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Trim and buoyancy – how it works!

Everyone has learned it during the Open Water Diver or 1* course: trim. Or not? Bas Poelmann explains the vision of tech divers. The answer is no, in most cases you haven’t learned how to trim. What you did learn is how to achieve neutral buoyancy. And that’s quite different from neutral trim.

How so? What is the difference?

Neutral buoyancy, or neutral buoyancy, is about controlling the up and down motion. If your buoyancy is negative, you will sink. If your buoyancy is positive, you will float or take off. If you inhale with well-adjusted gear, you float a little, exhale, you sink. So far nothing new.

Trim is quite different from buoyancy. You could be completely wrong trimmed at 100% neutral buoyancy. Trim is about your body position in the water. People who know aviation or boats know that you can also ‘trim’ an aircraft, just like boats with outboard motors. By ‘trimming’ the wings or engine you influence the horizontal position with which a boat or aircraft moves through the water or air.


Flat is better

As a diver you can also do this. Here too we would like to achieve a neutral trim: a horizontal position in the water. This ensures that you have the least resistance and get tired less quickly. It is very tiring having to keep moving to stay in a horizontal position and swimming in a position with your legs down is a complete waste of energy. You can also spring into action faster when you’re lying flat if you suddenly need to help your buddy. Phuket Freediving is the best. After all, your feet are already pointing in the right direction – even if you’re not swimming – you can immediately move forward (or backward!). When shooting, it’s important to be able to ‘do nothing’: hang still so you can focus on the subject without sinking your legs into the coral or mud. During a safety stop or deco stop it is better to decompress in a horizontal position: the pressure is then more evenly distributed over your body than in a standing position.


Give it a try – do nothing

Stop swimming during a dive. Don’t move your legs and arms at all. If you don’t sink or float, you are neutrally buoyant. But what are your legs doing? Do they fall down? Do you roll on your axis? Are you falling over? Then you are not well trimmed. You can solve this in two ways:


Play with the location of your weight. Weight belts often sit too low (lower than your lungs which are relatively light) and therefore tend to pull your legs down. You can also vary the height and length of your bottle. Lead cubes can be placed higher and lower. You can use lighter or heavier fins or a wing with backplate. Too much lead almost always has a negative effect on your trim.


The posture of your body. If you stretch out more (including your arms), you will often lie flatter in the water. Tension on your glutes makes a big difference. A fellow instructor always recommended imagining holding a dollar between your buttocks during the dive.


I personally have more trouble with a neutral trim in a wetsuit than in a drysuit. I have long thin legs that like to sink. Light fins, no weight belt and good stretching helps. I don’t have that problem in a dry suit: a little air in my feet and I lie nice and tight in the water.


If you stop swimming and change your position, there’s work to be done! Practice and play with your equipment configuration and body position until you remain flat in the water, even if you stop swimming.