Designing a Rudder

[John Stevens]

I sail a Precision 18. The rudder sticks down below the keel so I wanted to design a wing rudder that would not hit ground prior to the keel, in order to sail in shallower water.

I made one 8" shorter than the original and a wing at the bottom. It seems to be working ok but I'm getting some vibration from side to side. Any thoughts?

TIA,

John

[Raggi_Thor]

I can think of three things you may want to look into:
1) Is the bearings worn? Maybe they need to be changed?
2) Vibrations can occur when the rudder profile is not perfectly shaped. A naca foil section is normally used these days. Naca 0012 for example. If you do not want to plaster and fair the profile again, you can introduce small scale turbulence by "roughening" the surface (add sand to the paint) at the widest point. This can be better then the large scale turbulence that you (maybe) have now.
3) Also the trailing edge can be changed. try to make it perfectly flat, approx 2mm wide (1/12 inch or between 1/8 and 1/16).

[John Stevens]

Thanks for the reply. I'm currently looking into the NACA numbers.

Thanks,

John Stevens

[Raggi_Thor]

Some usefuld dxf files here:
http://www.davincitechnologies.com/A...tdAirfoils.htm

[asathor]

http://www.boatdesign.net/forums/att...achmentid=1951

the flow is much better than on a convertional foil.

[Skippy]

Ooh, weird. How does it work?

[asathor]

The flow in the grooves is substantially more efficient - the end result is undoubtedly so good because the water doesn't turn and spill off the bottom as it usually does on a foil.

The good professors made their test object flipper shaped and extremely smooth and symmetrical according to known NACA - the whales don't do that, and they have a lot of experience - so I figured that the concept executed in some smooth form leading the water in the right direcection would work, and it does.

You can buy the research paper as I did on the Applied Fluid website somewhere.

http://www.biol.rug.nl/itieleman/OutsideJEB4.pdf
http://www.vanaqua.org/aquanew/fullnews.php?id=1284