Diving without some sort of cutting implement is like eating fish and chips without the salt and vinegar. You can do without. But why try?
An equipment item that usually receives low priority both in its selection and its maintenance priority, a good diving knife is not a weapon but an essential tool and, in an emergency, a lifesaver. Rugged, strong, and tough enough to ensure that the blade won’t snap off when it’s being used as a crowbar, the perfect diving knife is a hammer, a screwdriver, an axe, a shovel and a saw. On occasions it’s even used as a cutting implement.
Despite a growing trend away from large blades towards smaller knives, or shears that can easily cope with monofilament line, the choice of knife should still be determined by the diving environment and the purpose for which it will be used.
In that regard diving knives intended for specialised purposes may be constructed from a variety of metals and materials. Non-magnetic beryllium knives, for example, are often used by military divers engaged in E.O.D work while, in the recreational market, it’s becoming more commonplace to see knives constructed of titanium and ceramics. Nevertheless, the metal used most often in the manufacture of diving knives remains stainless steel.
Steel is an alloy of iron, carbon, and other metals; the proportions determining the metal’s characteristics. The term, ‘stainless steel’, is a generic name given to those corrosion resistant alloys containing at least 10.5% of chromium. Generally speaking the more chromium a particular type of steel contains the more resistant it is to rust. (At this point it’s worth emphasising that stainless steels are not rust proof. They are, rather, rust resistant. For this reason there has been a tendency to produce dive knives that have excellent properties in terms of their ability to resist corrosion – but that often fail to hold a good cutting edge.)
As an underwater tool, the diving knife usually differs from those intended for above-surface use. Good knife-steel, with high carbon content, is often ill suited to the highly corrosive conditions encountered during diving. Although this type of steel can be ground to a sharper cutting edge, it will soon lose that quality after just a few dives, despite the protection of grease or anti-corrosive compounds. A strong, stainless steel blade, on the other hand, will usually retain a degree of sharpness without any special care or maintenance. The emphasis is on the word ‘special’. Stainless steel comes in a variety of grades that all require proper care if corrosion is to be prevented.
There is a common misconception that all that is required of a good diving knife is a sharp blade. There are other qualities to consider.
The handle, which may be made of metal, (as in the one-piece skeleton blades), plastic compounds, or thermo-rubber, should feature a comfortable and secure grip even when gloves or mittens are being worn. It should either be solid enough to use as a hammer or incorporate a solid metal butt at its end. A cross hilt, sometimes merged into the handle’s design, should separate handle and blade to protect the user from accidental injury.
The tang – the extension of the knife blade that secures it to the handle -should be wide and, for strength, continue the entire length of the grip. Because it usually remains hidden, the tang is frequently subject to corrosion and a number of knives now feature removable handles secured by a stainless steel crown nut that allows easy access to the entire blade unit. Like the blade itself, the tang will benefit from regular oiling and cleaning.
Blade shape and design are all important. Because the diving knife is intended as a tool – not a weapon – a needle sharp point is usually unnecessary. A rounded point being more than adequate and offering less risk of accidental puncture of flesh, BCD or dry-suit material.
The primary reason a diver carries a knife is to cut cordage either in the execution of their work or to clear themselves if fouled. A good cutting knife should be thin bladed, wedge-shaped to give weight to the cutting edge and, of course, razor sharp. Ideally one blade should feature a saw edge.
It is a knife’s length and weight that helps to determine its effectiveness. One of the things that lends a knife power is the mechanical advantage of leverage furnished by a longer blade. Small knives lack the purchase to make really heavy cuts.
A large blade, say 18+ centimetres, can make heavy slicing and chopping cuts and small, light cuts. A small knife, with a 10 cm blade, will not make the cuts that a larger knife will.
This is because there are different types of blade edges, with some types better suited for specific jobs than others. While the edge of a razor and a dive knife may appear the same they require different types of sharpness.
The choice of edge is determined by the sort of cutting you do. A scalloped edge is absolute dynamite for some cutting operations, likewise the saw edge, but conventional sharpening stones and files are useless in maintaining these types of edge.
Stainless-steel diving knives are best sharpened on a rough stone only. The rough stone leaves a microscopic edge that displaces the fine fibres during the cutting stroke, preventing a friction bind on the blade when cutting through thick rope and cordage. (Heavy rope is best cut by rolling the rope, cutting most of the way through one strand at a time and making the final cut through the centre.)
Different sharpening mediums will produce different results and determine how long a blade will retain its edge. In the main, with a stainless-steel diving knife, all that is required to maintain an edge is the periodic use of a coarse file or stone wielded at an angle between 17.5 and 25-degrees to the blade.
Following immersion in seawater the knife should be removed from the scabbard and allowed to soak for a while in fresh water. Any corrosive blemishes that appear on the blade should be gently and delicately removed using dampened fine emery or glass paper followed by a light spray with an anti-corrosive such as WD40.
With proper care a good diving knife will last forever; a trusted tool when the diver needs one most.