There is no better defence against the unknown than preparation, both in education and equipment. For that reason, part of the basic equipment list for any diver should include a knife, or two.

Knives haven't really changed much over the years but today there is a much greater selection of styles, sizes and materials. When I first started diving, the only knives available were big and heavy, some looked like they were better suited to hacking their way through the jungles of Borneo than diving. Today they are, for the most part, much more compact and designed to do what knives were meant to do, cut. And they don't necessarily have to be big to do that. In fact they don't even have to be knives. Many divers now use a variety of razor cutters such as the OMS Line Cutter and some even use a pair of heavy duty shears.

Knife blade length can vary from less than three inches to over seven and, for this reason, knives can be divided into two categories, primary and back-up. Many divers today wear more than one knife, a good idea when confronting a situation where there may be a high risk of entanglement. A primary dive knife should have a blade of at least four inches in length that is sharp on one side for cutting through monofilament and have a serrated edge on the other for sawing through heavier lines or rope. Some blades also incorporate a line cutter notch.

Dive knives, although primarily a safety tool, are rarely used for cutting. They are much more often employed as prybars, screwdrivers, hammers or bottle openers. One of the most common uses is prying and therefore knives with a blunt tip are especially useful. Pointed tips are designed primarily for piercing and more often used by spearfishermen when subduing their catch. Some of the larger knives also have a stainless steel signalling pommel on the butt end of the knife handle which can come in handy. The Oceanic Tiburon also features a graduated cut-out area in the centre of the blade that may look like a wrench, and is used as such by some, but is simply designed to reduce weight.

A dive knife does not need to be big to do the job and knives with a blade length of less than three inches such as the Tusa FK -10 are ideally suited to a backup role. Their only drawback is their small size which may make them difficult to handle while wearing thick gloves.

Blades can be made from a variety of metals but the most popular is stainless steel, either 304 or 420 grade of stainless steel alloy. The higher the number the lower the grade of stainless steel and the better the blade will keep an edge. A lower number means a higher grade of stainless and the more resistant it is to rust. Some of Underwater Kinetics line of Blue Tang and Remora knives are made of 316 grade stainless which offers a good compromise. A few manufacturers, such as Ocean Master, make knives from titanium which is light, very strong, keeps a good edge and is rustproof. The only drawback is that titanium is quite a bit more expensive than stainless steel. In any event, knives should always be rinsed thoroughly in fresh water after every dive and dried separate from the sheath. If the knife is not going to be used for an extended period of time then a thin coat of silicone grease could also be applied. Some knives, such as the Wenoka Sea Lazer are designed so that the blades can be removed for cleaning or replaced when necessary. The Underwater Kinetics knives can also be completely dismantled for cleaning. Some knives such as the small Zeagle Ranger have a one piece design with the handle being an extension of the blade, making for easy cleaning and drying.

It used to be a rule that knives were worn on the inner side of the lower leg but these days divers wear them on the thigh, upper arm, attached to a BC or mounted on the console hose, where they are easily accessible.

An important and integral part of any dive knife, and one which is often overlooked, is the sheath which should hold the knife securely but also release it easily when needed. Some have locking mechanisms that work well but can be difficult to operate with heavy gloves on.

DIVER Magazine would like to thank Aqua Sapiens, Great Pacific and the Diving Locker dive shops for their assistance in preparing this article.