thanks mike. i was not aware of the shallow draft requirement.
i did know that a tandem keel was retrofitted, but it is not clear to me how that helps with steering.
what was non-optimal about with lw hull mounted rudders? my only guess would be ease of access while underway for repairs or adjustments...
ben
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
>
> Ben,
>
> There has been much discussion on Blind Date's steering and luffing.
> For a full recounting, the best thing to do would be to search for
> Rudolf's posts in the past. He's better than most in terms of being
> concise.
>
> As a general summary: Blind Date is a shallow draft boat. Extra
> shallow. As a result, there's not as much foil area beneath the water
> as there could be. The boat was non-optimal in its original design,
> with the rudders on the lw hull, and when the rudders were switched to
> the beams (closer together, further from the lw hull), steerage was
> reduced and luffing tendency increased.
>
> There is an argument that deeper foils would solve the problem by
> providing the ideal amount of underwater surface area, but going deeper
> is not an option.
>
> A daggerboard could be a solution, but in a shallow-draft boat,
> probably not the best idea. Even if there's a crash box, and the boat
> doesn't sink, who wants to repair the damage? It could be an expensive
> proposition just to build it in the first place, at least in terms of
> finding an ideal design that works in both directions, doesn't get
> unstable or ventilate, and somehow manages to gracefully handle a grounding.
>
> A leeboard might work, but there's still the issue of designing it to
> be strong, efficient, stable in both directions, and able to kick up.
>
> Rudders with more lift probably won't do the trick with the steering
> issue, which really depends upon having a larger moment arm (rudders
> further apart) to generate turning force. And if the rudders are losing
> their laminar flow with the current design, higher-lift versions are
> likely to lose that laminar flow even sooner.
>
> A partial solution was a bi-directional tandem keel, which provides
> most of the lift of a deeper keel, but without the depth. Since it
> doesn't have to be lifted or flipped on each shunt, the keel is a plus
> when taking blind sailors out for a day with a limited sighted crew.
> This has helped the problem, but not fully resolved it.
>
> Rudolf definitely knows what he's talking about; he and Arrtu have
> more time on the larger Harryproas than anyone, I believe.
>
> There are many suggestions about what to try next, but the budget for
> modifications is limited. And since I haven't sent a check to Zeilen
> met Visie ( http://www.zeilenmetvisie.nl/EN/index.html ) to fund my
> personal theories, I can't complain.
>
> - Mike