All about Trim, so necessary, yet so little used

Nicolas Venance - Boat Technical Journal

Tags: motorboats, Mercury, Outboard motor, Trim

The Trim is a trimmer installed on powerboats equipped with outboard motors or Z-Drive bases. It allows you to change the boat's trim, thus increasing or reducing the wetted surface area of the hull, for greater performance or comfort, depending on speed and sea conditions.

What is Trim?

Technically, the Trim is a large hydraulic cylinder located on the mounting bracket of an outboard motor, or on the output of a Z-Drive baseplate, which allows the motor propeller to be moved away from or closer to the boat's transom.

Trim is controlled by a switch located on the throttle lever. Pressing the "Up" position moves the engine away from the transom (raising the bow); pressing the "Down" position moves the engine closer to the transom (lowering the bow).

Why Trim?

Using the Trim de facto means changing the propeller thrust angle, and therefore adjusting the pitch of the boat in motion. Trim is used in the various phases of navigation: lift-off, cruising speed. Depending on sea conditions, it can improve performance, fuel consumption and comfort.

Trim in calm seas

When starting out, trimming in the negative (bringing the base closer to the transom), will help lift the boat off the water, increasing lift and taking advantage of the hull's qualities. Once underway, trimming in the positive direction allows the bow to rise, reducing the wetted surface at the rear third of the hull, and boosting speed.

In calm seas, the desire to gain speed can encourage the yachtsman to work to the limit, to the point of venting the propeller! But this temptation must be curbed. Firstly, because the ventilation effect of the propeller reduces speed, but also, and above all, because depending on the boat's hull, it can quickly tend to pitch at the slightest chop, and in particular the wakes of other boats...

Caution: a positive trim setting will cause the propeller to fan out during a turn. It is therefore advisable to return to a neutral trim setting at the start of a sharp turn.

In calm seas, a good trim setting means lighter steering (except for hydraulic steering!), neutral helm (the boat pulls neither to port nor to starboard), and a very stable boat that has found the right balance between speed and stability.

Trim in rough seas

Trim is particularly useful when sailing in heavy seas.

In seas with tailwinds, the boat tends to sink. Trimming positively (raising the bow) relieves bow pressure and raises the boat's nose.

In seas and headwinds, the bow has a tendency to rise and the boat to slam into the rear of the hull. You therefore need to trim in the negative (lower the boat's nose), to allow the front of the hull, the V of the hull, to work and attenuate the boat's fall.

Using Trim requires real care

Using your boat's trim system requires real attention and a real apprenticeship on the part of the yachtsman, rather like a sailboat skipper who constantly monitors and fine-tunes his sails.

These conditions may explain why so many motorboat owners make little or no use of it. Yann Le Fillastreparts and accessories manager at Mercury Marine France : "Today, the majority of yachtsmen make little or no use of the trim on their motorboats, with the direct consequence of a sharp increase in fuel consumption, particularly before lifting off. This lack of knowledge is mainly due to the fact that they have generally not been taught how to do it. Trimming in itself is not complicated, but requires constant attention, depending on the points of sail and the fact that you're trying to keep to the air limit".