Anatomy of a Dive Knife

In California, a dive knife is an essential piece of dive gear. We California divers share the sea with thick kelp, fishermen, commercial shellfish trappers, and the oil industry. Whether natural or man-made, the threat of entanglement is real. And in diving, entanglement can be dangerous indeed.

Underwater hunting is popular in California, making a dive knife desirable tool. They are useful in finishing off a speared fish, prying loose a scallop, and cleaning game.

Our concept of a dive knife has, however, changed over the years. No longer is it necessary to have a eight-inch blade glistening chrome commando battle axe (although that kind is still available). Today, the dive knife is a finely engineered and machined multi-purpose tool. Understanding what goes into a dive knife, both in construction and features, will help you in choosing a knife that is right for your style of diving.

THE STEEL

Knives are made with different types of steels, depending on the qualities desired. There are three main types of stainless steels used in dive knives. Stainless steel is an alloy - - a combination of iron, carbon, nickel, chromium, and other elements, in varying quantities, to produce a desired result. In addition, the steel can be tempered (heated and cooled) and forged (beaten) to add strength and more qualities. The final combinations are given numbers that specify qualities (304, 420, and 440 being the most common for dive knives).

The main control element in the qualities of specific stainless steel is carbon. The more carbon, the harder the steel. While it is more difficult to put an edge on a high-carbon stainless steel (440), the edge will last longer. More carbon will make the blade harder but too much will make it brittle, prone to breakage when bent. And finally, high carbon in a stainless steel reduces its corrosion resistance. A high-carbon stainless steel will rust.

Conversely, a low-carbon stainless steel (304) will be very resistant to rust (given that enough nickel and chromium are added). It's easy to put an edge on a low-carbon stainless steel; it just won't last long. Pry with a low-carbon stainless and it will bend, ruining the knife.

The 420 stainless steel is a happy medium. You can give it an edge that will hold. With rinsing and light oil, it will never rust. And it is tough enough that you can pry full force with bending or breaking unlikely.

Dive knives made of titanium have arisen on the market recently. Titanium is a space-age metal that is very tough and virtually corrosion-proof. Recent alloys have made it practical for dive knife use. Its only drawback is its expense. Titanium makes excellent dive knives. Just be prepared to pay for it.

BLADE SIZE

How will you be using your knife? Prying scallops requires a good-sized thick blade. For cleaning game or dispatching a speared fish, a medium sized thinner blade with a sharp point is best. If your knife is for emergencies, keep it compact.

Where will you be wearing your knife? The most common option is on the inside of your calf. A knife should not be so big as to prevent you bending your knee or ankle (I've seen it happen with smaller divers). A dive knife, especially one only intended for emergency use, need not have a blade larger than five inches. Small emergency knives (or a back-up knife) can be mounted on arm, BC, or console hose.

BLADE DESIGN AND FEATURES

Point design is determined by intended use. If you will be chipping and prying, such as in scallop hunting, get a knife with a blunt tip. A blunt tip knife is less likely to break and the blade will last longer.

A straight edge is the most common and all knives should have it. Your classic knife edge, razor sharp, is used for cutting line or dressing out game. It should never be used to chop away at a reef as this will ruin the edge quickly. A serrated edge is for cutting heavy line and branches. It is a must for lake diving where trees may pose an entanglement hazard.

Another useful blade feature is the line cutter. This is a sharp notch cut into the blade that makes cutting small rope, fish line, even small wires, quick and easy.

HANDLE

The knife handle should be made of non-slip rubber with plenty of friction points. It should be comfortable in the hand, conforming to the user's grip. The "hilt" is a barrier between the handle and blade, keeping the diver's hand from slipping onto the sharp metal. A metal butt is useful as a hammer or in rapping a tank to signal a buddy, but this design has fallen from favor in recent years, most likely to accommodate better handle design.

SHEATH

The sheath is an often overlooked but important portion of your knife tool. The best knife, after all, is of no use if it is missing. The sheath should have a locking mechanism to solidly hold the knife in place. A solid "click" when the knife is locked is handy. The best locking devices can be released with one hand, at the touch of a button. Straps should be of a stretch rubber to hold firm and compensate for wetsuit compression at depth. Quick fasten/release and one-tug tightening is handy and convenient.