Contrary to popular belief, the diver's knife is not
used to kill sharks, Barracuda, and Moray eels. They are not held between
the teeth. The knife is a tool, not a weapon. It is an essential part of
an experienced diver's equipment.
In the old days the length of the blade seemed to be
in direct proportion to ego of the diver. It was not uncommon to find knives
that had blades longer than 7" and weighed 2 or 3 pounds.. Probably some
wore cutlasses. Today the knives are smaller. They are usually worn on the
inside of the leg or on the upper arm. The basis for wearing it on the inside
of the leg was the thought the knife could catch the weight belt if it were
ditched. This author did a lot of tests to try and get this to happen and
found it never did. Which leg or arm should it be on? That depends on the
diver's ability to remove it from the sheath using only one hand.
The main use for a knife underwater is to cut line. It
could be fish line that has become entangled in a fin. Trying to break line
such as this with the hands can result in cuts to the hands as well as damage
to wet suit gloves. Some knives have a "rope-cutter." It is a small groove
on the blade that has a very sharp end at the bottom of the groove. Using
that keeps the diver from getting cut from knife as the line is cut.
Surgical shears are becoming popular. They are now sold
with a sheath for underwater use. Some are capable of cutting metal. They
are usually worn on a BC strap. It is not unusual to find divers with either
the knife or the shears, or both.
The blade of most underwater knives are made of stainless
steel. There are a variety of stainless steels. Some are better at avoiding
rust than others. The ones that are the best do not hold a sharp edge as
long. In any case, all steel knives should be thoroughly rinsed with fresh
water and the blades coated with lubricant after each dive.
If you wish to spend about twice the price, a diver's
knife composed of titanium is really worth it. The metal will not rust, holds
a sharp edge for a very long period of time, and is about 1/2 the weight
of stainless steel. The author lost a titanium knife in a lake. It was found
over a year later and still looked new.
It is important to maintain an extremely sharp blade
for cutting purposes. It is dangerous to have a dull knife. It is also important
to obtain a knife that is non-floating. Some of the knives sold in the past
had hollow or cork-like handles that caused them to float. The idea was that
the diver could retrieve the knife from the surface if it were lost rather
than trying to find it on the bottom. That sounds reasonable until one thinks
of a diver becoming entangled, reaching for the knife, losing the grip on
it, and having it float up and away.