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stpetersburgsailor 09-18-2009 03:15 PM

Boat hum
 
one of my students asked a stellar question regarding boat hum...
why forces of drag vs. positive effect... keel issue
skiff issues... say a 420 or Laser

some say a boat is singin' & in perfect sych when she hums:cool:


any thoughts?

smackdaddy 09-18-2009 04:46 PM

Actually, that is a great question. I've heard people talk a lot about it - but know the physics behind it.

St Anna 09-18-2009 04:58 PM

I can vouch for the hum. Our Peterson sings when when going to windward (probably in at least 20kn apparent) and the sails are well trimmed.

The Peterson designs are a work of art so your question is valid. It has been reported that the 46 with a wider stern has some weather helm in comarison to the 44, but with headsail and main traveller trim, I havent found to much weather helm. Weather helm characteristics, (CE vs CLR & mast rake etc etc) may be worth looking at for the design causatives of 'hum'.

Its a beautiful sound - its just like she is communicating & having fun and its implanted in my memory.

imagine2frolic 09-18-2009 05:27 PM

I was on a test sail with Imagine. We were sailing from the Atlantic between St. Maarten, and St. Barts. The spinnaker was up, and as we got on the crest of the wave we got a good gust. We were doing 14 knots, and the boat seemed to dig in, and accelerate quickly. Just as we were peaking at 18 knots there was a small hum. I was in love, and my knees were weak. I was standing there thinking, and SHE'S MINE!........i2f

saildork 09-19-2009 11:16 PM

Quote:

Originally Posted by imagine2frolic (Post 524127)
I was on a test sail with Imagine. We were sailing from the Atlantic between St. Maarten, and St. Barts. The spinnaker was up, and as we got on the crest of the wave we got a good gust. We were doing 14 knots, and the boat seemed to dig in, and accelerate quickly. Just as we were peaking at 18 knots there was a small hum. I was in love, and my knees were weak. I was standing there thinking, and SHE'S MINE!........i2f

AHHHHHHHHHHhhhhhhhhh!! That special moment when man and boat are as one.:D :D :D

zz4gta 09-29-2009 04:17 PM

I was under the impression that the hum was caused by imperfections in the bottom and or foils. Fair the bottom, and sand to 600 grit, hum should go away.

philsboat 09-29-2009 05:08 PM

Boats hum cuz they don't know the words....hehe

Phil

US27inKS 09-29-2009 05:39 PM

When I was working on my friends keel I ran across a great video showing the cause of keel hum. Unfortunately I can't find it now. Basically it has to do with the width of the rear edge of the keel or rudder. As the foil moves through the water, an eddy forms on the rear edge of the foil. This eddy swirls first from one side then the other. As boat speed increases the eddy switches sides faster and faster until a harmonic vibration is set up in the foil.

The eddy on the back edge of the foil creates drag, and can be eliminated by reducing the trailing edge to a point. This is impractical at best, so reducing it as much as possible is the goal without sacrificing the ability to withstand a small impact. Besides, getting a pointed edge straight is a bitch.

Crapaud 09-29-2009 09:34 PM

In my experience, the hum is a function of speed and exciting the natural frequency of the rudder, keel or combination of the two. My "Fiz-ix" knowledge dealing with fluid flow is rusty at best, but to add to John's comments it is a vibration brought on by flow inconsistencies.

One of my thoughts has been - could this vibration be a result of flow separation (or eddies) coming off the keel that the rudder then contacts and starts the process? Almost like a flag flapping in the wind generated by a fan?

The nat frequency is determined by the object's geometry and stiffness. So, the frequency would be changed by changing the length of the rudder or the materials it is made from. Stopping the eddies completely would probably require NASA to design and build the foils to have no flow separations at the higher cruising speeds.

stpetersburgsailor 09-29-2009 11:13 PM

sanding with grit! gee I'm hopin' he was being comedic...
I've sailed BRAND new high performance skiffs & if u took a piece of sandpaper within 20 yards of one the maintenance director would have your head on a platter, a seafood platter

thanks for the foil discussion right above, VERY interesting
& Chapman's discusses this a bit too

cheers
-JD


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