Anatomy of a Fin

It's all about propulsion. The ability to move underwater and move fast, without pain and fatigue. You could move underwater without fins but not very well. Fins are simply an extension of the feet. They are a bigger lever to move more water.

The first fins were not much more than just that - - extensions of the feet. And they worked surprisingly well. With further fin evolution, there became three basic choices: jets, rockets, and ducks. Some of these choices are still available today. Now there are dozens, if not hundreds of choices. Most work quite well. With modern material and computer aided design, we are now able to get more distance with less energy than ever before. More importantly, you can find a fin that most closely matches your body and dive style.

The large number of designs and choices benefit the consumers, increasing their ability to find a fin that best fits their individual diving style. But amid the choices can be confusion. The purpose of this article is to discuss some of the basic features, their functions, what is important, what is not, and help you decide for yourself what fin features are most important to you.

There are three main sections to a fin: the blade, foot pocket, and strap with buckles. All are equally important, and it is just as important that they all work together.

BLADE

This is the extension of the foot. Arguments are intense on just exactly how it works, or should work, but a few items are generally agreed upon. First, the down stroke is the power stroke. It is the downward motion of the foot, pushing water backward as it flexes or "loads" the blade. The blade curls, then as it uncurls or snaps, water is further pushed backward, propelling the diver forward. The same thing can happen on the upstroke, but is usually very inefficient. It's generally best to make the upstroke the "rest stroke."

Blade stiffness varies considerably and is an important consideration in your personal choice. Powerful swimmers can flex a stiff blade, whereas divers with weaker legs will need a fin that flexes more easily. More important than a blade's stiffness is its snap. How quickly does it return to its original position? Bend the blade of a fin dramatically, over 90°. It should snap back into position quickly, and with force.

Blade length is also critical. The longer the blade, the more it can be "loaded" with energy. Expert freedivers have enjoyed long blades with only moderate stiffness. They can be very energy efficient if used properly.

It sounds as if a long, stiff blade would be best but don't be deceived. You have to have strong enough legs to use fins of this type.

Many fin blades are now designed to "cup" on the power stroke to assist in directing the water backward in an even more effective manner. This is usually accomplished by using a thinner, more flexible material in the center and stiffners on the outer edges. It can also be accomplished with a notch or "V" shape in the blade end. As the blade is thrust down, it partially collapses in the center "cupping" the fin, thrusting water in a straight channel backward. This also prevents the fin from slipping sideways, which is not only inefficient, it's hard on the ankle.

What about vents? Years ago, vents or slots between the main part of the blade and foot pocket where all the rage. Many fins still have them and users swear by them. Proponents state that this is a "dead" part of the fin that creates no useful power. Regardless of whether you agree, many still prefer this type fin.

FOOT POCKET

A properly designed and fitted foot pocket is key to the comfort of a fin. Try on a fin with the bootie you'll be using while diving. Foot pocket fit should be slightly loose. You should be able to get in at least one finger between your foot and the fin. A tight pocket will cause the foot to cramp and may be very painful. There should also be plenty of room for your toes to move. The force you apply to the fin will come through your ankle and top of the foot, not the toes.

The bottom of the pocket should extend all the way to the heel. If it only reaches the arch, your foot will cramp. The edges of the pocket should be soft rubber. Hard, sharp edges can chafe the foot, even through booties. Hard edges also wear out booties faster.

STRAP AND BUCKLE

Without the strap, the fin would fall off. That makes this an important part of the fin. Straps should be wide enough in order to not cut into the heel. With a fin that fits well, however, there should be no reason to over-tighten the strap. A pull tab on the back of the strap is helpful in putting the fins on. Straps that tighten with an easy tug of the end, and loosen with the push of a button are very convenient. Another useful feature are buckles that have a quick-release clip. A useful feature lacking in most fins is a way of securing the loose strap ends. For California divers, this is an important feature to prevent snags on strands of kelp. It is a feature, however, that can be easily added with tape, rubber bands or velcro strap.

Picking a fin that is just right for you is not difficult. Your professional dive store is a good place to start and finish. With their experienced help, you'll be moving underwater faster than ever.