Ben,
No worries. It's tough to read through all the old posts and know
what was already said.
I believe the tandem keel helps by taking some of the leeway
resistance off the rudders, as well as giving them a pivot about
which to turn the boat. With less force on the rudders, their flow
is likely to be more laminar, so they will work more powerfully at
lower angles. As soon as you lose that laminar flow, they become a
lot less powerful, both in terms of leeway resistance and in terms
of steerage.
There may or may not be an issue with using two foils for both
leeway resistance and steering. After all, many trimarans have just
two foils (daggerboard and rudder). And two foils versus three
means less structure, less weight, less drag (from a third blade
dipping through the surface), and so forth.
But using two foils where there's a shallow depth limit can be a
challenge, particularly if they aren't as far apart towards the
bows/sterns as the leeward hull will allow.
The original hull-mounted rudder design is actually quite nice,
but has limitations for shallow-draft sailing. The rudders are
nicely balanced when fully down, but are not as balanced when
partially retracted. So if you're doing a lot of shallow water
sailing, the helm will require more effort. And, as you said, they
aren't as accessible as beam-mounted rudders.
Also, the kick-up feature with the newer design is more robust and
easily reset when underway.
- Mike
bjarthur123 wrote:
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