FAQs about FSI and Wavegrinder Fins – FSI

This page answers some FAQs about FSI and Wavegrinder fins. If you have other questions, feel free to contact us.

Can I replace all my FCS fins on my 5-fin surfboard (as a thruster or quad) with FSI fins?  And if so, would I need different fins for either setup?

Yes, you can swap out a set of three or four thrusters and put in three or four FSI fins.  We have smalls, and larges, symmetric (double foil) fins.  We recommend starting small, and here’s why.

Our smalls produce more side force with less drag than the typical 4.25″ fins that they replace.

We measured the area of a thruster setup of three FSI small fins and compared it to the area of three typical dolphin fins that are the same size. Then we calculated the side force—or hold and drive—of our fin using fluid dynamics software. Our fins, in a head-high wave, produce 56.7 pounds of hold and drive and the dolphin fin setup only produces 49.9 pounds of hold and drive. You want lots of side force so you can plant a foot and make a turn. Not only that, the FSI fins produce .3 pounds less drag at the same time! Less drag means faster, more responsive surfing.

Side force is what we call hold.  Of course those numbers change with speed—they get more power with more speed—that increase of power with speed is what we call drive.  (More about lift, drag, drive and hold here:  https://finsciences.com/surfboard-fin-lift-drag-hold-drive/).

In the meantime, our fins produce less drag, so they are faster and accelerate more quickly. Here’s more about measuring fin performance by the numbers:  https://finsciences.com/why-not-use-numbers-to-measure-surfboard-fin-performance/.

Our fins already amp up the force, yet some surfers prefer even more.  Heavier surfers or surfers whose boards need more oomph to turn sometimes go with two smalls up front and a single large trailing fin for a thruster setup.  And for quads, some folks ride with two larges in the front, and two smalls in the rear.  More about this here:  https://finsciences.com/right-size-fsi-fins/.

Do I need sitebites with the longboard fin?  Will the Wavegrinder or WG2 work with sidebites?

Yes, the Wavegrinder (the most recent version is our new WG2) longboard-SUP fin will work either as a single fin or as part of a 2 + 1 fin setup.  But really this is a question of how much fin you need, in total, and your preference for speed versus turning. How much turning force do you need or want? How much resistance to turning, or how much tracking ability do you need?

As a general concept, smaller fins will be faster because they have less area, and thus less-skin friction drag. Think sports car versus SUV. On the other hand, larger fins will produce more power–more side-force/lift/hold for more turning ability.

We advocate a reductionist philosophy—less is more—and recommend the smallest surfboard-fin surface area that gives you the turning and tracking ability that you want.

So what is the right size? Fins with winglets, like airplane wings with winglets, behave as though they are about 20-30 percent larger than they are in terms of producing lift (hold and drive), while having less drag because of their smaller area. So if you have a typical 2 + 1 fin setup, a typical center fin of, say, 46 square inches, plus two sidebites of 14 square inches each, your 2 + 1 fin setup has a total fin area of about 74 square inches. You can pop out the center fin of 46 sq. inches, pop in the WG2 of about 30.54 sq. inches, and you have dramatically reduced your total fin area–a 21 percent reduction. If you’re like other surfers, you’ll feel a significant improvement.

But why stop there? We recommend that you try surfing with the WG2 as a single fin–it produces a lot of side force by itself, more than adequate for most surfers’ turning and maneuvering needs and, by removing the sidebites, you reduced your total fin area by 59 percent. Try the sidebites in, ride a few waves, pop them out, see what you think. Lots of surfers find that removing the toed-in draggy sidebites and using only the WG2 really improves performance.

Sizing of FSI fins compared to typical fins

Our fins have a lot of power. They generate a lot of hold and drive for their small size. [highlight]Winglets and other features we use make our fins about 20 to 30 percent more efficient than larger, older styles,[/highlight] so you need to downsize by about 20 to 30 percent as compared to the fin or set of fins you typically use.

FSI fins produce more lift than their size suggests—this is the key concept of FSI fins.  The smaller area still gets good hold–and the smaller area means less drag, more speed and acceleration, easier paddling, and longer surf sessions.

Example: If you typically surf a set of mediums in a thruster setup, fins that are usually about 14.5 sq. inches each, you are surfing 43.5 sq. inches of fin. Try our smalls. Our new small is 10.14 sq. inches, so a set of three is 30.42 square inches, a 30 percent reduction in fin surface area from your usual medium thrusters. That reduction in surface area means you’ve got a 30 percent reduction in skin-friction drag.

Another example: If your longboard or SUP board has a center fin and two sidebites, calculate the area of those fins, then try downsizing–try pulling off the sidebites and going with a single Wavegrinder or WG2 longboard-SUP fin. You will dramatically decrease the fin area, yet still get good hold and drive because of the winglets and other features of FSI fins.

FSI Fins, Areas in SetsBecause FSI’s current fins are symmetric, you can either get a set of the same fins, or mix smaller and larger fins to get the ride you want.
Fin type FSI size Area, each Area, set of 2 Area, set of 3 Area, set of 4
Shortboard fin Small 10.14 sq. in. 20.28 sq. in. 30.42 sq. in. 40.56 sq. in.
Shortboard fin Large 14.58 sq. in. 29.16 sq. in. 43.74 sq. in. 58.32 sq. in.
Longboard-SUP fin Wavegrinder Classic (discontinued) 30.54 sq. in. 61.08 sq. in.
Longboard-SUP fin WG2 (new!) 30.60 sq. in. 61.20 sq. in.


