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I recently bought some fins... Short, much stiffer than some fins used by divers these are more aimed at surfing.
I can feel they make me faster but am struggling with technique. I feel I'm fighting them and it's very tough on my ankles.
Can anyone offer any advice on how to use them properly?
I am a 25+ year scuba divemaster. Alas, poor fin kicking technique is very common, even among experienced scuba divers.
First of all, selection of a fin which is too flexible is better than one which is too stiff (within limits). If you sometimes use a full 7 mm (1/4 inch) wet suit and other times no wet suit, you likely need two sets of fins. Same for snorkeling/swimming versus scuba. The extra bulk of wetsuit/tanks/weights/BCD is appropriately matched with longer and stiffer fins than when skinny dipping. (I am not sure if it is proper when wearing only fins to call it skinny.)
It is nearly impossible to select the appropriate length and stiffness in a store (at least by a novice). Fins need to be test driven. One lap in a swimming pool is usually enough to determine a good match.
Fins are designed with a variety of parameters. One of the most important is their buoyancy. If thrown into the water, do they sink or float? Sinking is good for kicking technique—especially on the surface (swimming) where density helps keep the fins in the water. If you'll be at depth (scuba), non-buoyant fins are more comfortable and easier to control—unless your swim will have elements of Cirque du Soleil. However, when removing the fins, there is a definite advantage to buoyancy so that $150 fins do not immediately tumble 700+ feet to the bottom between the Galapagos Islands (darn!).
The most common fin technique inefficiency is "bicycling". The knees should not be significantly bending. This causes the fin angle to move a lot without moving through much of the water, so it provides little propulsion but a lot of wear and tear on the feet, ankles, and knees.
Efficient kicking motion comes from the hips with the knees kept stiff. If done correctly, it won't be possible to rapidly kick—and that's okay. This video shows great kicking technique. Though he "dolphins" as for a monofin, but it works well with two fins, too. Notice how he moves the fins through a great distance but hardly flexes the knees.
This video clip shows both frog kick and flutter kick (the latter I derogatorily call bicycling). Frog kick provides nice kicking variety because it uses an entirely different set of muscles. The flutter kick does have its use: the video demonstrates how it can be used to prevent kicking up clouds of silt and minimize coral reef damage when swimming near the bottom.
By using long, patient kick cycles and keeping the ankles relaxed, stress on the ankles should be minimal. If you do feel the ankles getting sore or tired, stop, relax, and then continue with gentle large motions instead of rapid tiny kicks.
Firstly, I must ask, is this for surface swimming (e..g snorkling) or for diving? I myself get so fed up with trying to use fins for surface swimming that I simply refuse to anymore, and I think for that case it's a personal thing. For me it just doesn't propel me much there, and constantly is an issue with breaking the surface and not getting much out of them, and they put heavy strain on my ankles. If I do have to use them, I'll swim sideways so that the fins don't ever break the surface of the water much. This works very well but is slightly annoying. This method seems to correlate with this video which shows swimming with and without fins for the stroke-of-choice for the Navy Seals (allegedly).
For diving, if I'm really trying to move using them I'll keep my feet locked in a 'toes-pointed' orientation and knees a little bent, but really make a full, smooth motion from the hips (allowing the knees to bend and using my quadriceps when needed) to move forward.
Just to clarify, short, stiff fins are often preferred by body-surfers (and I believe body-boarders, although I don't body-board) for several reasons:
Overall, if you have a weak kick and weak ankles, start with soft fins of medium length. If wishing to body surf big waves rather than ocean swim, you will eventually wish to move to short stiff fins, but build up with mid length soft fins first.
It seems like you buy bodysurf/bodyboard fins as @Brent point it out. Lifeguard (like me) enjoy it too, as it allow you to do medium length sprint (50 to 100 meters, like 100 to 300 feet) at record speed, while being easy to carry in your hand all day long.
These are great for strong acceleration, sharp turn, quick dive under waves and take waves to surf it. However, even for a skilled swimmer, it's not ideal for long swim, as it will put severe pressure on your ankles/knees, as they are really not flexible.
To deal with the technique itself, you may use freestyle stroke for surface swimming. If you want to do long swim, rely on your arm to rest your legs, while making long and slow yet poweful kick with your fins, keeping your legs straight (don't use your knees, you will lose power). For underwater swimming, I think you may look at @wallyk answer's.
As a side note, always test your material in a swimming pool, or worst case, in controlled sea/lake environnement with lifeguard, green flag lift.
