Dennis
If you are serious about getting to 30kts then you will need to give the rudders careful consideration.

One immediate problem will be cavitation. A NACA0010 will cavitate at 3 degrees AoA at 30kts. The maximum Cd before cavitation will be only 0.23 for a rudder with AR4.

The unique feature offered with two large rudders on a proa is the potential advantage of using asymetric sections for the rudders.

I have taken a look at the possibility of using rudders with fore-aft symmetry but asymmetry beamwise at an Re 10E6. For example a NACA4512-15 section with AR of 4 will have an L/D of 22.5 and Cl of 0.4 at 0 degrees. By comparison the NACA0010 with AR of 4 will have maximum L/D of 16.8 but only 16.4 if you are doing 30kts before it begins to cavitate.

Really you have two articulating keels rather than rudders. The actual control range is quite small because they will have plenty of power so you can use tall gearing. They will be very close to being in balance in either direction with the shaft centrally located. The pressure distribution is very even:
http://www.rickwill.bigpondhosting.com/NACA4512-15.png

At 30kts you would need an area of 0.35sq.m to generate 17kN with Cl of 0.4 so each rudder would be roughly 0.3m long by 1.2m deep and 0.036mm thick. Without doing the sums, I expect this would be too small for the loads. So next iteration would be to work with a lower Cl and thicker section - say NACA3515-15.

Due to the fine entry you will find that the Cd at zero lift will be similar to or better than a round nose rudder section.

I have no idea if anyone playing with proas has tried asymmetric section rudders but there are not many of these boats competing seriously to bring out the best from the concept.

Rick

--- In harryproa@yahoogroups.com.au, Dennis Cox <dec720@...> wrote:
>
> OK!  I am about to drill down to China.  (Hope that expression has the same
> meaning where you're from... I guess not... if you're in China)  I hope I
> can explain this without putting you to sleep... and hopefully you can slap me
> out of it.  There are two issues I want you to entertain... efficiency and
> strength.  We want to go fast... and we want to stay in one piece.  I think
> those are universal.  We are "thinking" about a HarryProa (nothing else really
> matters)...
>  
> Efficiency
> If you raise the front rudder, you have to kick the back rudder till the angle
> of attack (AOA) of the entire hull counters the leeward force of the sails.  We
> all know that low aspect foils are inefficient.  And the leeward hull is about
> as low aspect as you can get.  So even though its big and strong, there is a lot
> of side slip going on.  We are aiming for Europe and we hit Africa.  We also
> have a LOT of induced drag.
>  
> Now if we lower the forward rudder, we can get the two rudders (acting as highly
> efficient, high aspect foils) doing all the heavy lifting.  We can now set the
> AOA of the hull to be zero by having positive AOA on the rudders.  In this case,
> the hull is presenting the least drag it can possibly do and the rudders are
> taking all the load.  So we're kicking ass and taking names, until...
>  
> Strength
> ... you start realizing that we need to design for the "lifting windward hull"
> case.  As an approximation... the leeward force is about equal to the windward
> hull's weight.  Split that in two (for the two rudders) and for my
> case... that's 4000 lbs (17 kN).  (Forgive me Rob... you have the best
> pictures) http://www.harryproa.com/building_Vis/Vis_img_feb11.htm.  By the time
> you get the moment arms into the issue, the bottom rudder post would be taking
> somewhere around 11,000 lbs (50 kN).  I haven't analyzed these, but I think
> these supports would have troubles with that load.  Fortunately, they don't have
> to in Rob's design!  If you'll note the rudder isn't balanced so the lateral
> forces acting on the rudder are resisted... by... you got it... the helmsman. 
> If I were to take a WAG, I'd say Rob has a mechanical advantage somewhere around
> 100:1 on the wheel.  So to hold that, the helmsman would have to supply... 100
> lbs (500 N) On each wheel... not bloodly likely.  So, Rob has used the helmsman
> as a force limiting transducer.  Briliant!
>  
> Before I saw Rob's elegant brilliance, I was going to make my rudders
> (near) balanced.  I wanted some helm feel, but I didn't want a work out or blow
> the autopilot.  Then I realized using delicate members like Rob, I'd soon tear
> them off while sipping my afternoon tea and steering with one finger. 
>
>  
> Can anyone... tell me how I can have my rocket ship and eat it too?
>