Why are fins flat?

[jaredH20]

I have been wondering why are slalom ski fins flat? If you look at any other form of technology they have an aero dynamic shape. For example the wings on a plane, they are designed to move through the air with as little resistance as possible. So surely we should have fins with some sort of shaped fin similar to a keel on a yacht or something along those lines?

[Thomas Wayne]

Airplane wings are shaped to generate lift on the top side, and for that reason their cross-section is asymmetrical.  If a wing's cross-section were symmetrical on both sides - something that is fairly necessary for a ski fin - the wing couldn't generate lift and the plane probably couldn't fly.

On a yacht (presume you mean sailboat-type) the keel serves multiple functions, most notably to contain and position the boat's ballast at a low point in order to prevent capsizing.  Keels are generally shaped - in fact, its a rather complex science to determine a specific keel's shape.  However, the keel is always placed at or very near the pivot point of the hull (roughly centered fore and aft), which would equate to a fin being directly under the bindings on a ski.  I can tell you from experience that won't work.  A more apt analogy to a fin would be a sailboat's RUDDER, and those are generally similar to a ski fin (flat).

TW

[eleeski]

Actually, unless you are Horton who doesn't believe in wings, fins are not flat. The wing adds huge performance features to the ski.

TW, I've flown aircraft with symmetrical wings (for aerobatics where good upsidedown flying characteristics are needed). But you are correct about the design of the airplane wing being designed for it's lift and a sailboat keel designed for stability. And the rudder analogy is also right on - and reasonably flat.

Material advances have let some designers go very thin in the ski fin. But there are so many variables and the fin is just one. Horton gave us a lot with his choices of fins - and some ski/fin combinations were spectacular.

I prefer the funky HO pacman fin. Or the Horton soft fin. Both are a bit innovative.

Challenge to JaredFish: Try some innovative fins. It might be fun and who knows?!

Eric

[Dirt]

Interesting topic. I remember when a certain jumper built a pair of skis that had an (asymmetrical) airfoil shape on top to create lift. Great idea after watching Sammy Duvall (I know, the kids are saying who is that) cross his skis. The problem was that he built them out of balsa wood. They worked great right up until he hit the wakes. The skis flexed and the balsa exploded. I wish I had video. It was hilarious. Great idea, wrong material.

 Can anyone guess who the engineer was? Horton will give the first person to get it right a free BOS t-shirt.

  

[jimbrake]

Hey DSmart - off topic, but do you fly a helicopter in Sac? Was skiing at Bell Acqua yesterday evening and a helicopter flew over and Mike Winter said "that's Dave Smart".

 Anyway, interesting topic. Surfboard fins come double foiled (like a keel) and single foiled (like a wing) and there are endless shapes and dimesions and fin setups from single fins to the common three-fin thruster to the five-fin bonzer set ups. As far as I know there are no flat fins (blades) on surfboards. I've often wondered what some small side fins would do to a slalom ski's performance. I'm not much of a tinkerer, though, so I've never tried it. Would take a ton of experimentation on size, positioning, etc. Could be potential benefits to ability to hold angle, accel, decel, etc.

[skidawg]

It would have to be JTH, he has quite the history of making skis out of balsa wood.  Size XL.

[jaredH20]

according to my father who is a multi-time national sail boat champion, the shape of a fin on a yacht is very very import, it has to be specially foiled or you get cavitation.

[eleeski]

Drag is king for sailboats and airplanes. Drag is a minor factor for waterskis (just pull a bit harder!). I run my fin wing just short of where it starts to cavitate in an effort to maximize the drag.

Stability is also important for sailboats and airplanes. A slalom ski needs to be a bit twitchy. I run my fin as shallow as possible to where the fin won't skip out in the turn to counter some of the fin's stabilizing effect.

I fielded a ski with a small fin on the top of the tail which worked fairly well (helped give a bit more tip pressure). Side fins are something I've been contemplating but haven't gotten to. Double fins are a bit further down the experimental road. And moving the fin to underfoot was a failure. But fun research...

Were those jump skis the same ones attached to the top of the airplane wing?

Eric

[Dirt]

Jimbrake, I do, unless it was loud and annoyed you. If it did, it must have been someone else, probably CALSTaR.

 skidawg is correct and notice I said a jumper designed and built them. Horton send skidawg his t-shirt. He is waiting.

[Dirt]

Eric, these skis were painstakingly designed and built by JTH (when he was supposed to be studying) and looked great, right up until they exploded into a million pieces. I almost felt sorry for him except it was JTH.

[Horton]

Lies! All Lies!

[jimbrake]

DSmart,

Not annoying at all. Could tell you were checking out the lakes to see what was up. You should've landed - you were up next.

[Dirt]

Thanks jimbrake, maybe I'll take you up on that one of these days if I can get away with it.

[Horton]

SkiDawg can pay for his own freak'n t-shirt.

[Dirt]

Horton, he won the contest. Stop being cheap and pay up. Keep your word.

[Horton]

I put a few huge insects in a guys dorm room 20 years ago and he still holds a grudge.

 

And for the last time: that thing with Debbie was never going to happen.

[skidawg]

jth,

I did pay for my own t-shirt, that was my self provided fathers day present, but who should deny me of winning smart's contest, come on man, where's the hope and change!