I have not checked your calculation however aligning drive and drag is a significant factor as you have determined. A harryproa gets forces close to being aligned when the ww hull is lifting. At other states the windward hull is trying to round it up. This can be counted with rig as well as rudders.
The best way to analyse the performance of different combination of hulls, rigs and rudders is to generate polars using a good prediction model. There are are many interactions that are too complex to wrap the brain around unless they are dealt with sequentially in an iterative closed loop that can balance all the forces. As a first approximation the hull drag for a harryproa can be determined using the ITTC friction formula based on wetted surface and waterline length for the hull under different load states. My VPP approximates hull drag using a polynomial of best fit for data produced by Michlet over a range of displacements for each hull. This takes a few hours to generate.
For a performance boat the ultimate speed is a bit easier to determine because the ww hull only contributes windage - no water drag. The righting moment is known so the maximum drive for a given point of sailing can be determined. The trim needs to be checked to ensure the hull is not bow down under the maximum drive. If there is moveable ballast such as crew on a small boat then this could correct trim. A light boat will have a tendency to plane if the sections are conducive to that state - just another complication or simplification if you are only interested in top speed potential.
The preferred length of the lw hull may not be controlled by minimum drag but rather resistance to pitching.
A flat bottom on the ww hull will result in slapping when it is lightly loaded. It is not an issue from hull loading perspective but could be reduced with some V in the bottom.
Rick
On 28/02/2013, at 3:22 AM, fvonballuseck wrote:
Baring stupid calculation mistakes and wrong assumptions it seems that optimizing WW hull shape could change around 10% of total drag and 10% of rotating momentum challenge?
Rick would it be a lot of work to guess/analyze what say the drag curve of the WW hull of Harry is?
Rick Willoughby
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