1. Foil and Lift

Beyond the fun drawings and the eclectic art — all the simplicity and unseriousness — we do give a rat's ass, a big one, about building the best possible fin. It comes down to science, and we've been digging in hard. The goal is to leverage the physics so well it looks like magic.

So this journal is part tell-all, part middle finger, part notebook. I need to remember this stuff, and I figure some of you will find it interesting too.

Starting with the basics.

A fin is a foil

Before any of the surf vocabulary — drive, hold, release, pivot — there's an object underneath it, and it isn't specific to surfing. A fin is a foil. So is a wing, a sail, a rudder, a keel, a propeller blade.

A foil is a shape with four parts: a leading edge where the flow arrives, a trailing edge where it leaves, a chord line running between them, and a thickness distribution wrapped around that line. Symmetric if both sides match. Cambered if it's biased to one side.

Its job is to turn motion through a fluid into a force pushing sideways to that motion. That force is lift.

A flat plate makes lift too

Hold your hand out a car window at an angle and you get shoved. No foil shape required. So what is a foil actually buying you?

Cheapness. A flat plate makes lift and a huge amount of drag, and it stalls early and violently. A foil makes similar lift for a fraction of the drag, and it keeps working to a much higher angle before it quits.

That ratio — lift produced per unit of drag paid — is the entire reason foil sections exist. Every design choice we make is some version of buying lift more cheaply, or giving up cheapness in exchange for a failure we can predict.

That second half is worth sitting with, because it's where material comes in. A perfectly rigid fin holds its shape no matter what you do to it. A real one doesn't. Honeycomb, glass, the layup, where the thickness sits — all of it decides how the fin deforms under load, and deforming under load changes the shape that's making the lift in the first place. More on that in a later entry, because it deserves its own.

Why there's a pressure difference

The common explanation is that water takes a longer path over the curved side, so it has to speed up to meet itself at the back. That's wrong, the two halves of the flow are not racing. They don't arrive together.

What's actually happening is simpler. The foil turns the oncoming water. To bend a stream of fluid you have to push on it, and it pushes back on you. The faster flow and the lower pressure show up together as part of the same event — neither one causes the other.

Practical version: a foil at an angle deflects water one way and gets shoved the other way. The pressure on the two faces is just how that shove gets spread across the surface.

Two sources of lift, and one of them runs the show

Camber gives you a built-in bias. A cambered foil makes lift even at zero angle, always in the same direction.

Angle of attack gives you everything else. It's the live one, changing second to second while you ride.

You can see the difference in how fins get built. Center fins are usually symmetric — no bias, same behavior both directions, which is what you want from something that has to work identically on either rail. Side fins are usually foiled, and that asymmetry is on purpose.

Look at the sketch: the arrows point both ways off the fin panel. That's correct for a symmetric section. At zero angle it makes nothing, and which way it grips depends entirely on which way you've angled it.

What changes when you swap air for water

Same shape, different fluid. Three things change.

Density. Water is about 800 times denser than air. Lift scales directly with density, so the same shape at the same speed and angle makes roughly 800 times the force.

Speed. Lift scales with speed squared, and that cuts the other way. An airliner moves maybe ten times faster than a surfer, which is a hundredfold advantage in that term. Multiply it out and a fin at surfing speed lands in roughly the same force range as a small wing in flight. There's a reason fins are small.

Reynolds number. The ratio of inertial to viscous effects, which decides whether the thin layer of water clinging to the foil stays smooth or goes turbulent. A fin with a four-inch chord at surfing speed sits around 500,000. An airliner wing is up near ten million. Completely different regime, which is why you can't lift an aerospace foil section off the shelf and expect it to work on a surfboard.