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How to fit trim tabs on a power boat

Modern trim tabs for boats have been around for over 50 years. Charles Bennett developed the first adjustable trim tab unit, and the first hydraulic system came on the market in 1966. Electric systems have been around almost as long, although modern electronics and composite materials have driven the price down and the quality up.

Fitting trim tabs is a relatively simple job for a competent handyman, although there is something about drilling holes in your transom that sends a chill up a boat owner’s spine. However, as long as you take your time and double-check everything before touching the drill, you should be fine.


How to select the right trim tabs

So, having decided that you need trim tabs, the first question is whether to go hydraulic or electric. Each type has its pros and cons. Electric tabs are simpler to install, since the actuators that move the tab up and down contain all the moving parts inside them.

Hydraulic systems have a separate hydraulic pump mounted inside the boat, and pressure tubing connects the hydraulic actuators to the pump. Electric tabs are marginally cheaper, but the fact that the electric motor and gears are mounted in a component that is exposed to salt water means that they may have a shorter lifespan.


Where to fit them

Installation of both types is similar as far as the actual trim tabs are concerned. These should be located as far out from the centerline of the boat as possible, just in from the chines. It doesn’t matter if they cross a planing strake, or are at an angle, as long as the flat part of the tab lies parallel to the flat part of the hull. The exact location will depend on what is inside the hull — a fuel tank or bulkhead may necessitate positioning the trim tab closer to the centerline.

Once the position of the tab is chosen, it pays to check where the actuator mounting holes will be. These can be moved up or down to a certain extent, but check that the interior of the transom is clear for the cable or hydraulic hose to come through at that point.


Attaching the trim tab hinge plate

The first step is to attach the hinge plate for the tab. Trim tab kits come with instructions, but generally the hinge plate is mounted around 6-8mm above the hull edge. It may be best to hold a straight edge against the hull while positioning these. This ensures the tabs are completely clear of the water when not in use, but close enough that a slight adjustment will bring them into play.

After checking (again!) that the top of the actuator will be in a clear area on the inside of the transom, mark and drill the holes for the hinge plates. Use appropriate adhesive sealant (specified for below waterline) in each of the screw holes.

A product like 3M’s 5200 Adhesive Sealant or Sikaflex is good. Do not use conventional silicone sealant. Fit the hinge plate, the backing plate and the trim tabs and tighten it up. Check the tab moves up and down through its full range. Do this on both sides.



Now mark the top mounting point of the actuators. These should be positioned so the trim tab is just off the horizontal, tilting up slightly. Use the straight edge again along the bottom of the boat to check you have the right amount of tile. The rear edge of the trim tab should be about 15mm above the line of the hull bottom. Follow the manufacturer’s instructions as different size tabs have different measurements. Mark the position of the actuators on the transom with a marker pen.

Finally, use the template that came with your kit (or, if fitting second hand tabs, download a template from the manufacturer’s website) and mark the holes that are needed. After again checking they will be clear on the inside, drill the four holes that are required (three for mounting screws, one for the cable or hydraulic hose). Fit the cables or hydraulic tubes to the actuators, feed them through the holes, then use sealant again on the holes and screw the actuators in place. The exterior fitment is now finished.


Hydraulic pump

If you have a hydraulic unit, find a suitable location for the pump unit, usually in the transom area. Make sure the hydraulic tubing will not be kinked or bent in any way. The pump unit needs to be mounted horizontally and secured so it won’t bounce around, and you need to be able to access the fluid reservoir. Run the tubes from the tabs and connect them up to the pump, then fill the unit with the specified hydraulic fluid.


Trim tab wiring

The final step is the wiring, and most kits have a wiring harness that you simply feed through to the dash area. Follow your existing boat wiring (usually somewhere under the starboard gunwale) and feed the end of the wiring harness to the back of your boat’s dashboard.

You will need to decide where to place the control switch. For most units you will need to cut a square hole through the dashboard, and a bit of thought can make this job easier. If you can get a jigsaw to the chosen spot it becomes a simple matter, and this may require the removal of the steering wheel. Make sure the rear of the dash at the chosen spot is easily accessible, and again use the template provided by the manufacturer to mark the hole to be cut.


Cutting a hole

Before actually cutting into the dash you will want to protect the surface from being scratched by the jigsaw’s faceplate. One trick is to cover the surface with tape (marking tape works well), to an area at least 10cm all around the area to be cut out. Mark the cutout shape onto the tape. Use a drill to make a 3mm hole in each of the four corners of the cutout, then a 10mm hole anywhere in the middle of the space. Use the 10mm hole to start the job, cutting towards the edge lines, and then run along the line.

Once the hole is cut, sand the edges lightly, clean up the mess and remove the tape. If you’ve done it right the rest of the dashboard will be completely unblemished. Now fit the control switch for the unit. Depending on your model this may require screw holes in the front. Often there’s a mounting plate with nuts attached from the back.


Connecting the wiring

You’re almost done. Connect the wiring according the instructions, and connect a correctly fused and switched 12V supply to the unit. If you have an electric unit, the job is now done. Now you just need to test it and make sure the tabs go up and down correctly.

If you have a hydraulic unit the final stage is to bleed any air. On most models this is automatic — simply operate the tabs to their full down and up position three times. Leave them in the full down position and double check the hydraulic system for leaks. Finally, return them to the up position and top up the hydraulic fluid if necessary.


Test your DIY trim tabs

The last test of your DIY trim tabs is on the water, where you should experiment with their effect. Take a couple of people with you, let them sit on one side and then use the tabs to bring the boat back to a level riding position. Remember that the tabs only have an effect when underway — they do not affect the boat at rest. Similarly, experiment with the bow-up and bow-down positions as well as acceleration and low-speed planing. You should notice a difference in the performance of the boat.


This information is designed as a guide only. Norman Holtzhausen is a professional super-duper handyman-type guy. Always seek the advice of a professional if you’re not 100 per cent certain about anything.


Originally published in TrailerBoat #277, December 2012. Why not subscribe today?