Dive Knives
Dive knives are offered in a variety of materials, sizes and shapes. Below are a few tips that will help you make an informed decision on what kind of dive knife will best suit your needs as a diver.
Size
Large Knives (over 5 inch blade): Although many people think "the larger the knife the better," large dive knives can be awkward and can easily cut you or puncture you BC. You should buy a knife that will be useful to you as a diver- not an underwater warrior.
Medium Sized Knives (4-5 inch blade): The most practical dive knives are medium sized ones which are small enough to mount onto your BC, leg, or arm, but large enough for you to grip securely. A dive knife with a four to five inch blade and similar sized handle is easily managed and can be a very useful tool once underwater.
Mini Knives (2-3 inch blade): If buying a mini knife (which can be small enough to clamp onto a hose) make sure that the handle is longer than 2-3 inches so you can grip it securely, especially with gloves on.
Shape
Straight vs. Serrated Edges: Straight edges cut plastic better- this includes monofilament fishing line and nylon rope- while serrated edges are more efficient at cutting natural fiber rope and kelp. With a serrated knife, serrations should run almost the entire length of the blade, making it possible to use a good sawing motion. Many dive knives on have both straight and serrated edges, making them great in a variety of situations.
OnSharp Tip vs. Blunt Tip: Sharp tips most commonly used for spearfishing purposes. For other purposes such as prying, digging, hacking, and chiseling, however, it is easy to break a sharp tip off- this is where a blunt tip is useful. Sharp tips can also cut you or puncture your BC, so for a safe and effective tool, a blunt tip is the best.
Line Cutting Notch: You should also look for a line cutting notch, which can come in handy when cutting small lines.
Material
Stainless Steel: Most dive knives are made steel, either 300 or 400-series stainless steel alloys. All "Stain-less" Steel knives will show some signs of rust due to their high carbon content (carbon creates tensile strength). Because stainless steel knives are not completely rustproof, they will require maintenance. After each use, rinse well with fresh water and dry outside of its sheath. Coat with a light layer of oil or silicone.
300-Series Alloy: The lower the number (300-series alloy), the more corrosion resistant the knife is, but the less able it is to keep a sharp edge. If you buy a 300-series alloy knife, you can spend a little less time on maintenance, but you'll have to sharpen it after a couple of uses. Remember, if you buy a blunt-tip knife, it is not always important that the edge is extremely sharp.
400-Series Alloy: A 400-series alloy knife will hold an edge for longer, but is more susceptible to corrosion, and may break if used as a pry bar. If you purchase a 400-series alloy knife you will need to wash and oil it after every use, but you won't have to sharpen it as often.
Titanium: Unlike stainless steel knives, titanium knives are almost completely rustproof and corrosion resistant because they contain no carbon. The result is a knife that will hold an edge for numerous dives without sharpening, and requires almost zero maintenance.
