[divider_flat]Here’s a way to look at hold and drive—explained in terms of lift and drag, to relate lift and drag to surfboard fin and SUP fin designs, and why at FSI we think high-lift, low-drag fins are the most fun because they paddle more easily, accelerate faster, and turn better because they have more speed, hold, and drive—the way to go.
When we talk about surfboard fin design, we typically talk about a fin’s hold and drive. Often the discussion about fins seems pretty muddy, especially when you look at all the shapes out there and wonder what’s behind the shapes. Then you’ve got to try to memorize all the lore, so you can talk about it intelligently and with a straight face to friends in the lineup.
We typically refer to a fin’s hold as the ability to control side-to-side movement, like the ability to plant a foot and leverage off of fins when turning. We know that a bigger fin has more hold than a smaller fin in general—there is more area to plant a foot against. And as we make the fin smaller, it will lose some of that hold, it’ll be a looser fin, but it will probably gain some speed. But go too small, and you’ll spin out when you try to plant a foot against the fins—there wouldn’t be enough side force, i.e., there would not be enough hold.
And we typically refer to a fin’s drive as the ability to accelerate, and to maintain or gain speed though turns. But the way folks talk about all this, it seems like they think some sort of magical force is at work—some fins have good drive and others don’t, and making sense of it all so that fins can be compared to each other is pretty tough.
In terms of science, surfboard fins and SUP fins have two forces acting on them, lift and drag, and that’s pretty much it. The forces work the same way on fins as they do on airplane wings. But with wings these forces are mostly vertical, while in surfboard fins the forces are mostly horizontal.
In fins, lift is mostly a sideways-and-forward force. Drag is a hold-you-back, backwards force.
Technically, lift acts perpendicularly to the flow of water around a fin—at all locations as the water flows around the fin. Lift acts pretty much like the green arrows on the diagram above—some lift is sideways to the fin,and some lift is forward.
The side force is the component of lift that makes airplanes fly, and keeps surfers from spinning out–it’s hold.
So hold is the magenta arrow on the diagram below, one of two lift components, and it acts sideways.
But lift has a second component—a component that acts in the forward direction. If you’re a sailor, you know this forward component of lift that in concept is a suction force that pulls sailboats toward the wind. Without that suction force of lift, sailboats wouldn’t be able to sail toward the wind. So lift sucks! Aha! It’s drive!
In the diagram, drive is the turquoise arrow, the second of two components of lift. For those of you who remember science class, we can add the hold-and-drive arrows—or resolve those two vectors—and we get lift—the green arrow. Obviously the diagram is a simplification, but it captures the general idea.
It’s important to know that there is a speed component to lift. Faster-moving fins produce more lift than slower-moving fins of the same size. We experience this every time we take off in an airplane. As we gain more speed, the plane eventually takes off.
Lift in sailboats helps us understand surfboard fins. In sailboats, the sail essentially sucks the sailboat forward. And so the boat moves faster. Because the boat moves faster, it experiences more wind speed. The increased wind speed gives the sailboat more lift, and the boat accelerates because the wind is stronger. This process feeds on itself until the boat is sailing much faster than the wind, and toward the wind. If you’ve ever swung an umbrella around on a windy day, you’ve probably felt something similar.
The same thing happens with surfboard fins. Well-designed surfboard fins, fins that produce a lot of lift, will go faster, and that process will feed on itself. This is what we feel as surfboard fin drive.
So what about drag? Well drag is the force that holds you back, or slows you down—a force that acts in the backwards direction—the red arrow on the diagram. Some fins have more drag than others. But the takeaway is that drag reduces a fin’s ability to produce lift—i.e. hold and drive.
Not many fin companies talk much about drag, like it’s not even there. But obviously the less of it you have as compared to lift, the easier it will be to paddle and to accelerate, and the faster you’ll be. On the other hand, some will note that surfers can surf draggy fins and that draggy fins will help stick in the wave, as with classic boards, fins and noseriding. We try in our designs to maximize lift—the green arrow—while minimizing drag—the hold-you-back red arrow, so that surfers can easily paddle, catch waves, and place themselves in the wave where they want to be, as in noseriding where they want to bury the tail.
So how do hold, drive, lift, and drag all come together in surfboard fin and SUP fin design?
It’s important to recognize that there is a speed-and-direction element to producing lift and drag, and thus to shaping surfboard fins and SUP fins. Not all fins are created equally, or are designed for the same uses.
Some surfboard fin and SUP fin shapes are better at straight-line speed than at turning. And some fins might be better at turning than at straight-line speed.
And a given shape might be okay at slower speeds, but won’t work so well at higher speeds. Hydrodynamicists will know this speed issue is an issue of Reynolds number—a discussion for a future post.
In other words, some fins are high-lift and low-drag at some speeds, but not so high-lift and low-drag at others—another issue of Reynolds number for a future post. And some fins create a lot of drag when going straight, while other fins are good for turning, or are low-drag for turning but are draggy when going straight. And some might be better all around.
We can compare a fin’s foil section’s lift-producing ability to its drag. For example, the graph below on the left shows a fat hypothetical fin’s lift curves (Cl) and drag curves (Cd) at several angles of attack (alfas) and at several different speeds (Re for Reynolds number). On the right, the lift and drag are shown as a ratio for several angles of attack and speeds. We can see that the foil section for some angles of attack gets a better lift-to-drag ratio with speed, while at other angles of attack is gaining more drag than lift. Foil selection is very important in fin design.
[divider_flat]So we can decrease a fin’s overall drag, and it’ll give the fin more drive. We can increase the fin’s lift, and give it better hold and drive.
And we can optimize fins in various ways, making them better for turning, for lower speeds, for higher speeds, for larger surfers, and for smaller surfers.
There is more to designing a surfboard fin or SUP fin than just the foil and the template—and there are other design ingredients that’ll be discussed in future posts.
FSI fins are designed to give a lot of lift, a lot of hold and drive, for their drag. The idea behind our fins is this:
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