My first diving class beyond my initial open water certification was the SSI Perfect Buoyancy class (equivalent to PADI Peak Performance Buoyancy). This course was taught by Shyanne and Fred and had a full roster of students.
What is Buoyancy
Buoyancy refers to how you float (or don’t). In the context of diving, buoyancy refers to the upward force on an object (the diver) by the water. The amount of this force depends on how much water is pushed out of the way by the diver’s body. When the water pushed out of the way weighs more than the diver, the diver would move up. If it weighs less, the diver will sink. If it’s close to the same, the diver will sort of hover.
The Class
The entire purpose of this class was to help us, as divers, get closer to that ability to hover. A diver with good buoyancy control exerts less energy and is safer because they are more in control of where they are in the water. They are also less likely to hit or damage things they shouldn’t (like coral).
We spent a short amount of time in the classroom reviewing basic principles. Then we hit the pool and practiced it with guidance from Shyanne and Fred. The thing is, no one achieves perfect buoyancy in the class. What we did do is get a better understanding of what we need to get there so we can practice it.
The best part of it for me was verifying that I was using the correct amount of lead weight (18 pounds or about 8 kg). That’s actually a lot for a diver, but it matches what would be expected for my body size and composition and it really does work about perfectly for me.
Math Fun
I did do some rough calculations on my body density and possible displacement, but I realized I’d need to test the displacement of all my gear somehow as well. You can look up rough estimates of the % of a healthy person that is skin, bone, and organ with the rest being split between muscle and bone. Then you can also get rough numbers on the density of those various tissue types to get an approximate volume. Since we know the density of seawater and freshwater, it’s basic math to determine that and you can mathematically see how much weight you “should” need.
Maybe all the dive shops need a human sized graduated cylinder so they can accurately calculate displacement to determine the correct amount of weight.

Trim
While the class focuses on buoyancy, there’s another very important aspect of your position in the water and that’s your trim. Trim refers to your position in the water. Are you upright (vertical) or flat (horizontal) or somewhere in between?
Why does it matter?
1 – Proper trim reduces drag so it’s easier to move through the water. This means you’re not as tired and . . . your air lasts longer. This is not only a benefit to a good dive, it provides a safety margin as well.
2 – It’s easier to maintain buoyancy control. You’re more relaxed and breathing more naturally so you tend to stay at a consistent level.
3 – You’re less likely to have an impact on the environment. That could mean not damaging coral, not hitting something and getting stung or bit, and not kicking up silt and ruining visibility. This is a safety factor for wildlife and for you. People have died because of bites and stings but for divers, the visibility is the big danger, especially for cave and wreck divers.
4 – It’s more comfortable. When you’re in proper trim, it’s a relaxed position. My back doesn’t hurt when I’m in proper trim. The form of my kicks is better. I’m able to keep my hands in a relaxed state instead of flailing around trying to get into the position I want.
Adjusting trim is, in some ways, more challenging than buoyancy because you may need to move weights around your body. You may need to shift your tank higher or lower on your back by altering where the bands tighten around it. Most recreational BCDs have trim pockets on either side of the tank (possibly under the bladder).
For me, trim is a nightmare. My least dense (and thus most floaty) tissue is higher on my body, as are my huge lungs. So, I have to shift as much weight as possible higher on my body to get a decent trim. The standard recreational BCD is tolerable for this, but I’ll be shifting to a backplate and wing at some point to help with it.
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