What is the best way to use trim and trim tabs?  One of the most common questions we get asked are “how do I adjust the trim tabs”, and “how do I trim the leg?”. First things first, if your boat is a shaft drive, you don’t have to worry about trimming the leg. You will likely only have port and starboard trim tabs to worry about, if they have been fitted. Trimming the leg adjusts the angle that the bow sits out of the water. If you find the right angle, you’ll get optimum speed for the power used. On the other hand, trim tabs can be used both to ensure your boat stays level side-to-side and to set the bow angle.

What are trim tabs?

Most commonly, trim tabs are a metal plate attached to a hydraulic or electrically operated mechanical arm. The arm extends the plate which in turn pushes down against the water and alters the boat handling. Tabs are fitted on the port and starboard sides at the stern of the boat, and are operated from the controls at the helm.

I’ve already got power trim, what’s the difference?

Mercury Alpha Sterndrive

Mercury Alpha Stern Drive Trimmed Up

Mercruiser Sterndrive

Mercury Bravo 3 Stern Drive Trimmed Down

Power trim is the hydraulic adjustment of your stern drive or outboard, which will affect the direction of the bow. Trim down,  you will lower the bow, and cut through the oncoming water. Trim up, the bow is lifted assisting the hull achieving a faster, more efficient cruise. Excessive trim up, and one of two things typically happen:

  1. The propeller will lose “traction” and the engine will over-rev.
  2. The hull will start porpoising – which is when the bow bounces up and down as you are travelling along.

Now, just to confuse things – different brands have different features. Some to lookout for include:

Automatic trimming

Bennett Trim Tab Control

Bennett AutoTrim Pro Helm Display

A top of the range model can incorporate automatic trimming. Once activated, the system will adjust the trim tabs for you. If you have too much weight on one side, they will trim to compensate. They also position themselves down before the boat is powered up on the plane.

Automatically retracting

Linked into the ignition, your trim tabs will retract to the “up” position when the engines are switched off. This is helpful because it resets them before you head out, so you won’t have prior adjustments altering the ride.

Tab indicator

Some brands, such as Bennett, provide a dash panel with position indicator lights. This way, you can adjust them to your favourite settings every time – you can tell when they’ve been left down, and you know when they’ve reached their limit. It’s important to note that gauges like this need to be calibrated before you trust their readings, and it is usually a reading individual to the boat. For example, if your buddy has the same tabs fitted to his boat, his gauge may actually have them higher or lower on the same gauge reading.

How do you use your trim tabs?

  1. Trim your stern drive or outboard all the way down.
  2. Give your boat full throttle until it is “on the plane”. On the plane is when your boat is on top of the water, and skims across rather than pushing the bow through. Typically your bow will rise, until it’s going fast enough to “skim” across.
  3. Make minor adjustments to the stern drive / outboard trim in the up direction. Minor adjustments are achieved by pressing the up button for 1-2 seconds at a time. After each adjustment, give the boat another couple of seconds to settle and note any differences. If the trim is adjusted too far up, the propeller will lose “traction”, the boat will lose momentum, and the engine will over-rev. The aim is to find a sweet spot between the two, where the boat increases speed and the bow lifts out of the water.
  4. Once your boat is planing nicely and the power trim has been adjusted, we are ready to adjust the trim tabs. In this situation, they are used if the boat has a list to port or starboard. A list means a “lean”, so for example we have 3 passengers on the port, and only 1 on the starboard, it’s likely we will have a list to the port. Much like the power trim, minor adjustments are essential. Adjust the starboard side “down” for a couple of seconds, and then wait to see if the list is corrected. If not, further adjustment may be necessary.

The #1 rule for all trimming is to make minor adjustments. Your boat will take a few seconds to react, and too much adjustment will become unsettling.

Personally, I don’t use tabs to assist taking off, but most manufacturers suggest trimming down before powering onto the plane. This assists with getting the boat out of the hole noticeably quicker. It will be more evident if you have a full load, or the bottom of the boat is dirty.

 

Trim Tab Rocker

Bennett Rocker Style Trim Tab Switch

It is worth noting that popular manufacturer “Bennett” writes on their website that rocker switches are wired up in the reverse of what you may think. The port side tab lowers the starboard bow, and conversely the starboard tab lowers the port bow. This can be confusing initially, but will usually become second nature after some use.

What about stern drive trim gauges?

9 out of 10 times it is easier to adjust by feel. Acceleration onto the plane will be noticeably slower when you attempt it with the leg trimmed up – and it’s very common for trim gauges to fail. You must take into consideration when you are steering, that the boat will likely lean in that direction, so all trimming should be done whilst going straight.

The team at BigFishChannel on YouTube have an excellent video explaining the function of trim tabs.