Ask Andrew: Trimming – it’s just plane sense. Part 2: Trim Tabs

Trim Tabs

Aug 25, 2022

Last time, we looked at outboard and stern drive tweaking to get the boat planing. Further to that we look at trim tabs as an additional means.

The larger the boat, the more difficult this becomes through raising/lowering the outboard or sterndrive on its own. Trim tabs are added to aid in the deflection of water and to allow larger boats to reach the ‘planing’ position faster. By lowering the tabs into the water, they deflect (or drag) into the water coming off the aft end of the hull, forcing the bow angle to lower. Most trim tab controls (located at the helm position) are meant to be user-friendly but can be counter-intuitive if you overthink: they are labeled ‘bow up, bow down’. By pushing the ‘bow up’ position, the trim tabs lower into the water (‘bow up’ = trim tab down), so don’t get confused!

Trim tabs can also be used to fix listing: imagine all your guests sitting on one side of the boat – the boat will likely shift, and ride lower in the water on one side. By adjusting the trim tab down on that same side, the hull will adjust itself in the water and ride slightly higher. Don’t forget to re-adjust when the guests rearrange themselves!

Trim tabs can also be used to aid in quick or sharp turns, especially when engaged in events like wakeboarding.

These systems are fairly straightforward and trimming the engine works (electrically and hydraulically) the same as trim tabs. 

Trim Tabs Diagramtrimming to get on plane

Trim tabs are adjusted by a hydraulic (or electric) ram that pushes and pulls the tabs up and down. An engine-room-mounted reservoir contains hydraulic fluid that runs through lines to the ram. Common issues with these systems include low fluid levels (caused by leaks), seized components, damaged switches or electrical connections.

Troubleshooting common issues:

1) I push the trim button and nothing happens: confirm that the electrical connections between the battery and the up/down switch are good; confirm that the hydraulic pump is receiving electrical signals to move up/down; test the pump by applying power directly to the pump’s electrical connections

2) There is a ‘whining’ noise and the tabs don’t trim up or down: check to make sure that there is sufficient hydraulic fluid in the system; confirm that there are no hydraulic fluid leaks; confirm that the motor functions correctly; confirm that the trim ram isn’t binding on anything. In the case of trim tabs, the rams themselves are the least expensive components, and tend to require replacement more often than other parts of the system.

3) Only one trim tab works – confirm that there are no hydraulic leaks (or electrical breaks), ensure that the hydraulic fluid level is full

4) The trim tabs only move in one direction – confirm that the hydraulic fluid level is full, and that the rams aren’t binding.

If you’re new to adjusting trim, trying making small adjustments at both slow and fast speeds to get a sense of how the boat will react. Different boats will change their trim at different rates depending on hull shape, sea state, tab size, and ram length. It’s a good idea to become familiar with how these changes will affect performance before making drastic changes in questionable conditions.

If you suspect that your trim systems aren’t functioning correctly, consult your local marine technician for some advice and repair.

Andrew McDonaldAndrew McDonald is the owner of Lakeside Marine Services – a boat repair/maintenance firm based in Toronto. Andrew has worked in the marine industry for 12 years and is a graduate of the Georgian College ‘Mechanical Techniques – Marine Engine Mechanic’ program.

Questions or comments for Andrew? Email him directly via:


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