Why the water temperature matters

Warm water floats less

Water isn't always the same weight. Warm water is lighter than cold — less dense — and fresh water, like the river water that flows in at the top of the bay, is lighter than salty water. A ship floats at the depth where it pushes aside its own weight in water. In lighter water it has to sit a little lower to push aside enough. So the very same ship, loaded the same, rides deeper on a warm day than on a cold one.

The gap under the keel

That gap has a name — under-keel clearance, the space between the bottom of the ship and the seabed. The big ships that come up the bay's dredged channels, the ones the briefing tells you to keep well clear of, run with very little water beneath them. Sometimes less than a metre.

To send each one through safely, the port has to know exactly how deep it is sitting right now — and, as we've just seen, that depends on the water's temperature and saltiness. Read the water wrong and a loaded ship touches the bottom.

What it means for a small boat

What matters to a loaded ship matters to a small boat too: how cold the water is.

Port Phillip is cold-temperate water — roughly 10 °C in winter to under 20 °C in late summer. Cold water, not cold air, is the real danger after a capsize, and it decides whether you want a wetsuit today — cold enough to matter even on a warm day, because it's the water, not the air, that sets the risk if you end up in it.

We don't read the port's figure — it sits inside its own shipping system, if it's held as a temperature at all. So the number on this briefing comes from a satellite instead. How we read it from space → The Cold & Heat page covers hypothermia — the signs to watch in a crewmate and in yourself, and what to do.