed to assess cavitation. Doing that would require much more computationally expensive simulations with phase-change models.

As we mention in the manuscript, we're currently awaiting sea trial data. We plan to use that data to validate future, high-fidelity simulations that will tackle cavitation and rough-weather performance. We believe revision 2 now properly acknowledges how important cavitation is, while still being clear about the current paper's focus and our plans for future research. The manuscript has also been carefully proofread to fix typos and grammatical errors.

Reviewer 4 Report

Comments and Suggestions for Authors

Please see my review attached.

Comments for author File: Comments.pdf

Author Response

The paper cites only relatively recent publications. Fairness requires that the inventor of this type of energy-saving deviec (ESV) is cited: Schneekluth (1984). His followers fitted several thousend ’Schneekluth ducts’. 

**We agree on the necessity of acknowledging the original inventor. The paper has been revised to include a specific citation and reference to Schneekluth as the pioneer of this type of Energy Saving Device (ESD).**

>The current paper is nearly identical in method and topic to a dissertation of Ok Jil-Pyo (Ship Techn. Res. 2005). However, Ok tested only one such duct and found for that duct an INcrease of required propulsion power. And he found already that the duct does not INcrease (as is often guessed), but it DEcreases the inflow speed to the propeller behind (the duct. The same is found in the present paper. 

**We acknowledge the similarity in concept. We have introduced a discussion of Ok Jil-Pyo's finding in the Introduction, specifically contrasting his duct's performance (increase in required power) with our optimized design's positive result, while highlighting the shared flow physics observation (wake inflow speed decrease).**

>Detail comments: 

>What the authors call see margin, may be caused instead by the difference between the K_T und K_Q in open water-conditions from the actual inhomogeneous inflow speed. 

**The text clarifies that the CFD model explicitly accounts for the effect of inhomogeneous inflow via the wake fraction ($w$) and the Wageningen B-series curves. The $12.5\%$ sea margin is an independent, standard allowance for hull roughness and operational variability, not an attempt to correct for the wake effect.**

>The pre-swirl caused by the duct appears extremely small to absent; I am sure that it has no substantial effect on the required propulsion power. 

**The analysis confirms that the gain in hull efficiency ($\eta_{H}$) is the primary, dominant mechanism, while pre-swirl is a secondary benefit. The paper emphasizes the hull efficiency gain as the dominant factor.**

>The angle of attack should be clearly defined, either in the text or in Figure 6. 

**The angle of attack has been clearly defined in revision of the text, specifying the reference plane.**

>Figure 8 shows that the duct has substantial effect only above the propeller shaft. Therefore, it appears worthwhile to test, instead of the full-circle duct, a ’part duct’, for instance spanning the gap between the two shaft brackets. 

**This perceptive comment is entirely agreed upon and is now incorporated as a key finding and a primary direction for Geometric Optimization in future research, leveraging the new axial velocity and pressure contour visualizations to support this conclusion.**

>On page 13, the authors speak of ’a more homogenized and concentric wake’ for the ship with duct. That was indeed intended when such ducts were introduced. However, the opposite is clearly shown in comparing Figure 8a with 8b to 8f.

**The observation is correct. The new contours demonstrate that the duct creates a flow field that is more concentric and homogenized within the propeller disk by accelerating high-speed outer flow and decelerating inner flow, despite the severe overall velocity deficit it creates.**

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

accept

Author Response

Thank you