Ben,
Maybe a little theory might help...
The sound is obviously coming from vibration. And Rick was right on the points he made. That something (vortex shedding sounds perfectly reasonable) is exciting the rudders at their natural frequency. On many structures, if allowed to build up, this can be fatal.
Two common examples... breaking of a wine glass by voice. Or, an actual vortex shedding case...
There are several ways to eliminate such behavior. The main concept is the make sure the natural frequency doesn't coincide with the cyclic force exciting it.
Change the frequency of the exciting force. Rick's suggestion of changing the trailing edge would reduce the vortex shedding or change its frequency.
The other is to change the natural (resonant) frequency. This is the structure's "tone". Like striking a tuning fork. It's natural frequency has been designed to be in the audible range at a very certain frequency. Actually a structure (even a tuning fork) has many natural frequencies. But the first one is easiest to excite. The other frequencies are always higher and higher up. The thing is, you can change the natural frequency by stiffening up the structure as Rick suggested. You can also do it by reducing the stiffness. In that case, you would be lowering the natural frequency with the idea being... you would GO THROUGH the natrual frequency at a slower speed so that when you were at top speed, you would be above the natural frequency and thus NOT EXCITING IT... no humming.
The third way is to add damping. This is the shock absorbers on a car. Ever seen a car without shock absorbers? And how easy it is to "get it going" at about 1 or 2 Hz. In your case, it maybe as easy as putting in some rubber gromets if you have hinge pins. Eliminating any "slop" in the linkages, might do it... switching to ball end joints.
Anyway, hope this helps and didn't cloud the works more.
Dennis
From: bjarthur123 <bjarthur123@yahoo.com>
To: harryproa@yahoogroups.com.au
Sent: Wed, August 4, 2010 8:59:16 AM
Subject: [harryproa] Re: Leeway Prevention
thanks for the confirmation guys. the last time i heard the humming i was experiencing quite a bit of lee helm. perhaps the rudder was just stalling b/c of the AOA required to counteract it. i did not take the time to figure out whether it was the rudder or daggerboard though. i'll experiment with balancing the centers of effort better next time and see if it goes away.
class rules prohibit any modifications to the foils of course. the trailing edge is already quite sharp though.
just out of curiosity: high speed foils are generally thinner, right? precisely to avoid such cavitation/ventilation/vortex shedding/whatever it's called? the tradeoff is that there's less lift at lower speeds?
ben arthur
weta #358, "gray matter"
ithaca, new york
> It is almost certainly the trailing edge of your rudder/daggerboard shedding
> vortices and it slows you down. Make the trailing edge a 45 degree angle
>
instead of a flat and it will disappear.
> Probably vortex shedding off a bluntish trailing edge or flow
> detachment on a control surface.
> The vibration results from the supporting structure getting in tune
> with the shedding. So you have to avoid the vortices or stiffen the
> structure to eliminate the vibration.