Do hi-speed fins have a very sharp trailing edge? In keelboats generally there is a trailing edge with up to about 1cm across - this has been blamed often for cavitation and the solution has been to put a chamfer on the edge so that it is not square but actually on an angle across between 30-45 degrees (on one side, of course the other side the angle will be 45-60 degrees). Have also seen this often and used on dinghy foils, though the cross width of the trailing edge is much smaller. This supposedly stops alot of the cavitation though minutely biases the foil. Of course the targeted foil speeds are totally different for board speedsailing. If the speed sailing fin trailing edges are not sharp is a chamfer like this applied?

Hmm. I doubt that the trailing edge would have any effect on cavitation at all. Cavitation is created by very low pressure on the foil surface causing the water to vaporise and bubbles to form disturbing the flow. Even if this happened at the trailing edge I doubt it would cause much disturbance. But I may be wrong.....
Some of the best speed fins I have used had razor sharp trailing edge. I have the scars on my hand to prove it from foolishly trying to knock fins out with the palm of my hand. Ouch!! I have also heard recommendations to square off the trailing edge of Large course racing fins to stop them 'singing'.

Thanks SailQuik - you just jogged my memory - it was to stop humming when planing at speed - done to whichever foil required, rudder and or keel/centreboard. Apparently the humming created drag in some way (vibrations etc). So speed fins - the sharper the better (until a wipeout in close proximity to the fin )

I've got about 15 stitches in my left hand at the base of the thumb, due to a surfboard trailing edge, done back in the 60s.
Alby's got a nice gash on his calf from a stack the other day.

I no longer have sharp trailing edges, even if they do go a bit faster.
A thin square edge, seems to be the go. with the sides as close to parallel as possible.

I also think that a thick trailing edge produces a very low pressure, adding drag, but can also lead to ventilation if the fin is too close to the back of the board.

Just on the humming:

Every Select brand fin I've used (4 in total) has whistled. I've had a good look at the trailing edge, and they're all square as Decrepit describes. So it can't be the sharpness of the trailing edge that makes it whistle.

To be honest I like it, because the pitch rises in distinct steps every knot or two... I don't need to look at the GPS to see how fast I'm going

Hi Nebbian, I'm fairly sure the chamfer was to remove the humming on a squared off trailing edge - as with the constuction of many dinghy and keelboat foils (often done in 2 halves and glued together) it is not possible (or often leads to seams opening up - cracks working open) to have a sharp trailing edge.

I have seen countless times the ventilation effect, as Decrepit describes, on stern hung keelboat rudders and that was generally cruiser type boats - perhaps the chamfer would help prevent that as well. Maybe a chamfer would remove the humming you experience and possibly a little quicker?? (if the humming does have an effect on speed).

A mate had a Farr 42 racing yacht built, the designers were pretty specific on the angled chamfer on the keel fin and rudder. I think they do this to minimise vortex shedding (like swirls/eddies of water you see as it passes a stationary pylon etc) in the fin/rudder, these eddies of the water flow can induce the fin to start to resonate and vibrate. I could imagine this happening on a slow keelboat (<20kts) but on a windsurfer? maybe a question for Mal...

My mate used to sail alot on a S80 keelboat, this boats keel would vibrate all the time whenever it went over 10kts, the filler at the hull/keel join would drop out all the time, just a matter of time before the keel drops out..

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sailquik said...

I have also heard recommendations to square off the trailing edge of Large course racing fins to stop them 'singing'.

Sorry, I guess I misread the above line.

Could I restate it thus:

Square trailing edges make fins sing?

I suppose the main question then becomes - does the humming/vibrations cause a loss of speed. Certainly seems to be the reason behind the attempts at removal in keelboats and dinghies - there must be some proper research analysis on this somewhere (I know peer reviewed and all ). If so you might want to put a chamfer in those fins.

Nebbian, have you got a fin you can experiment with on this. Would probably be impossible to tell speed wise, but would be interesting to see if you can feel the differences. Would make an interesting test to pick a relatively constant wind day - go out with the fin as is and then come in and chamfer the trailing edge with a file and wet n dry, then see the effect.

Would also be very interesting to know exactly what is done in fin prep for the fins for the PWA guys.

If the trailing edge is too squared off it will sing . Thats when you add a small chamfer , it definitely works . I have had select fins that sing . It looks to me that when they paint them they get a build up on the trailing edge . A couple of passes with some 600 wet and dry and it,s fixed .