The biggest benefit comes from the advantage of the high L/D possible with a centreboard under the hull compared with the much lower L/D that a slender hull can produce.
For example taking the case of the design I have shown in 15kt wind at 60 degrees true the boat is capable of flying the ww hull but, assuming the wing is backed off to keep the ww hull on the water, forces are:
Wing thrust is 626N
Wing healing force 1255N
Centreboard drag to balance this is 43N so L/D for the board is almost 30
In the case of two large rudders, unless the twin rudders are cambered or they are set up to be independently tweaked to balance leeway the hull becomes the leeway preventer. I expect leeway will be as high as 6 degrees for a shallow long slender hull even with its hard chine. In this case the L/D is 10 so the extra hull drag associated with resisting leeway is 125N. This is 3 times higher than the cambered centreboard and an extra 13% drag on the 626N total drag.
The case for tweaking the twin rudders independently does not help much if they are ventilating because they have to be very large and draggy to generate the required lift.
In practice the above may not be as significant as the more stable control provided by a non-ventilating centre board compared with ventilated rudders with varying degree of immersion.
Rick
On 11/05/2011, at 11:54 PM, Arto Hakkarainen wrote:
--- On Wed, 5/11/11, Rick Willoughby <rickwill@bigpond.net.au> wrote:
I have done polars for this design and it looks quite good. There is a performance advantage in having small rudders and cambered elevating centreboard over two large rudders. I also expect the control will be more stable as the centreboard has a set level of immersion and is not likely to ventilate easily.Rick
*****That is exactly what my intuition tells me. How did you end up with this result? And is there a big performance difference? Curious mind would like to know...Arto*********************
Rick Willoughby
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0419 104 821
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