Neutral Buoyancy

Why it matters

Good neutral buoyancy makes nearly every part of diving better. You use less air because you aren't kicking to stay level. You can stop and look closely at something without touching it. You don't stir up silt that ruins visibility for others. Most importantly, you protect yourself and the reef: coral is easily damaged by fins, hands and knees, and a diver who can't control their buoyancy is also more likely to make an uncontrolled ascent.

The physics behind it is explained in Archimedes' principle. This article covers the practical side.

Step 1: get your weighting right

Most buoyancy problems start with the wrong amount of weight, and the most common mistake is too much. To check:

1. At the surface, in full gear, with a near-empty tank if possible, empty your BCD completely.
2. Hold a normal breath and relax, upright.
3. Correctly weighted, you float at about eye level and start to sink slowly when you breathe out.
4. If you're doing the check with a full tank, add the weight of the gas you'll breathe, about 2 to 3 kg (5 to 6 lb) for an aluminium 80, so you can still hold your safety stop at the end.

Write down what works for each suit, tank and type of water. Divers usually find they need less weight as they gain experience and relax more.

Step 2: use the BCD for big changes

Your buoyancy control device is for the larger changes: making up for suit compression as you descend and venting expanding air as you ascend. Add or release air in small bursts and wait a few seconds for each to take effect. Chasing your buoyancy with big inflations is the most common reason divers bounce up and down.

Step 3: use your breathing for fine control

Your lungs are your finest buoyancy control. A full breath in makes you rise slightly; a full breath out makes you sink slightly. Once your BCD has you close to neutral, use slow, deep, relaxed breathing to make small adjustments, such as rising over a coral head. Never hold your breath to stay up, for the reason explained in Boyle's law.

Step 4: trim

Trim is your body angle. A horizontal position, with your knees bent and fins slightly raised, makes you streamlined and keeps your fins away from the bottom. Weight placement affects trim: moving some weight higher, for example to trim pockets near the shoulder, can stop your legs from sinking.

Step 5: fin with care

The frog kick, a sweeping kick with bent knees that pushes water backward rather than down, is widely taught because it disturbs the bottom less than an up-and-down flutter kick. It's essential in cave and wreck diving, where a careless kick can stir up silt and wipe out visibility in seconds.

Practice drills

• Hover: stay motionless in mid-water for a minute, using only breath control.
• Fin pivot: lying flat on a sandy bottom, use breathing to rise and fall on your fin tips.
• Safety stop practice: hold 5 m (15 ft) for three minutes on every dive, as in safety stop.

Frequently asked questions