The Navy SEAL Physical Fitness Guide 77
Cold water is an issue to be addressed by SEALs training in open water. Although
the work of swimming generates heat, there is heat loss created by movement of the
swimmer into new “unheated” cold water. Thus, open water swimming may require
various combinations of passive thermal protective gear, in particular, wet suits. The three
determinants for passive thermal protection are:
" Temperature
" Length of the swim
" Effort level
It is important to remember that wet suits operate by allowing the body heat to be
transferred to a layer of water caught between the body and the neoprene material of the
suit. Convective heat loss from the swimmer's body is greatly reduced by this mechanism
and as a result, swimming at a high effort while wearing a wet suit allows the swimmer to
generate and retain heat. Guidelines have been developed for training and are presented in
this section.
Research has established some known “bench mark” facts about operating in a cold
water environment. In very cold water (below 40° F), the unprotected swimmer loses heat
faster than an immobile person immersed in the same cold water. Heat generation simply
does not keep up with losses. The immobile person warms the cold water immediately
around his body thereby limiting total heat loss. However, in moderately cold water
(around 68° F), an elite class swimmer may stay active, and the heat generated by
swimming keeps pace with overall losses (although the swimmer may develop cold feet
and hands). In this situation, the active swimmer outperforms the immobile person with
respect to maintenance of core body temperature. Currently we do not know the crossover
temperature point or the water temperature at which it is better to remain stationary than
active in the water for thermal balance.
Passive thermal protection modifies this balance by reducing the convective
component of heat loss. As a result, a swimmer may extend training durations beyond those
possible without passive thermal protection. Diving medicine specialists at BUDS have