Boating NZ : FREE TO READ March 2014
I like watching tug boats at work. It's when the little guy puts the big guy in its place. The tug Wakakume displaces 300 tonnes yet it pushes and pulls container ships displacing up to 50,000 tonnes. The tug is 22.45m long, yet its charges may be up to 266m, ie, a Maersk class post-Panama vessel carrying 15 containers across its beam and up to 100m high. To overcome that difference in mass, there’s an awful lot of torque going on. Wakakume’s bollard pull is 50 tonnes at 1800rpm. On general harbour duties, she uses 100 litres of fuel an hour per engine, but at 100% power, she uses 808 litres an hour, per engine. Despite the predictable man- humour on the tugging theme, jokes about men’s inability to multi-task are null and void on the bridge of a tug boat. Driving one of these water-borne Staffordshire terriers is the maritime equivalent of stroking your tummy while patting your head and dancing the quick-step. That was well demonstrated by Wakakume skipper Jamie McGregor as he manoeuvred backwards, forwards, sideways and diagonally in the course of his duties off Ferguson Wharf in Auckland. Wakakume doesn’t have rudders, because they provide only limited manoeuvrability. Instead, she has two Z-drives, similar to pod drives, which McGregor likens to the 2hp outboard on the back of a dinghy – as in, you can rotate the drives in any direction or even keep turning them around and around. These two Z-drives, each with a dedicated 3516 Caterpillar engine, reside well aft and opposite each other. Each spins a 2m diameter, fixed pitch propeller surrounded by a thrust ring. There is no neutral, and no gears. To hold a position, the skipper has the two Z-drives opposing each other, directing their thrusts out either side of the boat. Next time you see a tug stationary in the water, look for the turbulence aft on both sides. To go forward, both units are thrusting aft; to go back, both units are thrusting forward. When a tug does its famous sideways shuffle, the skipper directs, say, one engine’s thrust aft at 45 degrees to starboard and the other’s forwards at 45 degrees to starboard. The resulting vector is 90 degrees to port, propelling the tug sideways, with constant adjustments for wind, tide and the effect of the load from the ship. The angle of thrust is controlled with a turning motion on a chubby joystick, with a clutch lever-throttle at its base to control the revs. “You end up being quite ambidextrous,” McGregor says. Words by Rebecca Hayter Photos by Will Calver THE GUY little Whoever said men can't multi-task had never seen a tug boat skipper at work. subscribe online at www.mags4gifts.co.nz/boating-nz 83 subscribe online at www.mags4gifts.co.nz/boating-nz 83 Feature_Tug boat_March14.indd 83 18/02/2014 5:12:59 p.m.
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