HOW TO EVALUATE FINS
The following 5 attributes of a fin provide insights into its performance and should help you narrow down on fins that work best for you:
1) Thrust: This is a measure of how much propulsion a fin provides with a single kick, and depends on the length of the fin, its stiffness as well as the overall design.
2) Beat Rate: This is a measure of how frequently you have to kick in order to get the optimal propulsion. A direct analogy would be running, where your speed depends on your stride length and turnover or how many steps per minute you take. Beat rate is the equivalent of steps per minute here, with thrust being the equivalent of stride length.
3) Stiffness: This is a measure of how much force you have to (or can) exert per kick for optimal propulsion. In general, greater stiffness typically results in greater thrust, but manufacturers are always trying to find clever designs to improve the thrust:stiffness ratio.
4) Buoyancy: This tells you whether the fin floats or sinks in the water – which can affect your trim. These days, most fins tend to be more or less neutrally buoyant, although a few notable exceptions do exist.
5) Bite: This is a term i have coined to describe how well you “feel” the water when you kick – your proprioception, in other words. To use an analogy – when you do the front crawl, you learn to develop a feel for “holding the water” in your hand when doing the pull part of the stroke. Similarly, you have a better feel of the water with some fins than with others. That is bite. Why does this matter? This is essential when you are trying to make small precise movements in limited space – eg, inside a shipwreck or while engaging in underwater photography.
So what does all this mean?
Thrust and beat rate together give you a measure of the propulsion provided by a pair of fins. You can get the same propulsion by using a high-thrust fins kicked at a low rate (the equivalent of mashing a big gear on a bicycle) or by using low-thrust fins kicked at a high rate (high cadence spinning). The former is easier on your lungs but harder on your legs – the latter will increase your HR to some degree, but is easier on your legs.
Stiffness tells you how much effort is required to get that propulsion. Actually, to some degree, stiffness and beat rate are linked – stiff fins tend to lend themselves to lower beat rates, whereas softer fins tend to lend themselves to higher beat rates. But I feel it is worthwhile enough to keep stiffness as a separate category because it doesn’t just affect propulsion but also leg comfort. Also, it is possible to “outkick” your fin if you exert a larger effort than the fin’s stiffness allows it to handle – in such cases, it is better to increase the beat rate rather than effort per kick.
The last 2 characteristics aren’t about propulsion but about control and balance in the water. Buoyancy of fins tells you how it will impact your trim in the water, as explained earlier in this article. There is no right or wrong attribute here – a lot depends on your trim characteristics (defined by your body and your gear).
And lastly, bite gives you an indication of how much precise control you have in the water with the fins. I created this term while trying to understand why I liked some fins more than others even though both of them were equally effective in the water. The words that came into my mind were “mushy” vs “precise” – and it is a significant factor in determining how good a pair of fins feels while diving.
Hopefully, this framework gives you the ability to evaluate fins by yourself. As a short-cut, we have a recommended list of fins to cover the needs of more recreational divers as well as divers with special requirements. You can always contact us for more info, to try them in a pool or to narrow down a list of choices.