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Gearmiester answers your questions about KNIVES...

The Cutting Edge

 

One of the things that every prospective diver learns in basic open water class is “always carry a cutting instrument.” This cutting instrument is usually a knife - a tool for emergencies, not a weapon. (Thinking about how dense water is compared to air, you wouldn’t be able to swing a knife very hard any way…)

Why does every diver need a cutting tool? To remedy or prevent entanglement in fishing line, old fishing nets, or other line. A general purpose knife works well for this. Knives can be used for many other things – prying, spearfishing and fish filleting – but a more specialized blade is needed. For instance, a strong blade with a flat tip for prying, a thin blade with a very sharp point for spearfishing, and a sharp, thin, flexible blade for filleting.

What’s that Thingy?

First, we need to understand some of the basic terminology used to describe a knife. Then, we can talk about what to look for in a dive knife.

Let’s start with the business end – the blade. The blade is what you cut with.

The “belly” of the blade refers to the amount of curve on the blade. A curved blade cuts more easily on typical tasks than a completely straight one.

The type of  blade edge is determined by the things you want to cut. Most people are familiar with the smooth edge seen on kitchen knives. This edge is moderately useful for typical cutting needs in the water. Paradoxically, the blade will cut even better if it’s a bit rough rather than completely razor smooth.

A serrated edge is absolutely necessary on a dive knife. This portion of the blade is composed of small teeth that make sawing through line much easier than with a smooth blade. The serrated area also needs to be long enough to get a good sawing motion going. Serrations that are more “scalloped” shaped than sharp points also work better as they don’t snag the line fibers as easily. And looking ahead to sharpening the knife some day, it’s quite a bit easier if the serrations only come from one side of the blade. The ideal location would include an area towards the end of the blade so it could be used in tight spaces such as cutting a line off the propeller shaft.

The next most useful part of a dive knife blade is a line cutter, ideal for cutting monofilament fishing line. This hook-shaped area is sharpened on the inside of the hook so the diver only needs to slide the knife over the line until it drops into the line cutter and voila! It’s cut. Ideally, the line cutter is located towards the end of the blade so it’s easier to catch the line with, but, you can only have so many things on the end of the blade, so you have to make a choice. And in a pinch, a good serrated edge will easily cut monofilament as well.

On the opposite side of the sharp edge is the spine. This is usually wider than the blade and not sharpened on a dive knife. This adds strength to the blade to prevent flexing.

Moving back, we come to the handle. The handle needs to be big enough to get a firm grip on it in order to apply pressure and not drop it. Attached to the front of the handle are the guards that prevent the user’s fingers from accidently sliding forward onto the blade. It’s a good idea to make sure they’re big enough to do the job. At the other end of the handle, there may be a pommel, or knob to prevent the user’s hand from sliding backward and making it easier to hold the knife. And outside that, some knives will have a hammer end used to bang on your tank.

 

The tang in a dive knife is usually unseen but one of the most important parts of the knife. The tang is the continuation of the blade back into the handle. A “full tang” refers to the height of the tang rather than the length. If you can see the tang on the top and bottom of the handle, that’s a full tang. Most dive knives have a partial tang, meaning that the tang is tapered from the blade back into the handle. This can allow for better shaping of the handle, better balancing, and easier (and cheaper) machining. But in all cases, you want a tang that runs the entire length of the handle in order to be able to apply force without breaking or bending the handle.

And lastly, don’t forget the sheath where you’re going to keep your knife. This often overlooked part of a dive knife has resulted in my collection of slightly rusted dive knives I’ve picked up off the bottom. If I could just find some lost sheaths, I’d be all set.

To prevent losing the knife, the sheath should have some sort of positive locking mechanism with a definite “click” when the knife is secured. A retaining strap also works, but I recommend tying off the back side so the retaining strap can’t be pulled through. Remember also, that the time you may want to use your knife is when you’re entangled. So make sure that you can release and draw your knife with one hand - either hand - while wearing gloves.

And finally, be very careful putting the knife back in the sheath. More than one BCD has been punctured by an errant knife tip.

Hard, Soft, Sharp, and Rusty

The next issue we come to is what material our knife is made of. The two most common materials are stainless steel and titanium.

