Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec


One can see that upwind VMG is almost equal for both methods, with a little amount of higher VMG values calculated by RVPP for TWA <60°. Maximum VMG upwind is 2.31m/s at a boat speed of 2.94m/s and a TWA of 37.9° for the results gained from AVPP. The corresponding values calculated by RVPP are a VMG of 2.41m/s at a boat speed of 3.13m/s and a TWA of 39.6°. This results in difference in VMG 0.1m/s between the two methods.


3. RVPP results calculated with the same grid as the numerical towing tank data.


This will give indications about numerical errors by comparing results from the two towing tank testing. It will also allow better assessment of differences between a conventional VPP and RVPP by subtracting possible differences due to numerical errors or uncertainties about the test setup from the picture.


So at the current state, any assessment of accuracy is questionable. However, the authors will try to give some hints were differences in the results may be grounded


Generally, the results point in the direction that the sail carrying ability of the yacht as predicted by RVPP seems to be higher than that of the conventional VPP. Therefore, one might look at how both systems treat hydrostatic and hydrodynamic stability.


Conventional VPP:  Hydrostatic stability from heeling arm curve, usually based on the assumption of flat water


 Hydrodynamic heeling moment is calculated from hydrodynamic heeling force FH assuming linear wing theory.


 FH estimation is based on Delft Systematic Keel Series (DSKS, Keuning [9]).


It does


consider T-shaped keels and is only valid until Froude number of 0.6


Figure 9: VMG slope over TWA


Downwind VMG shows almost the same top VMG speed but at different TWA. This is reflected in the VMG max values, with maximum VMG downwind as calculated by AVPP being 2.85m/s at a TWA of 146.2° and a boat speed of 3.43m/s. Downwind results calculated by RVPP give a VMG of 2.86m/s at a boat speed of 3.71m/s and a TWA of 140.4°.


This gives a rather small overall difference in VMG of 0.01m/s between the two methods.


As mentioned above, it is hard to asses which method is more accurate, since the results of RVPP have been compared


with conventional VPP results based on


empirical data instead of towing tank data. (One has to keep in mind that this was only some initial testing on a prototype).


While validation of RVPP will remain challenging, in the authors’ opinion the best way to assess the accuracy of the method will be to use a comparison based on three different methods:


1. Conventional VPP results using experimental towing tank data


2. Conventional VPP results using towing data


numerical


RVPP:  Hydrodynamic and hydrostatic stability are both calculated from combined flow and rigid body motion simulation


 The forces used for the rigid body simulation are from calculated flow forces around the actual geometry


 Hydrodynamic Vertical Lift is implicitly considered.


 The resolution of the computational grid could be better, see section 7.2


The different ways stability is treated probably have the potential to be the source of the differences in the results. Furthermore, the optimum boat speeds calculated by the two methods are located around the critical Froude number of 0.4 (0.38 for AVPP, 0.42 for RVPP).


Adding that the downwind configuration investigated was equipped with an asymmetric spinnaker, the TWA at maximum VMG as calculated by RVPP looks a little bit more realistic. Perhaps heeling force and resistance are little bit overestimated by


the conventional however this cannot be sorted out clearly.


Finally, it must be admitted that the choice of model was perhaps unfavourable. It might have been a better idea to use a hull from the DSYHS fitted with a keel from the DSKS to gain more accuracy for the comparison.


VPP, not


©2011: The Royal Institution of Naval Architects


B-91


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