Hey list! I can see Doug's recent posts on yahoo.com, but I didn't get an email for his last messages. I just noticed this.
Anyone else?
https://au.groups.yahoo.com/neo/groups/harryproa/conversations/messages

| A cambered board can generate the lift without an AoA. The shape can be optimised to give the highest L/D for a given lift coefficient and thickness ratio.

And Renolds number.

| In the case of a board with an aspect ratio of 4, a cambered foil will have 15% lower drag for a given lift than a symmetrical board. It is the same reason wings on an aircraft are cambered rather than being uncambered. (some acrobatic planes have uncambered wings so they fly same upside down as right way up)

| The attached gives the data for the operating range of the swinging board on the 18m proa. It balances sail lateral force close to 50 degrees to true wind over a wide range of windspeed when at zero AoA where the Cl is 0.41 and the L/D is 31.4.

| With a proa, the benefit of using a cambered board to prevent leeway is that it can be designed to give highest L/D at the point of best VMG and at zero AoA. That means the leading edges that become trailing edges can be very fine as the board has a very narrow range of operation with regard to AoA..

| A rudder used to cancel leeway will be less efficient than a fixed board of the same aspect ratio and it must have continuous rotation or relatively large radius leading edges that become trailing edges if it is bi-directional so that makes them even less efficient.

Ah, I see. You are saying a board can be optimised over a narrow domain, so it can be better than bidirectional rudders over that narrow domain, and lifted when outside of it. But, you agree, not as good as unidirectional rudders could be.

And for a unidirectional rudder, one can rotate, which we have tried, or flip?
We have tried rotaters. Have we tried flipping rudders?
Isn't that what Rob is working on now?

Flipping makes sense to me as you already need the vertical rotation for kick up, both ways, so you got most all your needed 180° vertical rotation.

And unidirectional foils are where all the fun laminar stuff is. And lift devices.

I am curious how Rob is going to raise and lower the flippers.