 Where in the fin box should I put the longboard-SUP WG2 fin?

About 2 inches back from where you locate a typical dolphin fin (see diagram).

Because the FSI longboard/SUP fin is an upright (or low-sweepback or low-raked) fin, we recommend putting it back in the box about two inches from where you place a typical dolphin-shaped fin.

Placement of the WG2 within the fin box.

Start using your WG2 longboard/SUP fin by lining up the center of area at the same place as your old fin. This means that you put it about 2 inches back from a typical dolphin-fin placement.


Can the WG2 longboard/SUP fins carve wide radius turns?  Is the WG2 just a pivot fin for pivot turns?

The WG2 fin is an upright fin.  But thinking of it as a pivot fin that can’t go straight isn’t right.

First, fins generally have forward velocity, they aren’t just rotating by themselves. Second, in terms of hydrodynamics, surfboards don’t pivot around the vertical axis of the fin, instead, pivoting or rotation occurs around the Center of Lateral Resistance (CLR), which is center of the sum of the areas of the board-and-fin system immersed in the water.  Fins, of course, have a lot of lateral area and are a big factor in where the pivot point is.  We can imagine pivoting on a board with no fins at all–the board pivots around the CLR even without fins.  If we add a fin, or a few fins, the CLR shifts toward the fins.

Yes, a longer fore-and-aft fin will have more resistance to pivoting, by itself, but fins are attached to boards and are part of a system.  Being an upright fin doesn’t mean that the WG2 can’t do long turns or go straight.  Just because a Porsche 911 can turn on a dime and really hold the road on a switchback and a Buick can’t, doesn’t mean the Porsche can’t go straight on the freeway also.  [/toggle]

What difference does the WG2 FSI fin make?

The key concept behind FSI fins is their efficiency.  Because they are so efficient, they can be smaller in area, and because they are smaller, they have less drag, which means more speed and power–more hold and drive.

Most surfers notice several things.

First, they tell us they do feel a difference—in the past, many surfers bought other fins claiming this or that, only to find that those fins didn’t live up to the claims.  We’re not selling snake oil here—these really work.

Second, many surfers report feeling as though they are higher and lighter in the wave, especially when dropping in.  This is because they don’t have the tail-sink associated with a draggy fin or fin set; and they’re faster.

Third, many surfers report surprise at the ability of the fin to turn, not only in the wave, but also in the flat or in mush—summer California waves, and some East-Coast waves. This is due to the low-drag features of the fin and the foil shape, designed to maintain lift over a wide turning angle.

Fourth, surfers have described this as a feeling of “pop” coming out of turns. This is a product of the high-lift, low-drag design features–and a result of the material flex—the WG2 semi-flex has a moderate flex characteristic. The WG2 stiff fin does not.

Fifth, surfers report easier paddling, both on surfboards and SUPs, as well as excellent tracking ability on SUPs.

All these reports are consistent with our design goals—increasing lift per unit of drag—and make complete sense. See testimonials.

The FSI WG2 and Wavegrinder Classic fins look different.  Do they work?

Yes, our fins look different. And they work. We started with a blank slate; we didn’t just refashion dolphin fins that have been around for 50 years.  Science proves that winglets work, they make wings—and fins—produce 20-30 percent more lift without increasing drag.  The secret is not just adding winglets, but is FIRST making a major reduction in fin size, THEN adding back tiny winglets for an overall reduction in area.  A while back, we’d get on airplanes and wonder about the winglets on the end of planes’ wings. But now they’re pretty normal to see. Because they work.

Surfers are surprisingly resistant to change—they seem to like retro gear, and figure if it wasn’t adopted by the sport’s grandaddies or film legends of yesteryear, it must not work. This reminds me of a quote from Charles Duell, Commissioner of the U.S. Patent Office in 1899: “Everything that can be invented has been invented.”

Just because FSI fins are new and different doesn’t mean they’re bad or don’t work. Several prominent surfers experimented with winged fins over the years, but without the benefit of fluids dynamics programs or CAD/CAM.  So some designers added large wings to existing fins, which proved to not work well.  Now we know the secret is not adding area, but reducing area, and making the area that remains most efficient. This is the key concept behind our fins. [/toggle]

Does the WG2 or Wavegrinder fin help noseriding?

Yes, but only because the fin allows surfers to maneuver to the best spot in the wave.  The teeny-tiny winglets provide little tail-hold-down force—that’s not their intent—they’re about the size of a postage stamp.

Many surfers report excellent noseriding with the WG2 fin.

We attribute that noseriding capability to the surfers’ ability to better maneuver their boards to the right place in the wave.

Are FSI fins hydrofoil fins?

No.  The fins’ winglets are not designed to lift a surfer out of the water—they don’t.  For that, there would need to be a much larger horizontal area. The winglets are about the size of a postage stamp. The purpose is to keep laminar flow across the tip of the fin, making the fin tip more efficient.