For the last six months, I’ve been doing most of my diving in sidemount. I’m loving all the benefits of sidemount (better stability in the water, easier access to valves, thinner profile for going into small sections of cave), but getting started has been… challenging. I’m not the only one. Lots of sidemount divers talk about how much they like sidemount diving now, but also how much of a struggle it was getting everything set up correctly.
The main value of a sidemount class is not that it teaches you how to dive sidemount, but that the instructor can help you get all the equipment set up so that it all fits, stays in place throughout the dive, and makes diving easy. There’s no substitute for that experience. However, if do have to figure it out on your own, understanding the physics of sidemount helps cut out a lot of trial and error.
I did my sidemount training with Roger Williams in Mexico. Roger was great, but unfortunately my sidemount harness hadn’t arrived in time for the class, which meant I was on my own to set it up when it arrived. To complicate things, we were using “Mexico-style” sidemount, while most people here in Florida do it “Florida-style”. In this post, I want to talk about the differences between sidemount in Florida and Mexico, and in a future post (when I get around to writing it) I’ll talk about the specifics of setting each one up.
The fact that there are several geographic sidemount “styles” was confusing to me. Isn’t water the same everywhere? Divers talk about the styles as if they’re rival schools of diving thought, but they’re not. They both have the same goals: keep the cylinders on the diver’s side and in line with the diver’s body throughout the dive. The difference comes from the fact that steel cylinders (popular in Florida) and aluminum cylinders (popular in Mexico) have very different weight and buoyancy characteristics, so setting up a sidemount system to work with each of them has to be done differently.
Newton’s first law says that in order for an object to remain stationary, the total force on the object must be zero. There are (at least) five forces that act on a sidemount cylinder:
- The weight of the cylinder (which is constant)
- The buoyancy the cylinder gets by displacing water (which is constant)
- The force of the bungee pulling on the tank neck/valve
- The force of the bolt snap pulling on the tank from the bottom or middle.
You can’t control the first three forces (except by switching to different cylinders), but you can mess with the bungees and bolt snaps. The goal is to set them up so that the forces from the bungee and bolt snap cancel out the weight and buoyancy when the cylinder is in position at your side. It makes more sense looking at the forces one by one.
First, the weight of the cylinder. My steel HP100 cylinders weight 34 pounds. Each cylinder’s center of mass is near the middle of the cylinder, but slightly closer to the top because of the heavy valve and regulator first stage. That weight is supported by the bungee and bolt snap, but slightly more is held by the bungee since the weight is closer to the top of the cylinder.
The cylinders, when full, each hold 100 cubic feet of air. That air weights 7.5 pounds. The center of mass of the air in the cylinder is also near the middle, and is supported about equally by the bungee and bolt snap.
A 34 pound cylinder (plus 7.5 pounds of air) would be too heavy to carry on a dive, but the cylinders have about 25 pounds of buoyancy from displacing 25 pounds of water. That buoyancy isn’t enough to make the cylinder float, but it is enough to make the weight manageable underwater. The upward force of the cylinder’s buoyancy is opposed by the bungee and bolt snap pulling down.
When you put the weight of the cylinder, the weight of the air, and the buoyancy of the cylinder together, the buoyancy cancels out of most of the weight, and the bungee and bolt snaps carry the rest.
At the end of a dive when the cylinder is mostly empty, the weight of air in the cylinder isn’t a factor anymore. This mean that it takes even less tension in the bungee and bolt snap to support the cylinder.
Aluminum cylinders are different. They still have the same forces acting on them, but not in the same amounts or locations. An aluminum AL80 cylinder weights less than my steel HP100s, but displaces more water. The aluminum cylinder’s buoyancy is also closer to the bottom the cylinder, since the bottom is flat but the top is rounded. When you combine all the forces for a full aluminum cylinder, it looks about the same as the steel cylinder, although the weight supported by the bolt snap is very small because of the buoyancy near the bottom of the cylinder.
As the aluminum cylinder empties, the situation changes. Without the weight of the gas in the cylinder, the buoyancy’s effect on the bolt snap is larger than the effect of the tank weight. The bolt snap actually needs to pull down to keep the cylinder in position. If the bolt snap doesn’t hold the bottom of the cylinder down, the it will float up instead of staying in position. The bungee, however, still needs to pull upward like it would for a full cylinder.
That’s the main difference between sidemount with steel and aluminum cylinders – with steel cylinders, the bolt snap always pull upward, but with aluminum, the bolt snap needs to be able to pull upward or downward depending on how much gas is left. In the next post I’ll talk about how to use these principles to set up the bolt snaps and bungees without too much frustration.