Stacy:
So, Doug—Fin Sciences’ claim to fame is using science to design fins that improve surfing performance, right?

Doug:
Yep.

Stacy:
Isn’t that something you can’t really prove? I mean, if I’m looking at buying a car, I compare performance numbers like zero-to-60 times, stopping distance, miles per gallon, slalom times, or the number of Gs a car can pull on a skid pad. I like to know what I’m getting.

Doug:
Yeah, in surfing nobody really talks about surfboard fin performance numbers. They’re left to their own subjective impressions or just to the buzz on the beach.

Stacy:
So you have numbers that can show that FSI fins produce less drag than dolphin fins?

Doug:
Yeah.  I figure surfers might want to compare FSI fins to their favorite fins—with some hard numbers. The numbers show that our fins have better side force and less drag than typical dolphin fins out there.

Stacy:
I’m probably going to regret this, but can you explain that “better side force” in a little more detail?

Doug:
Sure, thought you’d never ask!  In general, “side force” is good and we want more of it—a bigger number is better.  Drag is not so good—we want a smaller number.

Stacy:
So side force is like power?

Doug:
Pretty much. Side force is really lift in terms of science.  More side force or lift means more power in turns–more hold and more drive.  What surfers feel when surfing is the ability to plant your foot and make a turn, or hold yourself in the wave without spinning out, or rocket out of bottom turns and cutbacks.  More powerful fins—fins with more lift—will do these things better. I wrote about hold and drive in another post (https://finsciences.com/surfboard-fin-lift-drag-hold-drive/).

Stacy:
And drag is like hitting the brakes?

Doug:
Right. You want less drag.  Less drag means easier paddling, better acceleration, better wave-catching, and higher speeds so you can reach other wave sections.

Stacy:
So we want as much side force, or lift, and as little drag as we can get?

Doug:
Exactly!  Feel like you’re in high school science class now? I calculated the side force and drag of our fins in pounds. Take our small shortboard fin surfing on a 3-meter wave—at about 20 knots. That fin will generate 19.23 pounds of side force—57.69 pounds of side force for a thruster setup with three fins. Typical dolphin fins are less efficient. A typical 4.25-inch fin on that same wave generates 16.63 pounds of side force—49.89 for three. Big difference.

Stacy:
So what about the drag on those same two fins?

Doug:
Okay, on that same wave our fin generates drag of 1.23 pounds—3.69 pounds for three. The dolphin fin generates 1.33 pounds of drag per fin—3.99 pounds of drag.

Stacy:
I bet you have a table that illustrates all this stuff.

Doug:
How did you guess? I love tables. (See below.)

FSI Small FSI Small FSI Small 4.25″ Dolphin fin 4.25″ Dolphin fin 4.25″ Dolphin fin
Speed (knots) 10 20 30 10 20 30
[highlight]Total side force (lift, in lbs.)[/highlight] [highlight]4.81[/highlight] [highlight]19.23[/highlight] [highlight]43.28[/highlight] 4.16 16.63 37.42
[highlight]Total drag (lbs.)[/highlight] [highlight]0.352[/highlight] [highlight]1.23[/highlight] [highlight]2.61[/highlight] 0.37 1.33 2.88
[highlight]Lift/drag ratio[/highlight] [highlight]13.67[/highlight] [highlight]15.7[/highlight] [highlight]16.61[/highlight] 11.24 12.52 12.98
Power Usage (hp) 0.0324 0.113 0.24 0.034 0.122 0.265
“Resistance” 24.16 84.26 178.97 25.35 90.98 197.61

Stacy:
Okay.   Fascinating conversation.

Doug:

In an upcoming post, we can more about drive, how it varies with speed, and with turning, and we’ll talk about SUP fin design and development.