Military fins
Natural rubber, Hardness: 75o, Black, Apollo Spring Strap

Fits : Amphibious boots and standard Military footwear.
Propeller-fin technology - proven to reduce fatigue and maximize endurance.
Secure stainless steel spring strap eliminate strap, lug or buckle breakage and are quick & easy to don and remove.
Soft comfortable foot pocket with anti-slip high traction sole (easy to walk in).
Negatively buoyant, if the operator is required to discard the fin it will sink to the bottom so as not to alert others of the operator's presence.
To understand the main difference between this and the commercial bio fin, see image below.