I’m assisting with a Peak Performance Buoyancy class next week. People like to make fun of this class for a variety of reasons, but there’s one reason that I’m very exited to help teach it: this is one of the only times you can talk at length with students about fundamentals like trim, buoyancy, breathing, and propulsion and expect them to pay attention. Students only pay attention to what they perceive to be important, and to a lot of students, if it isn’t a requirement to pass the class, it isn’t important. But this time, improving these fundamentals is exactly what they signed up for!
Teaching trim is tricky because the effects of things like moving weight around can be counterintuitive. There are a few common problems that look similar, but have opposite solutions. Here’s an example:
Our diver, in just a swimsuit, is lucky enough to be naturally neutrally buoyant, and their center of mass is in the same place as their center of buoyancy. All this means is that they can be in any orientation (head up, head down, perfectly horizontal, etc.) without having to work to stay there.
As soon as they put on a wetsuit, they become positively buoyant. The rest of their scuba rig (BCD, tank, regulator, weights, etc.) provide the negative buoyancy to counteract the wetsuit.
As you can see, there’s a balancing act going on, since the negatively buoyant scuba rig is on the diver’s back. In reality, this isn’t a big deal at all since the scuba equipment is only a few inches away from their body.
However, when this weight is too low on the diver’s body, it does cause problems. Single tank divers wearing a jacket-style BC are frequently foot heavy since the integrated weight pockets are on the waist strap. To compensate for this, divers usually drop their legs so that their finning is directed slightly downward.
This provides the torque to counteract their weight being too low, but wastes energy and kicks up silt on the bottom. It also means the diver has to adjust their buoyancy every time they start or stop swimming.
Divers who are head heavy have a different “solution”. Since humans can’t bend their hips backwards, they can’t fight the head heaviness in the same way they would fight foot heaviness. (At least with a flutter kick. Frog kicking does allow you to direct your finning upward, and this is one important reason to learn to frog kick, but that’s the subject for a future post.) Instead, divers who are head heavy try to balance the weight by being slightly head up all the time.
This has exactly the same problems as the foot heavy diver: it wastes energy, kicks up the bottom, and makes it impossible to maintain neutral buoyancy.
Notice how similar the last two pictures look? In both cases, the diver’s legs are below their torso, and their finning is directed downward. However, they have opposite solutions – the first case requires moving weight toward the head, the second requires moving weight toward the feet.
Weird cases like this are part of what makes achieving perfect trim difficult. Taking a trim and buoyancy class is a great way to sort these issues out!