On Ricks case about twin rudders he only mentions low draft hard chine flat bottom as the example. With what type of sail configuration? It will take more board area to correct for balanced steerage vs a more easily driven deeper draft V shaped hull. Deeper hull should provide better leeway prevention. Sail area configuration and typical sail grounds location with prevailing off shore or on shore winds would be a few other variables to consider.
I have developed a few V hulls that are only a few sqft different than the narrow flat bottoms. Attention to weight/displacement, draft depth of hull, along with beam water line width. Keeping prismatic coefficient high above .650 to smooth out drag and maintain enough buoyancy for the narrow waterline beam width. The block coefficients are not as good as the rectangle flat bottom but thats were the V frontal/ bottom shape and its ease of being driven may have some advantages if taken into consideration. Where does the need for buoyant lift and easily drive begin and end. Find the tight rope first,then walk on it:)
Here's a comparo of the V lee hull. Free sailing the same model with a flat bottom leeward hull.
http://www.youtube.com/watch?v=e-qiSXIDRIk
Todd
--- In harryproa@yahoogroups.com.au, Arto Hakkarainen <ahakkara@...> wrote:
>
> Thanks for the explanation. Seems like it is best to use a single fin (be it wing, board, keel, whatever) to take the majority of the load in resisting sideways forces and smaller fins to balancing and steering. Actually it is just like aeroplanes or traditional boats that never have big wings both bow and aft but main wing/keel/board in the middle and smaller balancing and steering set of foils as far back as possible. It is good to have an engineer tell a non-engineer why the intuitive approach was or was not correct...
>
> Arto
>
> --- On Thu, 5/12/11, Rick Willoughby <rickwill@...> wrote:
>
>
> From: Rick Willoughby <rickwill@...>
> Subject: Re: [harryproa] Design your proa
> To: harryproa@yahoogroups.com.au
> Date: Thursday, May 12, 2011, 2:32 AM
>
>
>
>
>
>
> 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@...> 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
> rickwill@...
> 03 9796 2415
> 0419 104 821
>