Ben
Leeway on a hull is added drag and loses VMG. Some keel boats are using flaps or even adjusting the keel or board angle on each tack to overcome leeway. If you have two large rudders rather than a keel/ boards and rudder then you can set rudder angles to avoid leeway.
Using asymmetric rudders means you can get the lift more efficiently with a smaller area than symmetrical sections that have to work in both directions. At zero lift (negative AoA) they have less drag than a symmetric section capable of generating the required lift on the wind. So downwind they will have less drag than symmetrical rudders and keels/boards.. You could also lift the leading one if set up to do so.
The rudder drag will be a significant portion of the overall drag on a harryproa using a long slender lw hull so anything to reduce it can have a significant impact on performance. You could reasonably expect to reduce the rudder drag by 30% using an asymmetric section compared with a symmetric section for the same lift.
I do not know how common asymmetric rudders are on proas but it seems the logical choice and another natural advantage of the breed.
Rick
On 03/08/2010, at 12:24 AM, bjarthur123 wrote:
> The unique feature offered with two large rudders on a proa is the potential advantage of using asymetric sections for the rudders.
what a brilliant idea! by "potential", do you mean to say this is NOT customary on modern proas? if so, why not!?
so upwind the more curved surface faces upwind. lift is generated even when the angle of attack is zero. so less leeway and drag. course is more upwind even though heading is the same.
but what about downwind? you maximize VMG by sailing on a broad reach re. true wind, which on a boat that can sail windspeed is a beam reach re. apparent. you don't want to prevent leeway (correct?), so do you flip the asymmetric rudders around so that the more curved surfaces point downwind? this would then generate more leeway, which when trying to go downwind is good. am i getting this right?
new member by the way. just bought a weta trimaran, and am dreaming about retiring on a larger boat. maybe a harryproa!
ben arthur
weta #358, "gray matter"
ithaca, new york
Rick Willoughby
03 9796 2415
0419 104 821