Eruttan,

  I think part of the issue is the definition of "better".

  The bidirectional will indeed generate more lift, but has penalties associated with it -- a third foil in the water, having to lift and lower it, and negative lift if you're sailing with the windward hull to leeward in very light winds.

  This works well for the requirements for the 18m proa, which has a shallow draft requirement , and draws less than 1m with the leeboard up.

  Is that better for a singlehanded boat in deeper water? 

  I don't mind drawing 5 feet, which should be a lot of foil for a lighter 12m proa, in which case I'd rather have taller rudders and skip the leeboard.  I'd want to be able to raise them for shallow draft areas, but not in an easy-up-easy-down sense -- more like every now and then, in which case a beefy cassette that locks with bolts or highfield levers would be sufficient.

  Plus I know that if I wait long enough, Rob and Steinar will have enough revelations to obviate whatever I was scheming on previously.

---

  As for rudders that flip through 180 degrees, I think you'd be challenged in finding enough clearance for the part that's out of the water to safely rotate if the rudder grounds out and kicks up.  You'd have to remove any decking or trampoline between them so they could go through a full 360 degrees.  Plus there's also the issue of windage on the section that's out of the water.

  Unless I'm imagining something different than what you intended.

        - Mike

 

No - I do not agree.  The bidirectional board is gives better windward performance than unidirectional rudders.  The bidirectional board is better than rotating rudders because the rudders are symmetrical foils so will be less efficient at generating lift than the board designed for a narrow range close to zero AoA.  We aimed for 3mm radius on the edges of the board and achieved close to that.  The unidirectional rudders were built at a boat yard and actually have thicker trailing edges thicker than that.   It is not unusual for unidirectional rudders with a 500 chord having a trailing edge with larger radius than 3mm.

Rick

On 27 Mar 2019, at 10:31 pm, '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

But, you agree, not as good as unidirectional rudders could be.