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The traditional hardhat diver's knife was a grotesque affair worn on the diver's chest. It was a heavy combination of brass and steel and was important, because if the diver got snagged in his own lifeline, he could hack his way through it.
He could also use it to lever his foot free if it got trapped in the jaws of a giant clam (a regular hazard, if Boy's Own was to be believed) or to prise out the eyes of any giant squid that might attack him (20,000 Leagues Under the Sea).
If he survived the dive, the classic diver's knife was also good for fighting off marauding grizzly bears but was more frequently used for sawing the bread that made up his sandwiches.
The enduring image of Crocodile Dundee in New York explaining to a would-be mugger "That's not a knife... this is a knife!", while slicing open his attacker's clothing with a blade the size of a machete, has done little to help the knife be regarded as an essential piece of equipment rather than a weapon. However, we've come a long way since the days of those hardhat divers' breadknives.
No longer is the first accessory bought by a would-be diver a big knife, closely followed by an equally big watch. Today's knives are more likely to be chosen for their conveniently small size and functionality rather than for impressing any likely attacker.
Diving knives do not even need a sharp point. In fact, in view of recent events surrounding the sudden popularity of fighting knives, such as among Galatasaray football-club supporters, one should be careful to select a knife that is specialised for the job in hand, or you could find yourself answering some difficult questions from the boys in blue, especially if stopped while travelling to a dive site on a match-day.
I had lots of suggestions from helpful divers about how to do a knife comparison. One said I should submerge all the blades in sea water for a week and note which went rusty first.
Another suggested I devise some method of checking for brittleness, to see how much pressure the knives took before they snapped, and a third thought that the ability to cut through thick rope was of paramount importance.
A helpful British distributor of diving equipment even suggested that I should do the honourable thing and fall on one of the knives he sent me!
Alas, as any chef will tell you, you need different knives for different tasks, and the same goes for diver's knives. It's a question of suiting your knife to the task in hand.
Most knives have blades of around 10-13cm long, and a wide choice is available in this size. For convenience, we have grouped the knives by blade length, whether standard, long, small or mini, ideal for stowing in or around the BC. We also separate out cutters, designed solely for dealing with monofilament mesh, and two (virtually identical) multi-function tools.
Almost without exception, each knife is made by a cutler under sub-contract to the manufacturer the brand of which it bears.
Many come from Italy and some from Japan, but the largest number come from the same factory in Taiwan. So you might well see similarities between the products shown here and some other brands also available in the shops.
So should we go into the water tooled up for any eventuality? I think not.
According to modern thinking, we should not go under water to plunder whatever we find, whether it be ripping non-ferrous metal off wrecks, digging up artefacts or simply flattie-bashing for the freezer.
The modern diver is a sensitive person who realises that it is a privilege to experience the underwater world. The modern diver takes only photographs and leaves only bubbles.
That means that the role of the knife is diminished. It is no longer to be considered a weapon or a wrecking tool. The knife is emergency equipment.
So what makes a good knife for modern divers? First, let's lay the myth of stainless steel. This comes in various grades, most manufacturers using 420. However, the more durable a steel is made against rusting, the more brittle it gets and the less easily it will hone to a sharp edge. You didn't think it would be that easy, did you?
So if you choose a knife for its resistance to corrosion, you will be opting for that against its overall ability to cut through anything. Also, the brittleness of stainless steel will make it more liable to snap if you do try to use it as a lever.
Rust-proofing is an illusion. It seems that even with stainless steel you cannot avoid having to clean and grease a sharp knife between dives or see it go rusty, though a superficial layer of rust will not affect a knife's performance.
Even so, rusting and the resistance to it seems to be the prime concern of the majority of knife-buyers, so most knives intended solely for use by divers have blades of this material.
Titanium goes one better. The modern wonder-metal, it is hard and light and is second only to gold and platinum for indestructibility, yet it is as hard as steel - at a price, and that can be considerable.
Because it is unlikely that you are going to need to cut through a big old hemp rope while under water, the big old traditional breadsaw is probably not going to be necessary. So what are you expecting to have to deal with?
Many knives have multi-purpose blades with the capability to slice, saw or cut net. Some have additional features too.
Someone using lifting bags might need to cut a cord in an emergency, which is when a saw blade is effective, but you are more likely to need a knife for use in low-visibility conditions, when it is possible to get entangled in unseen, discarded fishing nets or line. Unfortunately, many good British dive sites constitute wreckage of some sort and these attract fish, which in turn attract fishermen.
Trawlers snag nets and anglers lose lines. Wrecks often end up draped in monofilament of some sort and it is this fishing line which is the most likely hazard a diver faces. It is probably the only valid reason for most divers to carry a knife at all.
Consider the situation in which you might become entangled in an invisible web, and the position in which you stow your knife becomes important. The usual location, strapped on your calf, might look good and certainly suited Ursula Andress as she climbed out of the sea in Dr No, but the knife can be hard to get at, and the act of getting at it can increase the entanglement problem. Knives for cutting net are best stowed conveniently in the BC, or strapped to the arm.
Then again, you might consider that a conventional knife is not the best tool for this job.
Many knives now have a mesh- or net-cutter built in to the blade, but scissors or snips might be more useful.
The modern lightweight mesh-cutter has evolved to deal with the problem. Remember, monofilament line has the same refractive index as water and becomes almost invisible, so you will be best off with a tool designed to gather up the line, in order to cut as much as possible.
A knife's sheath can be of utmost importance, too. The fact that most active diving instructors have a large collection of knives found on the seabed, all sans sheath, bears testimony to the fact that many knife-holders are hopeless at doing their job of keeping it safe until the moment you need it.
The security of the sheath could be even more important than that of the knife itself. Then again, if the knife is so secure that it's impossible to get at it without cutting yourself, that's a minus point.
Many knife handles now have an eye through which you can thread an additional lanyard.
Some fold penknife-style, dispensing with the need for a sheath altogether.
Knives generally have either a pointed end or a chisel end. Few divers really need a blade with a point. Knives can have a multi-function blade with a slicing edge, a sawing edge, and a net-cutter or combinations of any of the three.
Some have a capped handle-end used for hammering, and some can also be used as shears.
Years ago, most diving knives were calibrated so that they could be used for measuring - the length of your buddy's blade, perhaps? Today, only one of the knives in this selection, the Technisub Diablo Tool, is calibrated in this way.
A couple of others float. This is more useful on a boat than under water. If your knife floats away under water it will never be seen again until washed up far away.
How do you keep a knife in good condition? Rinse it in fresh water after use, dry it carefully, and coat it in silicone grease. A few last words about choosing a knife: Don't cut yourself!
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