If you’ve looked at dive knives already, you’ve probably noticed that titanium knives tend to be quite a bit more expensive than stainless steel knives. For starters, titanium is a more expensive metal. But its characteristics make an ideal dive knife. Titanium is very hard and will hold a sharp edge for a very long time. Second, titanium is very light, so it’s easy to carry. And finally, titanium will not rust, which is perfect for a knife that’s going to be in the water. The only real downside to titanium, other than cost, is that it is more brittle than stainless. Don’t even think about trying to pry something with a titanium blade  - it’s likely to snap.

So that leaves stainless, which we’re about to learn, isn’t. Stainless steel is an alloy made of iron and a variety of other minerals such as chromium, nickel, molybdenum, and vanadium. The mix of elements in the alloy determines the final characteristics of the product, and are denoted with a number such as 301 or 440. In a nutshell, different types of stainless steel strike a balance between hardness to hold an edge, and corrosion resistance. The higher the number, the harder the metal but the less corrosion resistance it is. Conversely, a lower number will resist corrosion better, but not hold an edge as well. It’s all a trade off.

Get the Point

Initially, you may think that a nice sharp point is a good feature. But if you remember that a dive knife is a tool and not a weapon, there’s really not much need for a really pointed end. In fact, a thin sharp point allows a lot of force to be transmitted to a small area often resulting in snapping off the tip.

Consequently, a lot of knives these days have what is known as a “tanto” point. This type of point is essentially another sharp edge running at a sharp angle from the main blade. It can even be perpendicular to the main blade. Originally designed to pierce armor, these tips allow the knife to have a sharp, yet substantially stronger tip.

Goldilocks had it Right

How big a dive knife do you really need? The short answer is, “big enough to be useful”. A knife that is too small will be difficult to hold and apply force and the blade will be awkward to use requiring many small rips. A knife that is too big, just gets in the way, creates drag, and gets tangled. So, in most cases, a medium size knife fits most users and uses. What’s medium? A blade in the 4-5 inch (10-12cm) range with a handle in the same range works well. These are still small enough to fasten to a BCD.

Where?

Before you can draw your knife with one hand, you have to be able to reach it. The original location for mounting your knife is on the inner calf using straps. I strongly recommend the inner calf rather than the outer calf to reduce the chance of snagging the knife on things.

In recent years, BCD manufacturers have begun adding grommets to their BCD’s, typically near or on the pocket. Dive knife makers (often the same people), then began making knife sheaths that come with screws and nuts to fit into those BCD grommets. All in all, a very convenient solution.

Care and Feeding

Every dive knife needs maintenance after use. A titanium knife usually only needs to be taken from the sheath, rinsed in fresh water, and both the knife and sheath allowed to dry before storing.

A stainless steel knife, regardless of which alloy it’s made of, absolutely, positively, must be rinsed in fresh water and allowed to dry after every use or it will rust. Actually, it will rust anyway, but in a controllable manner. Light rust can be removed with fine sandpaper or a fine file. If the knife is going to be stored for a while, you can coat it with silicon or silicon spray. Petroleum products and oils are not recommended since they’ll wind up adding pollution to your dive site.

When your knife needs sharpening, rather than using a stone, which will leave a very smooth edge, it’s better to use a fine file that will leave small “teeth” on the blade that will make cutting stuff underwater easier.

Alternatives

As your dive instructor said, you need a “cutting instrument”, but not necessarily a knife. Some divers carry only a line cutter, which, is great if their diving environment doesn’t have anything but monofilament fishing line. But, you never know when you’ll come across some larger line wrapped around the coral that you’d like to remove. A line cutter alone limits your options.

A very common alternative now are dive shears. Originally modeled on Emergency Medical Technician shears, these scissors are at an angle that provides leverage for the transmission of a lot of force. And the hardened blades will cut through a penny. I’ve used these to cut through heavy gage wire, one inch (2.5mm) polypropylene line, and chicken wire fish traps. These shears also require the same care as a dive knife or they’ll rust, too.

Hopefully, we’ve all gotten the point on dive knives now. We need to carry a cutting tool and there’s one out there just for you. You just have to find it….