foil and skeg profiles

hey all, I've reached a new impasse in my boat building adventures and figure you guys could probably provide some pretty straight forward answers. I'm looking to design a skeg for a kayak and am not sure what sort of profile would be best. I know it's pretty standard to use a NACA 0012, but I don't think this would be ideal. As far as I can tell, the skeg on a kayak isn't doing much other than moving the CLR back to prevent weathercocking. Generating lift therefore seems pretty irrelevant, and I'm thinking I should simply go with whichever profile will minimize drag ( a narrower entry with max thickness @ 50%?). If this is right, I presume getting the thinnest foil possible would also be ideal, right? Let me know if I'm totally off base and thanks in advance for the help!

 

I expect your deductions are close to the mark.

One additional consideration is that you will have some leeway depending on the strength of beam wind. This means the boat is not tracking the way it is pointing. Hence there is a slightly angled flow onto the blade. A slightly rounded nose will ensure attached flow on the blade under these conditions.

The blade needs to be thick enough or flexible enough to avoid damage in the normal day-to-day use. A good test is that the hull will sit unloaded on the skeg without damage.

Rick W

 

Hey Rick, that's essentially exactly what I was thinking... taper the leading edge to a thin-ish edge but keep it rounded to prevent detachment and keep the maximum width at 50% with the trailing edge as sharp as possible. anybody else got suggestions?

The other thing I'm not sure about is whether it's better to have a deep skeg, as in this picture:



or a longer shallower one like this:



If i remember correctly, I should expect less drag (and a more significant effect on the CLR?) with a shallower longer skeg. I also figure that since the whole skeg has to rotate up into the skeg trunk, the second design should have less of the opening in the skeg box exposed when the skeg is down, meaning less turbulence and drag. Does this seem reasonable?

Also, does anyone have any good centerboard design links? Since the skeg has to be trimmable (i.e. work well at various levels of down, not just all the way down), I'm not sure how I'd shape the board so that water flowing past it is encountering a smooth foil shape regardless of the angle the skeg is deployed at.

Thanks!

 

I just came across this:

http://www.paulzander.biz/Centerboards.html

which has me thinking that a thicker foil will have less drag when the angle of attack increases above a certain level... If this is so, then I guess I'd have to calculate the expected angle of attack of the foil in order to determine which foil is expected to have the lowest drag. Now, I know the angle of attack is a matter of calculating the combination of the velocity vector and the leeway vector, and the velocity is easy-enough to calculate, but how would I go about calculating leeway under various conditions (i.e. wind direction and speed).

am i talking to myself?

 

Your skeg should be working at lower angle of attack than a conventional rudder or a even a sailing dinghy centre board.

Given that it will be low speed you are probably best with something around 8% thick, more or less as you described initially but with the slight rounded nose to give it adequate working range. If it is working at more than a few degrees then you could be working on your paddling to track truer.

Typically you end up do some experimenting and then settle on what works best with you. That may change over time as the paddling changes.

Rick W

 

If they have the same area, the deeper one will have less drag. The drag due to lift on the skeg is inversely proportional to the square of the depth, so when it is producing lift, the deep one has a lot less drag.

Yes, unless the deep skeg is a dagger foil that is pulled up vertically instead of pivoting aft. Then there's no open cavity at all when it's down.