Scuba Diving Knives FAQ

1. What are dive knives used for?

The biggest misconception about dive knives is that they are used for defence.  In reality dive knives are a tool used for practical and safety reasons.   Many divers do not realize how much debris is scattered throughout the oceans.  Fishing lines, nets and rubbish is present in every ocean and divers can get tangled up in this debris.  You need a knife to be able to cut yourself free as entanglement could be a life threatening situation.  You only have a certain amount of time underwater with your scuba cylinder and if you do get caught up you may not be able to return to the surface before your air runs out. Dive knives can also be used to pry, dig cut and pound. Also, when knocked against metal surface make excellent signalling tools to gain your dive buddies attention.

2. Can I use dive knives against sharks?

When you are diving it is important you do not antagonise underwater predators such as sharks and barracudas.  They will not attack you unless they feel threatened and then they would most likely just snap at you and then flee.  Water pressure at depth makes your movements very slow and clumsy so it is highly unlikely you would be able to use your knife against lightning fast attacks from underwater predators.  If you did manage to strike back this would only make things worse as blood in the water would just attract the attention of more underwater predators.  It is important that you understand how to dive safely with sharks and if you see that the situation has changed and become hazardous it is time to leave the water.

3. How many knives should I carry?

Most experienced divers carry two knives.  This is just in case they get tangled up and are unable to reach one of the knives.  Secure one knife within easy reach of your right hand (such as on your inflator hose) and one within easy reach of your left hand (such as on your weight belt of BCD waist band).  This will mean if one of your arms gets tangled up you will still be able to reach a knife to free yourself.  Avoid carrying too many knives as this can start getting awkward and will weigh you down.

4.  What type of dive knife should I use?

There are a few different types of dive knife available and these are meant for different purposes.  If you are going to choose just one knife then it should have both a serrated blade and a normal blade as these are the most effective at cutting through the tough rope that is used in fishing nets. It is also useful if this has a line cutting notch for thin plastic fishing lines.  If you are going to carry a second blade then you should think about what sort of diving you are doing.  Spear fishers will find a stiletto blade useful as this will kill your catch quickly and prevent it from floundering around attracting predators.  If you are collecting underwater samples from wrecks or reefs then you can get more robust blades that are designed for a wider range of tasks such as prying and sawing.

5.  Should I choose a black blade for my dive knife?

Shiny dive knives can alternatively attract and scare away marine life.  In most cases your knife will remain firmly in its sheath so this will not be a problem.  However if you will be using your knife a lot then you might want to consider opting for a black blade to avoid attracting any curious underwater predators.  The obvious drawback with a black blade though, is should a diver drop their knife, they are harder to locate than shiny metallic ones.

6.  Are knife straps that go around your leg safe to use?

Provided the knife is designed so that it is low profile and sit flush against the leg. Many experienced divers, especially those who penetrate wreck and other underwater environments prone to entanglement, choose to mount their dive knife on the inside of the lower leg. By doing so, the knife is in-between your two legs, and cannot easily become entangled. Smaller diving knives can be attached to your BCD or inflator hose so that you can reach then  more easily should you get tangled up. Many divers choose to carry two or more knives or cutting tools, one on the inner leg and one on the BCD

7.  How do you care for dive knives?

Dive knives are robust and if you care for them well they should last for years.  After every dive soak the knife, sheath and strap in fresh cool water.  This will help to soak away grit, salt and chemicals which could degrade the materials.  Dry everything thoroughly before you put it away.  Rub a little silicone or oil into the knife.  It may need to be sharpened before storage if you have been using it a lot.  It is a good idea to sharpen knives after every dive so that you can be sure they are sharp and ready to use when you next go diving.  Store knives in a safe place out of the reach of children.

8.  What size dive knife do I need?

It can be tempting to choose the biggest knife you see, but you will have to carry this around with you on a dive so it does need to be a manageable size.  Dive knives are for safety and functional purposes.  A shorter blade (less than 5 inches) will be much easier to use and carry underwater.

9.  What knife sheath should I use?

Most dive sheaths are made from plastic.  This provides a durable, safe way to store your knife when it is not in use (plastic will also not blunt the blade).  For the strap section choose a durable, high wearing material such as double stitched nylon.  This will stand up well to any rubbing from abrasive rocks or sand.

10.  What are diving knives made out of?

The majority of dive knives are made out of good quality 300 or 400-series stainless steel alloys. This provides a reasonable level of protection from rusting through exposure to salt water.  However, stainless steel knives are not totally impervious to rust so you will need to look after them carefully.  Generally, the trade off with stainless steel knives is between high stainless qualities and ability to keep a sharp edge. High grade stainless has excellent rust proofing but a poor ability to stay sharp, whereas grades of stainless steel with higher levels of iron in the alloy, have better ability to keep a cutting edge but show signs of rusting quicker. To minimise rusting, make sure you rinse them thoroughly after every dive and dry both the knife and the sheath before putting the away.  You should coat the knife with a layer of oil or silicone grease to help prevent rust.