How a lifejacket floats

Anything less dense than water floats. A lifejacket adds volume without adding much weight, so the person wearing it displaces more water than their weight requires. The spare displacement is the lift.

Foam lifejackets hold that volume all the time, in closed-cell foam that does not soak up water. Inflatable lifejackets stay slim until a gas cylinder or a blow tube fills a bladder. Once filled, the bladder is a bubble of air.

Buoyancy

The bladder or the foam sits on the chest and behind the head, not on the back alone. Lift high on the front rolls an unconscious person towards face up, so the mouth is clear of the water. A cushion tied around the waist does not do that.

The force is limited. A jacket rated for a certain lift will support that load and no more. Heavy clothing full of water, or a person holding a weight, uses up the margin.

Why the fit matters

A puncture or a failed cylinder removes the bubble. That is why inflatable jackets are checked, and why a foam jacket is preferred where a spike or a fire risk would destroy an air bladder.

The jacket does not swim for you. It buys time by keeping the airway up. In cold water the limit is heat loss, which a thin jacket does not stop.

Buoyancy

A lifejacket floats you face up because it holds buoyant material, or inflates, high on the chest. Anything less dense than water floats. A lifejacket adds volume without adding much weight, so the person wearing it displaces more water than their weight requires.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a lifejacket floats needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

Why the fit matters

A lifejacket adds volume without adding much weight, so the person wearing it displaces more water than their weight requires. The spare displacement is the lift.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

The spare displacement is the lift. Foam lifejackets hold that volume all the time, in closed-cell foam that does not soak up water.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

Foam lifejackets hold that volume all the time, in closed-cell foam that does not soak up water. Inflatable lifejackets stay slim until a gas cylinder or a blow tube fills a bladder.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

Inflatable lifejackets stay slim until a gas cylinder or a blow tube fills a bladder. Once filled, the bladder is a bubble of air.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

Once filled, the bladder is a bubble of air. The bladder or the foam sits on the chest and behind the head, not on the back alone.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.