Back to Basics: How to Set Up Your Spearfishing Weight Belt
Taken from Ultimate Spearfishing Magazine volume 12
The weight belt is a standard piece of equipment for the freediving spearfisherman and its primary purpose is to offset the buoyancy of the diver and his neoprene wetsuit. Basically, what this means is that your wetsuit floats and makes you float, while the weight belt is heavy and will help you sink. If you did not have a weight belt you would battle to swim down and stay down.
The problem comes in when trying to balance out the buoyancy of the wetsuit with the weight belt. This is compounded by the fact that there are variables such as salinity and the deeper you dive, the less buoyant your wetsuit becomes. These variables need to be accounted for. The neoprene wetsuit is made up of rubber bubbles that have gas in them. As you go deeper, those little bubbles compress and become less and less buoyant. This means that you can be positively buoyant on the surface and negatively buoyant once the suit has compressed at depth.
Simply put, positive buoyancy means that you float, and negative buoyancy means you sink.
The best solution is to add just enough lead to the weight belt so that it assists you in submerging to the depth where the wetsuit is most buoyant. Now... You could use lots of weight, making the descent quick and easy, and use less energy going down, but you need to remember that swimming back to the surface with all of that weight will be difficult, energy sapping and potentially dangerous. So, you need to find the balance where you don't use too much energy going down or returning to the surface.
The way to do this is to find the depth at which you are neutrally buoyant. This is absolutely key to safe and effective diving. Finding this depth and setting your weights accordingly will not only help you dive more efficiently but, in the event of a black out, will probably save your life.
Generally, you want to be neutral at about 8m and positively buoyant above this depth. You should float to the surface from around 7m. This means that when you are below 8m, you should become more and more negatively buoyant and be able to glide down to the bottom without finning.
However, what if you are diving in less than 8m of water? If the water is 5m deep, then you will not be able to rest on the bottom as you will float back up to the surface. In these shallow conditions you will need to add more weight so that you are negative when on the bottom. In rough conditions you might have to add even more to keep you on the bottom with the surge of the waves pulling you up. Be careful when adding weight though as it's easy to add too much weight and become negatively buoyant on the surface, which is very dangerous.
The converse is true for deep diving, here you want to have as little weight as possible. The reason for this is that the greatest change in buoyancy occurs in the first 10m, where the atmospheric pressure is double that of the surface. This halves the volume of the air in your lungs and the buoyancy of your wetsuit would also approximately be halved. This means you are probably half as buoyant as you were on the surface, but your weights are just as heavy.
The deeper you go, the more the pressure increases and the less buoyant you become... to the point where you are not buoyant at all. This is important to understand because at the point where your suit is no longer buoyant, it means that you will have no help from the buoyancy of your suit to swim the weight of the weight belt back to the point where you are positively buoyant again.
For example; if you are neutrally buoyant at 8m and are diving to 30m, it means you have to swim the weight of your weight belt 22m back to the point where your lungs and suit buoyancy will kick in and help you to the surface.
If you are diving in a thin wetsuit that requires very few weights to compensate for its buoyancy, 22m is maybe not too much of a problem. But when you are diving in a nice new (has not compressed and lost some buoyancy) 5mm or thicker wetsuit that requires more weights to compensate for its buoyancy, it becomes a problem. The difference between a thick and thin suit can amount to a couple kilograms of weight - which becomes difficult to swim to the surface.
To remedy this problem, guys diving deep generally dive as light as possible. This means that the depth at which they are neutral is deeper, sometimes as deep as 15m. The downside to this, is that they need to swim hard for the first 15m to get to the point where they are negatively buoyant.
Working out your buoyancy
Working out the amount of weight you need is fairly simple with this basic equation.
1kg for every millimetre of wetsuit + 2kg (based on an average man of 80kg)
So if you have a 3mm wetsuit and weigh around 80kg then you will need 5kg (3kg +2kg) of lead weight.
If you are of a smaller build then add on 1.5kg and if you are larger, add on 0.5kg
This is a very generalised equation and variables like your wetsuit's buoyancy, your body's natural buoyancy and your lung capacity will be other factors that will affect whether you add or subtract from this calculation. It will however serve as a good starting point.
Testing your buoyancy is fairly easy. With your full diving kit on, float vertically in the water. You should be totally relaxed with your fins straight down and your hands at your sides. The water level should come just over your mouth. If your mouth is out of the water then you are too buoyant. Conversely, if the water level is past your eyes and you are sinking, then you are too heavy.
The next test is to make a few dives to the depth where you want to be neutral. If you are battling to tell if you are neutral then swim up a metre and relax. If you float then you are positive at that depth. Do the same 1m below and see if you sink. If you don't have a dive watch, mark off the depth on your float line and use that as a reference. Note: Don't do this in fresh water and then go and dive in the sea, as salt water is more dense than fresh water and you will be more buoyant in the sea.
Once you have worked out how much weight to use to make yourself neutral at 8m, you can start adding weights if you are diving in very shallow water and taking off weight if you are diving deeper. Drop weights are good for this as they are easy to take on and off using the elastic cord to attach them to your belt. They are also not that heavy (approximately 500 to 600g) and allow for those subtle changes that will help your buoyancy regulation, and hence your diving.
It is good to experiment with your weight and see how it affects your diving. If you are diving deep, however, do this in small increments as the deeper you dive, the harder it becomes to swim every gram off the sea bed.