I have brand new 3rd generation rudders on my n20. They make an incredible amount of humming noise. I tried a seach but couldn't come up with a solution. I saw at some point in the past here, a remedy for rudder hum, but can't find it now. Can anyone help me out?

Have a professional fair them. did that with the rudders on my HT and doing it with the rudders on my blade (though they don't hum).
well worth the money IMO. I figure it will get me the extra 1/10 of a knot to push me onto the gold podium <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

is there someone you recommend for the fairing?

If your new rudders are anything like the new dagger board we got this year, you've got some work to do!

There may be a lot of extra material on the leading and trailing edges where the two halves are sandwhiched together ("casting flash" is what I call it). On the leading edge, file/sand this down until you have a fair shape on the leading edge. Look for similar "casting flash" on the trailing edges, and file that down to as well.

Don't fair the trailing edge to a fine razor-like edge - instead, square it off a bit. If you have any knicks fill and fair them (our brand new dagger came with a little chunk already missing from the trailing edge).

You should be able to find previous posts on this - it's a very common question and issue. Another thing - make sure you take care of any looseness in the system, tighten and shim to reduce the slop.

Am I alone here. I would liken the rudders not arriving absolutely ready to sail is just wrong. Like me delivering a painting that just needed the buyer to finish the last details.

The rudders might not be aligned correctly. I got hum when there was a little weed caught on a dagger board.

I have checked the rudder alignment and they are spot on. Set identically to the way the old rudders were. Old rudders...no hum, new rudders....hum like h***!

I did find this in my search:

http://www.ahpc.com.au/pdf/Noisy%20Foils.pdf

Do your search on this forum. You should find lots of info, including all the jokes, like "why do my rudders hum? Because they don't know the words! Ba-dum-bum"

Warbird - welcome to modern manufacturing and service. I had to do the same thing to the new Hobie racing rudders I bought in the past for my 18 as well as the work on the new N-20 dagger. Contrast that to the absolutely beautiful ready to use dagger that was recently purchased for a Blade F-16 by one of our Fleet members...

I can wholeheartedly second the recommendation to send your foils to Jim Bowman at Recreational Composites. Send the rudders in the castings and get the teflon lining, too. Jim is a craftsman and takes great pride in his work.

What about the trailing edge, did you malke a small about 1/8" flat spot. Most people use a pair of scissors.

Doug

The hum is caused by vortex shedding off the trailing edge of the blade. The vortices shed alternating from side to side across the trailing edge and resonating through the blade. It is usually worse when sailing downwind when the blades are lightly loaded and the pressure on both sides of the blade is the same. The fix is to bevel the trailing edge at an angle so it is not symetrical causing the vortices to shed from only one side. Shaping to a 'v' as with sissors suggested above seems to work also. Theoretically a square trailing edge would be the worst possible condition but I have had blades with a square edge that never made a peep, go figure.

Vortex shedding:



Even though that's a cylinder, the principle is the same. Whether it causes the foil to hum depends on the natural frequency of the foil and the frequency of the vorticies. If one is a multiple of the other, the foil resonates = hum.

By making your trailing edge asymmetric, you change the frequency of the vorticies, hopefully to one that is not a multiple of the foil's natural frequency.

Here's another cool image of votex shedding caused by wind flowing past an island. These are called "Von Karmann Vortices":

I am not kidding, I am really disgusted by that being accepted as a norm and I would simply return them and tell the supplier to sort it or refund the money.

Understand your feelings completely - however, when class rules say things must come from the manufacturer and that's what they send, you're kind of stuck. I suspect you could send boards back until the cows came home and the next set they send you to replace them wouldn't look any different.

Very good guy. Faired the HT rudders and actually made some carbon rudders for the HT.
What is the teflon lining? I may ask him. He has my boards and rudders right now.

Hobie should be ashamed of themselves. They have a captive group and are not respecting their own product enough leaving it to others to finish the kit.
It's only a bit of finiishing for gods sake.
But I understand that if that is where the competition is........

Warbird, since when does Hobie make NACRA 20 rudders?

BTW, Hobie EPO2 rudders don't hum.

That's the part I was missing. Thanks Matt

Don't just blast Hobie - I've had this experience with both Hobie (carbon reinforced racing rudders for my 18) and Performance (new daggerboard). I haven't bought a new Hobie blade in awhile, maybe they're better now, can't vouch for that, suspect it's probably no different.

And yes, I agree, for the price charged I think it's pretty lame.

I gotta tell ya, after building my own rudders (for my 1 meter R/C boat, this is not easy. Still though, I do finish mine with a clear coated carbon finish and they don't hum. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />