The Essential Function of Fins

Let’s assume you’re buying a new single for yourself. You choose the size of the boat according to your weight. Of course, you pick a nice color, and also give your shoe size. Maybe you even select a certain seat top. But did you ever think about the fin size or its placement? Most of the time, you don’t have a choice, since boatbuilders usually offer only one type of fin, and the fin box is in a fixed position. What if you want to change things?

Before you do, think about the functions of the fin. Its main purpose becomes obvious as soon as it’s lost by hitting a submerged lo: The boat no longer goes straight. Since every boat rolls a little during the stroke, the submerged boat shape is, for parts of the stroke, no longer symmetrical, which tends to turn the boat. Once the boat is set to spin, it will continue to do so increasingly. 

The fin keeps the boat going in a set direction, absent any side forces. Small roll movements have no effect on the course of a boat with a fin. The boat tracks straight after completing a steering intervention, and the fin reduces the impact of side winds. 

The second function of the fin is to stabilize the boat; a deep fin helps balance it. 

All these functions are dependent on the fin’s area, height (vertical distance from hull to tip of fin), and placement along the boat’s longitudinal axis. The height and area of the fin determine its ability to keep the boat straight and influence its stability. Where the fin is placed affects its ability to turn the boat; the closer the fin to the stern, the harder to steer.

So what’s the optimal fin design and where best to put it? It depends on various factors—current, side wind, whether a course is straight or twisty.

Is a high fin best? Not necessarily. High fins tend to “sing” or vibrate from the turbulence at their tail edge, which generates a whistling sound that gets amplified by the boat’s hollow canvas. Not only is it annoying but also it slows the boat. To avoid vibrating, a high fin needs to be longer and thicker, which increases drag. 

If you want the boat mainly to go straight and be balanced, and more drag is acceptable, then a higher fin with considerable area is desirable. Ditto on a straight racecourse with loads of side wind. On a head-race course such as the Head of the Charles, however, as well as when training on a winding river, a smaller fin is best. 

 A fin needs to be able to shed weeds, which means its front edge should be rounded and smooth. It also has to be angled toward the stern. Caveat: A more triangular shape means increasing the height of the fin will also increase drag. Reason: the fin’s longer base.

The ideal? If your new boat came with several different fins that could be swapped out easily depending on conditions. There could be a “training fin” for occasions when drag is not an issue.  A “straight-race fin” that’s high and hydrodynamically maximized.  A smaller “head-race fin” that sheds weeds well. A larger “side-wind fin,” and so on. The fins would be simple to attach and remove and would break off in an accident without damaging the hull.

Next wish:  A fin box that would enable you to move the fin longitudinally toward the bow or stern. Better yet would be a mechanism that would allow you to move the fin horizontally from within the boat, so that you could position the fin optimally while sitting at the start and observing actual wind conditions. 

Since we’re dreaming, why couldn’t we move the fin while rowing, pulling it toward the bow as a turn approaches, pushing it toward the stern for a straightaway. Of course, this interchangeable fin-positioning box would not generate additional drag or add extra weight to the boat. 

 A big ask for boatbuilders, I realize, and of course it would increase the cost dramatically. So how about two fin boxes at different locations? You could choose the most desirable fin position for a given situation, and the unused fin would close nicely so that it wouldn’t drag.