Tuesday, January 12, 2010

Rudder foil planform area.

So Amac won the Australian Nationals (due in a large measure to his talent) but it seems part of his kit was a "smaller" than normal rudder horizontal. So just how large was it? How large is yours? (We're still talking area of the horizontal, here.) For a couple of data points here are the dimensions of my two rudder horizontals:

My first rudder horizontal, a Gui super-large (the one I used at Weymouth)

Span 40 inches, Max chord 4.75 inches, Chord two inches from tip 2.75 inches. Approximate area 142.5 sq inches (920 cm^2). This foil has a minimal bulb. It's mounted to its strut at a 90 degree angle. In practice it seems to balance best with a slight negative AoA, perhaps due to it high lift cross sectional shape.

My 2nd horizontal, a Bora built, Beiker designed (the one I used at the Gorge)

Span 30.75 inches, Max Chord 4 inches, Chord two inches from tip 2.375 inches. Approximate area 94.2 sq inches (608 cm^2). The foil has a rather large bulb and "tail." It is mounted to a forward canting strut, similar to the M2 rudder. It was with this rudder, before the gantry exploded (another story) that I clocked my 20.3 kt avg at the Gorge (in all likelihood a greater speed over the bottom since I was sailing downwind but up current.)

Lastly, here's a pic of the two back-to-back. I don't have a reliable micrometer, but the Beiker foil is much thinner than the other.

I suppose that smaller and thinner is better. Since I don't have the "stock" BR, M2, or Assassin dimensions, could someone please provide them? It would be interesting to see the range of what's actually being used out there. My suspicion is than thin foils with a shallow AoA are the way to go fast. As Simon says, "discuss..."


Unknown said...

Hi Joe,

Firstly glad to see you are back out of blogging 'retirement' you had us all worried!

Jonnie Hutch and I have been talking about foils for some time and the consensus is that foils should be looked at in much the same way as windsurfing fins which are designed to work efficiently at speeds of around 25kts.

I guess what we will start seeing is rudders being designed in much the same way... thin and small with the majority of the lift coming from the main foil. Mike cooke is doing the same with the Ninja rudder... lean and mean.



Joe Bousquet said...

Shucks, at first I thought I was going to get the inside dope on the M2 foils from a Chinese turncoat. But then I find out that perhaps there was a mis-translation of the "AssAssin" dimensions. Oh well......