Adjusting a boat’s trim alters the running attitude of the hull in relation to the water. Trim affects the handling, onboard comfort, speed and fuel consumption of planing craft. Hulls perform best when they are properly trimmed; that is, balanced in two directions – fore-and-aft (pitch) and side-to-side (roll). NZ Fishing News contributors offer some tips on getting it right…

Powerboats rely on adjustable thrust angles, trim tabs and ballast to adjust their running attitude. Ballast is often in the form of fuel or water, but can include passengers and other heavy objects. Moving passengers to different seats or asking someone to stand closer to one side of the boat or the other often helps to keep the boat level, improving its ride and performance.

Drive trim

Most outboard-powered craft and sterndrive vessels enjoy drives with adjustable trim. So do some jet boats. Adjusting the drive angle gives the operator a great deal of control over the boat’s fore and aft attitude.

Most outboards of 30hp and bigger come with power trim and tilt. Power tilt is used to lift the leg clear of the water for storage or trailering; power trim adjusts the angle of thrust while the boat’s underway. The majority of outboards and sterndrives use a two-stage hydraulic ram for both tasks, but some systems are electric.

Trimming the engine leg outwards, away from the boat’s transom, thrusts the transom downwards into the water, raising the bow. It’s often called positive trim. Trimming the engine leg inwards towards the transom – negative trim – lifts the stern and pushes the bow down.

Although trimming is a fundamental powerboat skill, some boaters never come to grips with it – the oscillating ‘waaa, waaa, waaa’ of a trailer boat with its outboard trimmed out too far is a distinctive and familiar sound!

Some beginners drive powerboats with the bow in the air because they are unaware how the trim function works.

Power trim basics

The basics of using power trim are simple: tuck the leg in to keep the bow down before accelerating onto the plane and then trim it out a bit once you get going. But judging how much positive or negative trim to apply takes practice. For instance, if the bow is too low in the water it can catch a wave, pushing the boat around and making it difficult to steer (bow-steer). Raising the bow by trimming the outboard or leg out (away from the transom) once the boat is up on the plane gives a smoother ride.

For best speed and fuel efficiency, trim the engine leg away from the transom as the boat gathers speed. This lifts the nose, allowing the boat to plane on the flatter sections of the hull towards the stern. Less wetted surface means less friction, more speed and better fuel economy. Positive trim is also used to keep the bow up in a following sea.

The faster the boat travels, the further the engine can be trimmed out, but there is a point of equilibrium: too much positive trim can leave so little of the hull in contact with the water that the boat becomes dangerously unstable and difficult to control. Take care in windy conditions, too: wind getting under the bows can cause a loss of control.

In rough weather you want to do the opposite: push the bow down so deepest part of the hull’s vee slices through the waves first, rather than the flatter hull sections behind the bow, which will slam. But don’t get carried away: too much negative trim can result in bow-steer, sluggish performance and excessive fuel use. When travelling in a following sea or overtaking waves, use enough positive trim to raise the bows and avoid stuffing into the back of the next wave.

Ventilation, where the propeller sucks air from the water’s surface, is common symptom of too much positive trim, as is ‘porpoising,’ where the boat’s bow constantly rises and falls.

Turning at speed

Negative trim is applied to enhance safety when turning the boat at speed. Tuck the leg in a touch before beginning the turn to ensure the propeller and hull maintain their grip on the water. When pulling water toys or skiers, keep the bow trimmed in a bit more than usual to help the boat maintain speed in sharp turns. Once the apex of the turn is reached, add some positive trim as you straighten the wheel and accelerate away.

Be careful not to overdo negative trim when turning because the boat’s chine could grab, throwing the boat occupants about and potentially pitching the vessel end for end.

Trimming Small Boats

Adjustable trim isn’t something usually found on small RIBs, planing dinghies and sub-4m runabouts: outboard motors under 25hp seldom feature power trim and hydraulic or electric trim tabs are expensive and unnecessary on such small craft.

But small craft still benefit from trim adjustment. The easiest way to adjust the trim is to shift weight around inside it, including the bodies of its occupants. Shifting a spare fuel tank forward can help keep the bows down when negotiating a head sea; moving it back can help when running the boat down-swell. Moving your buddy to one side of the boat can keep the boat riding level in a cross wind while shifting him forward or back can greatly improve the boat’s ride.

Drive trim can be adjusted too. A pin on the outboard bracket adjusts the angle of the outboard leg in relation to the transom. The leg locks on to this pin so you can reverse the boat without the propeller climbing out of the water. Simply change the pin position to change the trim angle. Operators generally select a trim setting somewhere in the middle of the range to suit average load and water conditions.

Even small boats benefit from trim. In this case the motor is trimmed up using a manual adjustment in order to negotiate shallow water.

Practice makes perfect

All boats react differently to different trim levels and conditions, so practice manoeuvres at different trim settings while slowly increasing your boat speed and repeat them as necessary to get a feel for the boat. There will be a sweet spot: a combination of speed and drive trim where the boat feels well balanced and responsive in the conditions. Often a lessening of steering resistance felt through the wheel indicates the boat is trimmed correctly.

Trim tabs

Many larger outboard craft and virtually all planing launches have trim tabs fitted at the junction of the bottom of the hull and the transom. Conventional trim tabs are like extensions of the hull: individually adjustable plates that can be tilted down to alter the hull’s angle of attack by lifting the transom and pushing the bow down. Even when retracted, tabs effectively extend the hull’s running surface without adding substantial weight. A longer hull gives a smoother ride.

Trim tabs are used to adjust side-to-side as well as fore-and-aft trim, but don’t use too much trim tab to straighten the boat because tabs cause drag, slowing the boat down and wasting fuel.

While there are probably still a few boats around with mechanically operated trim tabs, most now use hydraulic or electric trim tabs. Fine adjustments are possible, using one tab or the other, or both together.

Interrupter/interceptor trim tabs are quite different. Moderately popular with larger launches, they are becoming more common on trailer boats too. Interrupter trim tabs work by interrupting the laminar flow of water passing over a planing hull by extending a rectangular transom-mounted plate vertically into the water flow. The interruption to the laminar flow of water lifts the transom and lowers the bow.

Interrupter-type tabs have certain advantages over conventional tabs: when retracted they disappear completely into inconspicuous housings mounted flush against the transom where they are protected from damage. Conventional plate-type trim tabs can be bent or broken by being stepped on, contacting the ground, or hitting objects in the water.

Trim tabs are particularly useful for adjusting side-to-side trim, but operators need to remember that depressing the tab on the starboard side lifts the stern on that side, which then pushes the bow down on the port side, opposite. The tab on the port side acts on the starboard bow. This is not intuitive for many people. Fortunately trim tab controls are usually labelled: port and starboard bow, up and down, or similar. It’s not too difficult to get the hang of, provided the controls have been wired the right way around, which isn’t always the case.

Trim tabs are only effective when a boat is planing. The faster the boat goes, the better trim tabs work, so make adjustments in small increments to avoid throwing the boat over on one chine or the other. Interrupter tabs work less well than conventional tabs at lower hull speeds.

Used in combination with drive trim, trim tabs help provide the safest, most comfortable ride