THE UNDERWATER KNIFE
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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.
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