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. 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.
The cambered foil at a set depth has a limited range of operation. Sailing higher than the design point creates leeway and sailing lower causes negative leeway. There is benefit in being able to reduce the immersed area once sheets are eased. The cats can usually set the depth of their cambered boards to reduce area ; foilers automatically reduce area as speed pics up and they rise. On the 18m proa we just swing the board out of the water once sheets are eased. The rudders provide good control once above 4kts and more than 60 degrees to true wind. At angle to true wind of 70 degrees, lifting the board is like releasing the hand brake in a car; immediate jump in speed.
Rick
On 27 Mar 2019, at 1:16 pm, '.'
eruttan@yahoo.com [harryproa] <
harryproa@yahoogroups.com.au> wrote:
ick, can you clarify this for me.
I don't understand how a cambered board is most efficient, BECAUSE it can be made bidirectional.
I am not even sure a cambered foil is most efficient. Do we know that is always the case?
And would it not be odd for a foil to be at 0 AoA when the boat makes best VMG?
| There will be a little positive leeway when pinching up and negative leeway if sailing low and raised once reaching.
Specifically regarding positive leeway while pointing. I have read about this a very small bit. I was thinking it was because most boats dont have two rudders, and this was as best as could be done. But, I was thinking, with two high performance rudders one would be better to adjust them for zero leeway. My thinking being better to load the foils than drag the hulls.
Is that wrong?
