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Foil hum #32608
04/24/04 11:44 PM
04/24/04 11:44 PM
Joined: Feb 2004
Posts: 461
Sydney Australia
Berny Offline OP
addict
Berny  Offline OP
addict

Joined: Feb 2004
Posts: 461
Sydney Australia
Ok, does anyone know why my foils hum? One rudder and one d/board get a real sing song going as soon as the boat reaches a certain speed. It can't be fast and I'd like to eliminate it but I'm at loose end having checked the alignment and I've also tried fairing but no result. I have a suspicion it might be that the gelcoat isn't bonded to the glass laminate in some places. That's about all I can think of as being the culprit.
Any other ideas?
Bern

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Re: Foil hum [Re: Berny] #32609
04/25/04 08:38 PM
04/25/04 08:38 PM
Joined: Mar 2002
Posts: 284
Norfolk, VA
Dan Berger Offline
enthusiast
Dan Berger  Offline
enthusiast

Joined: Mar 2002
Posts: 284
Norfolk, VA
Berny,

The old Hobie rudders used to do that until you shaped the trailing edge of the rudders (daggers, too, in your case). It is my understanding that the blunt edges made the water cavitate and therefore created the humming. Sounds pretty cool and you know you are going fast, but it actually slows you down. I just got a new dagger for my A-cat and either it or the rudders were humming like crazy last weekend. I'm going to sand down the edges this weekend and see what happens.

I helped W.F. put some F18HTs together last year and he had us shaping rudders and daggers. We started wet-sanding (using blocks) with about 400 grit to knock off the edges, then came in with 600 to make it smoother and increased the angle, then we hit it with 800 and finally 1000+ to give it a nice smooth edge. We came back about a half inch from the edge of the blade. When we were done, they were razor sharp. Remember to sand in the direction that the water would go over the foil! On the leading edge, we really just knocked off the mold line to give it a smooth edge.


Dan Berger
Norfolk, VA
A Cat USA139
Supercat 15
Re: Foil hum [Re: Dan Berger] #32610
04/26/04 05:07 AM
04/26/04 05:07 AM
Joined: Feb 2004
Posts: 461
Sydney Australia
Berny Offline OP
addict
Berny  Offline OP
addict

Joined: Feb 2004
Posts: 461
Sydney Australia
Hi Dan.
My foils are taken from moulds that I had built from plugs I made employing sections developed using NACA design principles which call for 'squared off' trailing edges. They are vac. bagged carbon/foam laminates, very straight and very strong. To sharpen the trailing edge would seem counter productive given that one rudder and one d/board are noiseless so it's probably not the squared trailing edge causing the problem. They are also finish blocked with 1200 wet&dry and their surfaces are fair and straight.

Bern

Re: Foil hum [Re: Berny] #32611
04/26/04 08:50 AM
04/26/04 08:50 AM
Joined: Jun 2001
Posts: 778
Houston
carlbohannon Offline
old hand
carlbohannon  Offline
old hand

Joined: Jun 2001
Posts: 778
Houston
Be careful with what you do to get rid of the hum. I remember a big monohull that had a "hum". The owner went out and applied "common sense" and the hum went away. The problem was we were instrumented, after the owner applied common sense we couldn't go fast enough to hum anymore.

Starting with easy.

Check the leading and trailing edge. They should be smooth, no bumps or humps. Check the foil specs to see if the trailing edge is blunt or square edge, there is a difference. Also check the foil specs to see what thickness the trailing edge should be. It is normally 1/16" to 1/8" or a % of the foil thickness.

Check the foils for stiffness. The stiffer the foil the higher the resonance frequency. If the foils that hum are noticeably less stiff than the foils that don't hum that could be your problem.

Look for local delaminating. A local delamination will vibrate.

Make sure it is not the housing that is vibrating. Tighten the bolts on your rudder assembly and stuff some foam between your rudder and dagger board well. Swap rudders and dagger boards side to side.

Check your dagger board well to see its loose, tighten and check your gudgeons, check your transom.

Look inside your boat. If the problem is only with one hull you might be like me and find that drill bit you lost.

Carl


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