The sound of its engines, up here on the bridge, is a mere grumble.

It's illusory: below decks two great diesel motors, each with 12 cylinders and capable of producing 1400 horsepower, fairly bellow. Big funnels baffle the sound.

The Cape Grant's twin, the Cape Nelson, is sliding into the pre-dawn, too, under the command of Garry John. Work to be done.

Together, these two tugs and their various masters and crews herd ships 20 times their size from the ocean into the port of Portland, nudge and bully them to moorings at one of several wharves, and haul and push them out again.

The two stubby vessels are the port's heavy-duty labourers. Portland, Victoria's first European settlement and the only deep-water harbour between Adelaide and Melbourne, exists largely because it lies within the western crook of Portland Bay, offering shelter from often cruel seas at the far reaches of what is known as the Shipwreck Coast.

Space at the wharves is rare. The port is the world's largest hardwood wood chip exporter. The world's thirst for mineral sands, grain, softwood logs, aluminium and live animals also keeps the harbour in restless movement, and several large cruise ships arrive each year.

This morning the tugs must send the bulk carrier Royal Forward, loaded with wood chips, on its way to China. The Cape Grant nudges up to the ship's bow and a rope snakes down, enabling integrated rating Paul Westlake and marine engineer Henry Kraak to secure a tow-line.

The Cape Nelson does much the same manoeuvre down at the ship's stern, and soon both tugs are in reverse, hauling the loaded ship off the wharf, turning it to face out to sea. Radio commands go back and forth from the Portland pilot, Altaf Hussain, who is commanding the Royal Forward from its bridge. Once the ship is out to sea, Hussain will clamber down a ladder and return to port aboard the small pilot's launch.

Soon, with the Royal Forward steaming away, the Cape Grant drags from another wharf the China Spirit, loaded high with softwood logs.

Allan Geraldene's light touch on the controls makes it seem so simple, the gentle nudging and the full-power reverse towing in a tight section of the harbour. He's been working on tugs since 1980 and skippering since 1984.

A windless dawn reveals a perfectly calm sea. It is often not like this.

In winter and spring, westerlies howl. Big depressions out in the Southern Ocean send swells of five metres into the bay. In summer and autumn, the big blows come from the south and the east, and there is no shelter from them even in this sheltered harbour.

Shepherding cargo ships in a tight harbour with a sudden squall ripping in, turning the towering steel sides of those ships into giant sails, is a tricky science requiring calm nerves and decades of experience. A single ship is typically 55,000 tonnes dead-weight, and is often loaded with another 50,000 tonnes of cargo.

The tugs need to be nimble. They are: two propellers driven by those big engines sit within pods that swing 360 degrees, enabling these workhorses to spin on the proverbial sixpence.

It is a ballet.

Photographer Dean Koopman was granted rare access to document the dance of the Portland tugs and the skills of those who operate them.