After slicing half my foot off on the trailing edge of my fin, I am now considering sanding it a little to make it a little blunt.
This thing is like a knife edge. How sharp does it need to be?
I remember reading advice from Robby Naish to "Take the sharp edges off your fin". Once bitten no doubt, you're in good company Harrow. And just checking the fin that came with my Hybid 110 - it's not at all sharp, nor does it work.
I sand mine back generally to about 0.5 - 0.75mm wide flat on the back
Had a couple of nasty incidences where the trailing edge has worked like a potato peeler.
yer i sand mine as soon as i get them from the shop, missed a cranking 40 knot day, was stretching my arms before i hit the water, the wind picked the board up and dropped it on my foot, new fin= massive gash on my toe..
yes they don't need to be that sharp, but sand them flat at the back not round..
They should be as sharp as possible - if you are going to hit the town straight from the water you can give your chin stubble a quick shave. And a small notch in the trailing edge makes a useful stubby opener, for when your fingers are cold.
Not trying to take away from the safety aspect, but how much extra drag does it make? They obviously have sharp trailing edges for a reason.
If I took half a mm off the trailing edge and made it dead flat, would I notice the difference in drag? Can I assume that nobody no-one has measured it?
On GPS Surfing I remember reading a forum article about this, and some guys said a 1 mm trailing flat was fine, even desired for performance. Not having tried it myself I cant say !!!
i wouldn't think for speed sailing it would be good because that would induce cavataion (i think that's the spelling) and that would be bad.. i would think.
The law/theories of hydrodynamics are not as obvious as one would think :D
I de-sharp all my fins, particularly my waves fins as I'm interested primarily in the preservation of my limbs etc rather than doing 25kts as opposed to 24kts.
Have a read of these, a flat on the trailing edge up to 0.5 mm seems OK and iterates what Decrepit says about parallel outline
www.gps-speedsurfing.com/default.aspx?mnu=forum&forum=1&val=2205
www.gps-speedsurfing.com/gps.asp?mnu=forum&smnu=1&val=3161
flatten the sides going to the trailing edge, then sand it square.
The flow separates either before or at the traling edge.
Basically where you have too large convex curvature the flow can separate. This means you get a pocket of highly turbulent flow extending behind where the flow separated. The thicker the pocket the higher the drag. So the thicker the fin where it separated, the higher the drag. Yes, insert golf ball graphic here... its relevant because the dimples delay flow separation on a golf ball by making the flow turbulent before it detatches. (note: dimples not required on a fin!!!). This has the effect of keeping the flow attached for longer, where it separates closer to the back of the ball. In a fin, the flow is likely to be turbulent before it gets near the trailing edge without the assistance of visible dimples (all to do with the viscocity of water compared to air, and the speed we go). But the effect on drag is the same- if the flow separates where the profile is thinner, the drag will be lower. On the golf ball, delaying the separation decreases the turbulent wake thickness and makes a huge difference to both stability, and efficiency. Same for fins.
So, if our fin hasnt been finished properly in the factory (gee I wonder if that ever happens 
- picture short dude with a big buffing wheel in overalls smoking a fag and 20 fins on a rack waiting for "precision hand finishing" 
) the trailing edge will be rounded, so the flow separates before the trailing edge, the turbulent wake is thicker and the drag is higher. So by sanding out all the tight curves near the trailing edge (can involve a lot of sanding on some fins), you shift the separation point closer to the trailing edge where its at its thinnest. Squaring off the trailing edge creates a clean separation point. Making a fin perfectly sharp (if it were possible) offers very little improvement in drag. Its already really thin and there are other effects that increase the turbulent wake thickness due to the combination of the flow from both sides of the fin. *Trying* to make it perfectly sharp makes the flow separate early because you will have introduced a tight curve in there even if you might not see it. Early separation leads to vortex shedding and other separation effects => whistling, drag increase, and possible loss of lift. Has anyone ever used a stock Select?
Simple 
But always use a cover over the fin when pulling it out of the box 
You now have a double edged blade thats still quite sharp and becomes serated with a few nicks... If you feel safer with a thicker trailing edge, then its really important to square it off properly so it doesnt add unnecessary drag.
0.5mm is more than thin enough IMHO
Where was this thread 12 years ago when I kicked my fin? Back then I heard of no-one nobody
doing any real damage until I had stitches between my toes (ended up with one really long pinkie toe).
Now I make 3 or 4 90 deg. passes with wet and dry, just enough to take the cutting edge off the bottom two thirds of the fin.
That seems to make sense to me Slowboat, it's got me once again checking fins with straight edge against the morning sunlight. But how far forward of the trailing edge are we looking for a problem? I've been looking at the last 20mm.
Comparing my incessantly whistling fin against one that doesn't, the whistler is flatter over the last 20mm before the trailing edge.
The whistling fin has been sanded before according to various past theories but no change, it still whistles and, as far as I can tell, still performs well. I tolerate the whistle, sort of musical , it jumps an octave at 28 knots.
