Sport Science: Fin Tuning

Fins on our boats make taking a boat on and off the racks more difficult, require special attention so they don’t hit the dock when we launch, and, last but not least, cause resistance in the water. So it’s no wonder we want to get rid of them—until we lose them because we hit a log we didn’t see. Then we realize how well they serve us.

I remember a situation like this when we were in an eight a few years ago. The crew had done well in previous regattas, and we were preparing for the big race at the end of the summer sprint season. Because our experienced cox couldn’t make practice, one of the lightweights offered to step in and help.

All went well until the boat began swaying side to side suddenly and making long, violent turns, throwing off our rhythm, balance, and, of course, speed. I could see the coxswain’s desperate attempts to steer as I stroked the boat; she was moving erratically and pulling on the rudder lines frantically. The poor volunteer coxswain lost her confidence quickly, and so we cut the practice short, went back to the boathouse, hauled the boat out of the water, and discovered…our fin was gone!

Even the best coxswain in the world can’t keep a boat straight without a fin, and so our replacement coxswain was redeemed.

We need fins to steer our boats, obviously, but why not make them smaller? The advantages: less risk of bumping the fin in the racks or on the docks and less drag, which would make our boats faster.

It’s not that simple, however. If a fin is too small, the boat will not track well, and crosswinds will disturb its directional stability. Finless rowing requires more steering effort, which increases drag and decreases speed. The fin also helps balance and stabilize the boat; the larger the fin, the more stable the boat.

Because a smaller fin doesn’t always make a boat faster, what’s called for is compromise, tailored to the situation for which the fin is needed. A straight sprint race requires a larger fin than a winding head race. A less experienced rower would benefit from a larger fin. A larger fin is useful also when racing on a sprint course known for its strong crosswinds. For all these reasons, it would be nice to have a boat design that allows fins to be changed easily.

The efficacy of a fin depends on two factors: depth and surface. Since surface area is the most important drag factor, boat designers like to make fins deeper, as reflected in all modern fin shapes. Hydrodynamically shaped fins also increase the function of a fin, but here, too, compromise is necessary. A thicker fin increases its efficiency, but also its resistance. In addition, such shapes can be achieved only with special plastics, which are more expensive than aluminum.

For boats used to train beginners, the choice of fin hardly matters, but for highly competitive crews, the right fin can make all the difference.

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