TRIM SYSTEMS

EVERYTHING YOU NEED TO KNOW

- Dave Marsh

What are trim systems?

Trim systems are devices, invariably electro-mechanical, which are attached to the transom. They help the skipper to control and optimise their boat's running trim (attitude) and in some cases the real-time dynamic behaviour underway. These systems vary immensely in their level of sophistication and control. Although these are not dedicated stabilising systems such as gyros and fin stabilisers, the most advanced trim systems are so powerful and fast acting that they are capable of providing a meaningful stabilising function, especially on small fast boats.

`Never-ending change, that's why. A strong crosswind will induce unwelcome heel; away from the breeze on a long-keel semi-displacement boat, and into the wind on a deep-vee planing boat. That heeling adversely affects visibility, steering and handling. Sea state typically has an even more discomforting effect on boat motions. Driving upwind into big head seas, most planing boats will benefit from having their bows pushed down to reduce pitching and slamming. In a beam sea, even the best designed boats can roll around, especially the big ‘uns.

Finally, it is all but impossible to position all the tanks - fuel, water, black and grey waste - directly over the centre of gravity, so real time changes in tank levels will change the trim and therefore the behaviour of a boat underway. On a small boat, so will crew moving around the decks. The weight and positioning of cruising stores and tenders can vary enormously. And pity the poor designer whose 10-metre RIB hull might have to cope with everything from a single 450hp outboard weighing 480kg to a pair of 380hp diesel sterndrives topping 1,520kg. So the notion that a perfectly designed boat might not need any help is a complete myth.