ahah, i see how the tandem keels help now. to re-word, they reduce sideslip and hence AOA on the rudders which helps keep flow attached. leeway isn't a problem presumably if the rudders are fully extended, so would probably want to retract the keels in deep water.
question about the rudder balance when partially retracted: couldn't a foil planform be designed for which the balance remains the same no matter how much they are extended?
ben
--- In harryproa@yahoogroups.com.au, Mike Crawford <mcrawf@...> wrote:
> 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.
--- In harryproa@yahoogroups.com.au, Rob Denney <harryproa@...> wrote:
> I also know the original rudders work on Rare Bird and consider this a far
> better option than keels, rudders on the ends etc. The problem is not
> shallow draft steering. This can be done with the original set up. The
> problem is if the boat is sailed at high speed in shallow water, when the
> rudders become unbalanced and steering loads get high. I think this is
> unseamanlike so the unbalance on the raised rudders will force the boat to
> slow down, which is a good thing.