Hydromechanics of fins

Looking for info on how fins work. How do different foils create different effects?(Hold, drive, lift, drag, etc.). What does toe and cant do? How do different bases and tips of the fin effect things? Just looking for an overall knowledge dump on everything fins. thanks.

That is an incredibly broad topic. And I’m definitely not a fin expert.
Your best bet is to search through past Swaylocks threads.
I did a quick search for “Fin Design” and got this:


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Id be happy to see this thread expand.

The only book directly aimed at fins that I know of is Dion Patelis Surf Fin Design. If someone else knows articles from older mags or good sources please post.

Like stoneburner mentioned , there are tons of discussions on fins. I spent hours and hours on reading stuff , but I didn’t translate into practically applyable knowledge in terms of if I do A , B will be the result- the only thing I’m quite sure off is that there’s plenty of room for experimentation.

It’s a sad that 4 wfs and Larry Allison’s boxes seem to have vanished from the market. They were/ are still ahead of the time , I’m pretty sure with surfings new playground, the wave pools, there will be a comeback of those adjustable systems.

Hans from finfoil.io said a while ago that foils with a low Reynolds number will work. I don’t understand the math behind Reynolds numbers, and I dont understand Naca digits in depth , but from just starring at fins when sanding them - and the belief that Hans knows his stuff- I think foils should have a blunt front edge when you want to turn( as opposed to have a sharp blade for directional stability on a SUP race board for example.) -and have that low reynolds number.

From what I read here on sways my own direction is away from " half- foils" to thicker fins, and I try to build them myself.

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Reynolds and Naca digits?!?! WTF are those?
The world of fins is infinite I think. Too many possibilities. Thanks for the response!

Hell yeah thanks for digging that up!

:wink: Well , it’s better you read that online …Im too stupid to function as the messenger.

Theres a ton of scientific research on wings and even if not all and everything applies to fins in surfing most of the discussion starts there.

Check out the Theory of Balance thread here, a very good one, I don’t know how to link it…

Interesting thread topic. Like so much of surfboard design, it seems to me that the big decisions are arrived at by intuition coupled with trial-and-error, very anecdotal in nature, and then theory comes more into play in the refinement stage. I almost wrote a brief thesis just based on observation, just to get the conversation started. Which would be pretty unscientific, and no doubt riddled with errors or incorrect assumptions.

You can google the subject, and find some interesting stuff, and some stuff that seems pretty antiquated. I remember reading some good stuff by Rusty Preisendorfer on his quad fin theory, which simplified the subject to make it more accessible. I’m not sure where to find that information now, but I’m sure a little googling would likely turn it up. Also some good stuff out there by Simon Anderson on thruster fins.

It seems to me that the toe-in and cant of rail fins was arrived at anecdotally in response to the problem of tracking with multiple fins.

Again, its a good topic, and I hope you get some more responses in this thread.

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You may find http://www.airfoiltools.com/ informative: especially for the calculation of Reynolds Number.

Some general info at this link: