rick, could you please compare here more quantitatively the case of two large rudders vs. one big leeboard and a small rudder? you give numbers for the latter, but not the former.
in rob's typical designs they ARE tweaked independently to prevent leeway. at least when going upwind. yes, they break the surface of the water and so could ventilate, but wouldn't necessarily do so unless the AOA were large. so yes they need to be large to keep the AOA small, which would create more drag. but how much more? specific numbers would be great.
the tradeoff is that two big beam-mounted rudders are lighter (b/c the hull doesn't need reinforcing etc). lighter boat is faster (and cheaper). but does the lighter weight compensate for them being less hydrodynamically efficient? hard to tell without concrete numbers.
rob-- do you have specific numbers on how much lighter?
ben
> 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.