The tides at Port Melbourne

A small tide behind a narrow door

Port Phillip is a huge bay joined to the sea by one narrow, shallow gap — the Rip, at the Heads. Only so much water can squeeze in and out through that gap each cycle, so the tide inside never gets to swing very far. The range at the top of the bay is often under a metre, where outside the Heads it is far bigger.

And a late one

The tide also has to travel. The rise enters through the Heads and works its way up the bay, so high water at Melbourne arrives about two to three hours after it is high at the Heads. Same tide — just smaller by the time it reaches us, and later.

Why the tables are often out

Because the swing is so small, the wind and the air pressure push the water about as much as the moon does. A northerly blows water out of the bay and the level drops below the prediction; a southerly, or a high-pressure day, piles water in and it sits higher than the table says.

The timing, though, stays reliable — high and low fall close to the predicted time; it is the height that wanders. So a Port Melbourne tide table is a fair guide to when, and a poor guide to how much.

A long record — and where it started

We can only be sure the timing is reliable and the height wanders because someone has been writing the water down for a very long time. A few kilometres across Hobsons Bay, Williamstown keeps one of the oldest tide records in the country — a self-registering gauge has measured the water there since the late 1850s. Lay those decades of observed levels against the predicted ones and the pattern falls straight out: the predicted times of high and low sit close, while the measured heights ride above and below them with the weather. The long record is the proof.

The gauge lived in a small octagonal bluestone house, built by convicts from the old prison hulks and first set at Point Gellibrand, the tip of Williamstown. Its readings became the datum for Victoria — the zero that land heights were measured from. The automatic gauge ran until 1943, when the site silted up; in the mid-1950s, when the pier was rebuilt for tankers, the little house was moved along the shore to the Commonwealth Reserve. It stands there still — heritage-listed, the gauge itself long gone, beside the anchor of the Nelson of the old Victorian Colonial Navy. You can walk up and read the plaque. Worth a look, if you like this sort of thing.

What to do about it

Read the water, not just the table.

Watch the sandbank: on a low it is barely covered and shows itself; on a windy high it can all but disappear. Leaving the beach against an incoming tide in light wind is hard going — the water keeps setting you back on, so wade the boat out past the bank and start from there.

And if it turns into a battle — it can — don't wear yourself out fighting it. Signal for assistance, the low one-hand “come closer” wave, and let the safety boat tow you out to where the wind fills in. It catches everyone at some point; there is no shame in the wave.

And it is why the briefing page reads the Bureau's observed gauge at St Kilda Marina rather than a prediction. A real measurement already has the wind and the pressure baked in — it tells you what the water is doing now, which on this bay is different from what the table said it would.