The Navy SEAL Physical Fitness Guide 73
Fins and Fin Selection
There are three factors to keep in mind when you are selecting a fin; the specific
design characteristics of the fin, the physical attributes particular to your body, and
operational constraints. Fins, by increasing the surface area of the foot, serve to magnify
the thrust delivered by the legs. Kicking with fins involves a forward stroke and a backward
stroke. Several examples will illustrate how the three factors interact to influence the
SEAL’s fin
selection.
" SDV operations require an operator to sit with fins on for a long period of
time. The operator may require thermal protection and he may have a flexible
ankle. Space is limited, so a shorter fin is necessary. For this situation, the
operator should select a short, wide fin with a strap that can be adjusted for
varying amounts of thermal wear." Surface swimming with gear will involve a sidestroke flutterkick. In the side-
stroke, forward is toward the front of the swimmer's body and backward is
toward the swimmer's back. Applied force is typically much greater during
the forward stroke, and fins are often designed with this fact in mind. When
power is needed for both forward and backward kick, like in sidestroke, a
straight symmetrical fin may be more efficient for you." Underwater swimming involves a face down position, where the forward
stroke is down and the backward stroke is up towards the surface of the water.
In this position, gravity assists with the downstroke and produces a less sym-
metrical type of thrust than seen with sidestroke kick. You should select a
high-tech diving fin with moderate flexibility and an integral type footbox.
The fin should have an offset design to allow a more efficient transfer of pro-
pulsion force." For bodysurfing, use short surf fins.
The main principle of fins is to increase
the surface area of the foot.