Airfoil shapes under water

[Sakari Aaltonen]

I know rudders and daggerboards should be shaped as airfoils, and have indeed done this myself a few times. No NACA, though, just drawing a line one-third from the front and aiming for a drawn-out tear shape (in cross-section.)

However, all those previous underwater appendages had vertical - plumb - sides, front and back. Now I have a daggerboard whose sides slope forward (12 degrees, to be exact.) Should the airfoil shape also slope forward? Or should it be vertical? In the latter case, the board will not be uniform in cross-section.

Thank you,
Sakari Aaltonen

[brian.cunningham]

It will still be an airfoil, it will just be elongated in shape when set in place.

[Russ]

There is an excellent article the Sept/Oct 2002 issue of Good Old Boat magazine written by Ted Brewer that deals with hydrofoil shaping. The issue of tapered edges is dealt with and in a table of offsets he gives the shaping percentages. Would be well worth the price of a back issue just for that.

[pjwalsh]

What I have done when faced with this is to set the foil cross section parallel to the waterline. If the board is tapered then the bottom section is a scaled down version of the top by the ratio of chord length.

The naca foils were tested with the direction of flow perpendicular to the section so when applying them to a boat making the sections parallel to the waterline accomplishes that (assuming a more or less level attitude for the hull).

[mark ward]

I found this article a while back when I was looking for answers to similar questions:

http://www.vacantisw.com/keel%20design.htm

Mark