Why does my windsurfer sometimes "sing" when I'm really moving quickly?

Firstly, which bit's making that singing/moaning sound? I thought it was the rig singing in the wind -- some kind of aerodynamic effect -- but someone on the side mentioned that it was actually the fin... which doesn't seem intuitive but I'm prepared to accept anything.

Is it really related to speed or something else? My impression is that it happens on my freeride board when I'm really moving quickly but it seems to happen more often on my smaller board (JP MagicRide Pro 119) than my bigger one -- different fins, of course, so that could be relevant if it is actually the fin that's doing it.

I've heard other people's craft making similar moans (... of ecstasy... hur hur...) as they fly by but some seem to be silent (... in their pleasure... hur hur... ahem... sorry... )

Next: is it a good sign or a bad sign? I sand-papered all my K10 G10 fins until they were smooth, earlier this season, and, in our next sessions after the T.L.C., our rides seemed to be even *more* prone to vocalising their velocity -- this would be further evidence that the fin is somehow linked to this phenomenon. But all our fins were properly knackered (by previous owners... I promised myself that I could only replace the beaten-up second-hand fins that came with our practically-pristine second-hand boards (yeah. European inland waters have pebble beaches) as a reward for learning to water-start properly. I have since mastered the latter but I'm waiting for next season (Europe) to invest.) and so I could well believe that the singing sound is a bad sign.

Is it a technique thing -- something related to my sailing style? If so, what is it? What should I "fix"?

Is it a material thing -- something that I can change/fix about my kit?

Unless it's a good sign, in which case I'll just enjoy the eerie noise.

I'm fascinated. It's a cool sound but I'd really like to understand it.

Fins certainly can "hum" it's caused by turbulence off the back edge, especially if it's thick and square. If both your boards have fins with a thick trailing edge, try thinning them down a little, around 0.5mm should be OK, but sand them on a slight angle, about 10 or 15degrees off 90, don't leave them round.
If that doesn't help, it can be the holes in your mast extender, especially if they aren't covered by a mast protector.

Any turbulence is probably slowing you down a bit, you may be struggling to notice it but.

I get this regularly, more likely when going my fastest. I always assumed it was the fin but on a previous thread on this topic it was suggested it was the boom which would probably make more sense. I still don't know for sure what it is but I don't mind it because it tells me I've reached a certain speed, whatever that speed is.

The boom is another possibility, but I think you'd pick that by the direction the noise is coming from.

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decrepit said..
Fins certainly can "hum" it's caused by turbulence off the back edge, especially if it's thick and square. If both your boards have fins with a thick trailing edge, try thinning them down a little, around 0.5mm should be OK, but sand them on a slight angle, about 10 or 15degrees off 90, don't leave them round.
If that doesn't help, it can be the holes in your mast extender, especially if they aren't covered by a mast protector.

Any turbulence is probably slowing you down a bit, you may be struggling to notice it but.

Decrepit is spot on I think here the only time I've experienced this exact issue was when I took the sharp edge off my weed fins after getting a particularly nasty cut - squared/ rounded trailing edge then 1st sail afterwards the board sung / whistled like a flute.... fixed trailing edge & sound went away ...

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decrepit said..
Fins certainly can "hum" it's caused by turbulence off the back edge, especially if it's thick and square. If both your boards have fins with a thick trailing edge, try thinning them down a little, around 0.5mm should be OK, but sand them on a slight angle, about 10 or 15degrees off 90, don't leave them round.
If that doesn't help, it can be the holes in your mast extender, especially if they aren't covered by a mast protector.

Any turbulence is probably slowing you down a bit, you may be struggling to notice it but.

Thanks.

I guess I'll conclude that it's damage on the trailing edge, then, because the JP fins are actually sharp, by design -- my feet and shins can bear witness -- but ours carry a fair amount of damage.

My sand-paper job was just done by hand so I couldn't really get a pristine edge, again. I used a block because I explicitly wanted to avoid rounding that edge. I'll probably do a more thorough number on them, during the off-season, and start with more aggressive sand-paper, this time.

I knew, when I bought the boards, that these fins were going to be replaced sooner or later. They've served for two seasons -- excluding time under their previous owner -- and, during that time, I've learned to beach start and water-start and had many a great session. They've served their purpose.

North Platinum Booms (with the tail end that sleeves OVER the frond end, and the clips that slide up and down the boom) sing beautifully - something to do with all those holes into the hollow rear end exposed to the wind.

I've had singing fins but these days it's usually the sailor..

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eckas said..
North Platinum Booms (with the tail end that sleeves OVER the frond end, and the clips that slide up and down the boom) sing beautifully - something to do with all those holes into the hollow rear end exposed to the wind.

And if you pull the tail in and out it plays a tune.

The AFS-2 foil was a "singer" that could be heard from far away. More like a "howl." Bruce Peterson and I worked on the stab TE (trailing edge) to reduce, but not quite eliminate, the singing. There is information out there about shaping the TE with a 1.8-times-thickness curve to a sharp 45 degree edge on the low pressure side. Don't shape the high pressure side. People who engineer turbine blades in hydro power dams came up with this. The idea is to reduce TE vortex shedding. Some slalom racers like this noise because it announces their location to other racers.

I think there is more to it than just the TE shaping. You can always get resonant harmonic vibration that is caused by the particular shape and stiffness of the structure. The whole structure when assembled. AFS uses the same stab for the AFS-2, AFS-85, and AFS-95, and the latter two do not sing. Their assembled structure is different from that of the monocoque AFS-2.

In the "old" days of finning, some fins used to sing, too. The solutions for them, in my case anyway, was to get out some 400 grit sandpaper, and de-edge the TE a little.

It hasn't been mentioned yet, but if the fin box (for a single or the middle of a thruster) has a slight angle to it (not pointed perfectly straight ahead), that can also make the fin hum.