Hum?

P

pez

Member
Great winds in southern Michigan over the past weekend and through the independence day holiday...

Had spectacular sailing... at one point had two people in the boat hiked all the way out on a starboard tack... outrunning a few of those pontoon boats that people buy for the funniest array of reasons...

Anyway... my laser will hum at a certain velocities/points of sail... I have no idea where it's coming from... the entire thing emits a sort of resonance... details get so fuzzy after you blast back and forth, but... seems like it happens more on a broad reach, and it seems to happen more often on port tack.

This may be a bit more detail than this forum wishes to hear, but I had two sessions where I was out so long I had to practice capsizing the boat so I could take a pee... Like I said, great weekend.

PS... side note... I received a new mainsheet block while I was away... I didnt actually realize that they were twice the diameter of the original 1971 mainsheet block... no wonder my hands are killing me.
 
pez said:
Great winds in southern Michigan over the past weekend and through the independence day holiday...

Had spectacular sailing... at one point had two people in the boat hiked all the way out on a starboard tack... outrunning a few of those pontoon boats that people buy for the funniest array of reasons...

Anyway... my laser will hum at a certain velocities/points of sail... I have no idea where it's coming from... the entire thing emits a sort of resonance... details get so fuzzy after you blast back and forth, but... seems like it happens more on a broad reach, and it seems to happen more often on port tack.
Click to expand...

Read somewhere that imperfections in the board can cause "humming".
 
It's to do with the shape of the foils, particularly the trailing edges of centreboard and rudder (when you're really belting along, they'll both hum unless they've been tweaked).

I did know of some guys who used to work the trailing edges to improve the flow across their foils (helps reduce the tendancy for the boat to "round up" as well), and the noises vanished.

What it is about them that makes the actual sound, I don't know, but the above is how it was explained to me 22 years ago.
 
It's to do with the shape of the foils, particularly the trailing edges of centreboard and rudder (when you're really belting along, they'll both hum unless they've been tweaked).

I did know of some guys who used to work the trailing edges to improve the flow across their foils (helps reduce the tendancy for the boat to "round up" as well), and the noises vanished.

What it is about them that makes the actual sound, I don't know, but the above is how it was explained to me 22 years ago.
 
The humming is caused by vortices leaving the trailing edge of the center board (a phenomona known as 'vortex shedding').

While you are going fast, heeled slightly, the board is both cutting though the water longitudinally, and also resisting lateral motion. The blade is shaped like a wing, and that lateral force is making it 'fly' similar to the way your sail is behaving on a beat or reach. If you take your sail battens out the trailing edge of your sail will also hum (flog) - the board is doing the same at speed, but being stiffer (and water being more dense than air), the frequency is higher - and is heard as a hum. Reshaping the blade changes the fluid dynamics, and hence alters the speed at which it hums.

Engineering degree paying off at last...
 
Nice... so google yielded a tidbit where as soon as the frequency of these shed vortecies matches the resonant frequency of the object that causes them, said object will resonate...

So there is nothing 'wrong' with the boat, it's that the sideslip coupled with the water moving across the board causes eddies on the trailing edge of the board, as the water does it's best to refill the area of low pressure created behind it...

So to apply what I have learned, it must be true that such humming requires energy. And to hear that hum most likely means that you are not sailing the boat as efficiently as possible, and it needs to be heeled less, such that the resistance on each side of the "double wing" shape of the dagger board is equal.
 
Hi,

I've cured quite a few "hummers" with one or two simple steps.

First, and easiest, is a very simple squaring off of the trailing edge of both boards. Use 120 or 220 on a wood block, and stroke lightly 3-6 times down the trailing edge. When the entire edge just turns brown, use that as an indicator to stop. You should have about a 3/16" flat back edge. This creates a quick exit for the flow, and a space for the vortex to do it's spin without hitting the board. Kind of like the whale tail effect on most fast cars these days. Sail with this, to see if it works before going to step two.

Step two. Sand the last 2-3 inches of the foil, to straighten the line the water flow takes just before the exit. Again, med. paper on a block. You don't really want to make the trailing edge thinner. What you want is to make the exit straighter, meaning the last 2-3 inches. so the flow can remain attached, and not spin off too early, and bang into the board before the board has a chance to leave.

Few laser foam boards require this step, but many of the old wood boards did, so I don't recommend doing Step two, until step one fails to cure the problem.

I don't repaint after step one, as it might round off the edge again. I would repaint after step two, because I'd have used the same wood block to plane off other high spots, and the result is pretty ugly. Then I'd use the finer grits to finish off the paint job. Note many paints need a week or more to harden off, before a good finish sanding.

JMHO though.

Al Russell 182797
 
older hobies were famous for their hum, because the CB molds they used had imperfections. The boards will hum if both sides aren't exactly symmetrical (which of course they probably aren't.) you're correct, it does use energy, and sanding it helps, but that's about all you can do.
 
one tip for peeing in high winds is just to get going really slow and rounding up into the wind and letting your sail out alot and just hanging over the side of the boat.
 
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