Motorboat Trim
Book Extract
The correct running trim not only makes the boat’s motion more comfortable, it saves fuel too. The first consideration is to concentrate moveable items in the centre of the boat and not shove them away in the bows or stern lockers. Extra weight in the ends only accentuates the sickening hobbyhorse effect in a seaway.
With planing hulls, the prime objective is to get the boat up on the plane quickly, then running level over the water. If the boat has a stern-down attitude, it may struggle to get on the plane. If it is bow-down, then it will be heavy to steer – and very wet!
Trim adjustors
Outboards and outdrive legs have electric or hydraulic rams to adjust their running angle up or down from a rocker switch sited either on the dashboard or back of the control box.
- Trimming the power leg down forces the bow down and will reduce slamming when running into head seas.
- Trimming the power leg up will lift the bow and reduce the chance of burying the front of the boat when running down a following sea.
- Many planing powerboats are also equipped with adjustable trim tabs mounted on either side of the transom to fine-tune the boat’s running angle.
These tabs can also level the boat laterally when running through beam seas or when the effect of the wind lifts one side of the hull. Pushing one tab down will raise that side of the boat back onto a more even keel.
Trimming the outboard/outdrive down has the effect of pushing the bow down and reducing slamming in head seas.
Outboard/outdrive trimmed level for flat water conditions.
Outboard/outdrive trimmed up pushes the bow up to reduce the chance of it burying in when running in following seas.
Typical outdrive and trim tab installation.
Keeping a powerboat trimmed right requires constant adjustment. Tabs or power trim down may help to get the boat on the plane faster.
Trimmed correctly, the helm will be lighter without pull to either side and engine revs may rise slightly.
Tip
Tab down when running into a head sea. Tab up when running into a follow sea. Experiment to see what suits your boat best.
© Not to be reproduced without written permission from Fernhurst Books Limited.
Motor Boating Start To Finish is written by Barry Pickthall. Barry Pickthall is a respected sports writer and photographer and has been covering all aspects of sailing and boating for the past 35 years, 20 of them as a yachting correspondent to The Times newspaper. He has been covering the international stage for the past 4 decades from the Olympics to America's Cup and Round the World Races. Barry is also a qualified sailing instructor – having graduated from one of the first courses held at the UKSA.