The non-whistler has an easily seen convex radius of curvature to within the last mm or two of the trailing edge where my eyesight can't really see the detail.
I used a stock Select once but was ordered off the beach for disturbing the peace. It turned out to have a big concave on one side of the trailing edge, ( over the last 20mm ) a manufacturing problem, looked like it had distorted coming out of a mould. I sanded that out and it went completely quiet, but I don't use it much , it's good but spinout comes on too sharply for my comfort.
So my observations suggest too much concave rather than too much convex in the last 20mm causes whistling. Am I looking in the wrong spot?
Hi Ian,
I think the last ~10mm is the most critical.
If you look closely you'll probably see there is a bump just before the TE. All the Select fins I've seen have this. I have a large select that I've tried a few times and I need earplugs to use it. Its still pretty fast, even faster after I sanded the trailing edge area, but its still the noisiest fin I've ever used.
Some simple calcs- guessing the whistle frequency is about 1500Hz. Given the speed of sound in water is ~1500m/s this would imply a half wave resonant cavity size of 50cm... In air this is ~11cm. One effect of air bubbles in water is to reduce the speed of sound. This rough calc shows that the coupling effects on the vortex source extend quite far behind the trailing edge.
I've read somewhere that some guys solve the whistling problem by sanding off the edge on one side. Perhaps this is enough to prevent the separation point from wandering back and forth on one side (consequence of flat trailing area?), and prevent the vortex source from being influenced as strongly by downstream events. Effectively dropping the gain enough in this feeback loop to make it stable. I might try it on the noisy select. Loads of speculation here...
cheers
I've got 2 select fins, but neither of them sing. 
What should I do to remedy this?
I think the water density has a bit to do with it. I get noise from the fin at about 44kn in salt water but not until 46kn in fresh.
Interesting.
To add further to the speculation I dusted off the old fluid dynamics text and whistled into the music analysis software that comes with my computer. I estimate my fin whistles at 1760 hz.
A chapter in my fluid dynamics text discusses wake vortex shedding from cylinders. The phenomenon may or may not be appropriate to fins. But if it is wake shedding the equation is St = nd/u. St, the Strouhal number, is a slow enough varying function of the Reynolds number that it can be taken as a constant 0.2.
n is the frequency of vortex shedding d is the diameter of the cylinder and u is the speed.
The text says that between 200<Re<400 and 300,000< Re < 3,000,000, the vortex shedding becomes less regular, which may explain the stepwise frequency transitions we sometimes hear. Fins are in that last range I think.
Solving for d though with u = 15 m/sec (my speed not Slowboat's) and 1500 hz to make division easy gives d = 2mm. OK that's for a cylinder, but it is in the ballpark of the width of the trailing edge??
For 15 m/sec boardspeed and 1500 hz the spacing of the vortices left behind the board, if that's what they are, is 10 mm.
I'm glad we're all speculating, and how does the sound get from a vortex to your ears? From the water surface? Back through the fin and up through the deck? Mast base? There's a lot we don't know about whistling fins.
Nice one Ian.
If the flow separates before the trailing edge, then 2mm is pretty close.
Yeah the fin Re is ~500K to 2M depending on chord and speed (and water temp and salinity)
The deck of the board would make a nice acoustic radiator. Its pretty tightly coupled to the fin.
Many mysteries in the whistling of fins... At least it doesnt seem to kill the performance. I think the general consensus is its most likely due to vortex shedding. How to stop it or make it play sick tunes is another mystery...
So still on a musical note, are the trailing edge vortices always associated with whistling. Or maybe vortices are always being shed from the trailing edge and it is only under special circumstances that the vortices set up acoustic vibrations in the fin?
I looked up sound power on Wikipedia, the source strength, and picked "loud speech, vivid children " as the category most matching the loudness of my fin, they have a sound a power of 0.001 watts.
Sound power appears to be only a small fraction of the mechanical power associated with the source.
I remember once estimating, (OK wildly speculating) the drag on a fin as 1 kg.
Power = force times velocity so the fin at 15 m/sec is consuming 10 newtons *15 m/sec = 150 watts.
So the actual sound power is negligible by comparison. So unless we know it's associated with another inefficiency, an nobody's detected them being slower, yeh we might as well enjoy it.
Ill bet my Lightning Evo 37 against anyones elses whistling fin
The whistle starts around 24-25 knots and the faster you go the louder it gets. Mates reckon they hear me approaching at a big distance. At 30 knots and above its like standing behind a 747 spooling up
I finally got SamP to check it and he said its all over the place.
I even contacted the factory guys and got 2 different opinions, have tried everything to no avail.
Feel better that Chris knows about Select fins
"I've read somewhere that some guys solve the whistling problem by sanding off the edge on one side"
Well I tried that select fin again this morning... Noisy as hell. Came back to the beach, 2 or 3 wipes of one side at about 45 degrees with 400 grit and...
silence...
no more whistling.
Speaking of fin finish, which we weren't, what's the current received wisdom? - polished leading edge and 400 grit finish in the fore/aft direction?
