fin characteristics and their performance attributes

General Discussion
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there are aspects that give you performance gains fast, and other aspects that offer less gain with perhaps more effort...law of dimishing returns

wrt thrusters, here are the quick gainfull aspects:

  1. Setup: location, spread, toein, cant, etc

  2. Template size/design: bigger, smaller, all fins symmetrical, bigger fronts v. rear, vice-versa, etc etc; keep it simple using medium rake designs - ie what’s on the market

3a. Fin Flex: all stiff, all soft, some stiff, some soft etc etc

3b. Foil design: very esoteric shit but you can break it down to thick vs medium vs thin; and if youre bored add pushed or pulled, sharp edges v rounded

when you figure those out, you’ll be very satisfied if not delighted and everything else will become trivial in comparison.

my current favorite setup: stiff TC redlines front, flexible AM rear

cheers

I’ve been playing with distribution of chord lengths lately for thruster

rail and center fins…

Rail fin base chord length has a pronounced impact on thrust. Rail fin tip chord length has a pronounced impact on hold. You can almost independently change the two by going long base small tip, short base large tip.

The rotating fin prototype has large base small tips, not good hold in bigger waves, but generates thrust very well in smaller waves. Make the tip larger and you lose a little thrust (draggier in turns) but the hold improves. Make the base shorter and you lose thrust.

Rear fins don’t get much out of having a long base chord…try moving the fin area further from the board, like a hatchet. In the middle of the board the water close to the board is being dragged with the board. The fastest water is a few inches deep (unlike a rail fin where the fastest water is perhaps 1/2 inch up from the board).

Lends a lot of credence to using small tip larger base rail fins in smaller waves, and always using a fuller-tipped smaller base rear fin.

Most people never play with rake, it has a big impact too. And certainly if you work with a fixed template and just change the foil you can feel that easily too.

Course I’m biased, I’ve felt for years that fins are a hot area for tweaking your ride.

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I like it Dave!

The hatchet center theme makes sense. But I have to disagree on your assessment that longer base chords are not as important in the center. My experience is just the opposite. One example, take two FCS centers, G3k and a AM…the tips and depths are not very different but the bases are very much different…try each on one of my sticks and there’s no comparison…plenty of drive hold on bottom turns with the AMs…not so much with G3ks. I use both fins regularly.

Another noteable…those templates adjustments you mentioned…ive tried a couple experiements like that and unfortunately, the flex of the fin goes haywire…and then its just not a fair comparison…the base of the fin is like the root system of a tree…some trees have strong roots, some weak.

fortunately, the Futures hatchet fin is pretty stiff…it has to be or it’d be a dud.

been giving some thought about rotators too in the center…thats why Im trying flex centers now…i think a rotating center fin would be kickin’.

and addressing the original post…popular fin analysis is a lot like board analysis…usually the real key things are ignored…very few guys talk rocker…more talk tail shape…shamefull…now double foils are all the rage…ha! Like I said, esoteric shit. You can have the most advanced fin design on the planet but get the toein wrong by just one degree and fahgettaboutit…

Things like foil theory is rocket science…making fins work on a surfboard isnt. You just gotta spend to play and learn…I have a large rubbermaid storage box for all the fins that I own…thats tuition expense. And besides, buying fins is good for the economy.

Slater’s using mass produced off-the-shelf stuff…think his surfing has suffered? Like I said, not rocket science…

fun fun fun…

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Speaking of rocket science, I thought this was kind of fun.

http://travel.howstuffworks.com/framed.htm?parent=airplane.htm&url=http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

This can help to het a handle on the ideas of foil shape and thickness

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The hatchet center theme makes sense. But I have to disagree on your assessment that longer base chords are not as important in the center. My experience is just the opposite. One example, take two FCS centers, G3k and a AM…the tips and depths are not very different but the bases are very much different…try each on one of my sticks and there’s no comparison…plenty of drive hold on bottom turns with the AMs…not so much with G3ks. I use both fins regularly.

As stated, not completely true.

But fin area at the tip has a disproportionately large effect on hold, and a disproportionately small effect on thrust, and vice/versa.

Take any fin, decrease the root chord, and you will lose some hold. Take the same amount of area off the tip in that rear fin and leave the base alone and see what happens to hold and drive…

My credo is that fin action is maximized by making chord lengths longer where water flow is faster, and shorter where water flow is slower. On a rail fin that means the root chord needs to be the longest, and on a center thruster fin the chord about 2 inches up needs to be the longest.

It fails to explain, though, the poor hold of a small tipped rail fin. The tip gets s a disproportional control over hold. The thrust is maximal by putting longer chords where water moves faster (and tapering chord lengths SLOWLY to depths where it is slower).

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Another noteable…those templates adjustments you mentioned…ive tried a couple experiements like that and unfortunately, the flex of the fin goes haywire…and then its just not a fair comparison…the base of the fin is like the root system of a tree…some trees have strong roots, some weak.

fortunately, the Futures hatchet fin is pretty stiff…it has to be or it’d be a dud.

The stiffness coming from the base is a strong function of thickness…you had to keep the main chord close to 0.25" thick (in glass) or all bets are off, you are making a flex fin. The point I was trying to get across is that I can see the logic in the hatchet style rear fins, and why you want big, eddy-inducing, tips on fins when you need a lot of hold. Purely from a hydrodynamic point of view you may prefer an elliptical or raked ellipse shape, but the fat tips of surfboard fins are there for a reason.

But you can leave them off in smaller surf and benefit in increased thrust when you do not need the hold.

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been giving some thought about rotators too in the center…thats why Im trying flex centers now…i think a rotating center fin would be kickin’.

The Waltie Swivel fin was a rotating center fin. Loosened up the tail a bit. Herb sent me one a while back, it is a neat design, doesn’t let you change the fin though. And it lacks a lockout angle and preload to keep it from oscillating. It didn’t solve the problems I was pursuing in making thruster fins rotate - removing nearly all fin drag when you want to go straight, and making every turn a two-fin thrusty turn.

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Things like foil theory is rocket science…making fins work on a surfboard isnt. You just gotta spend to play and learn…I have a large rubbermaid storage box for all the fins that I own…thats tuition expense. And besides, buying fins is good for the economy.

A grinder and sander from Harborfreight, a few blanks from Probox, and you could save an awful lot of money.