Installing Trim Tabs
There are several manufacturers that market trim tabs. Here's how to install them.
by Steve Noury
December 1, 2004
Trim tabs are everywhere. You see trim tabs on everything from bass boats to bow riders, ski boats
to center consoles. But all boats from small to large benefit from trim tabs for the same reasons -
they improve planing and fuel efficiency, correct for uneven weight distribution, and improve
speed, safety and overall boat performance. There are several manufacturers that market trim tabs,
and prices can range anywhere from $150 and up. Here's how to install them.
Step 1 -
Bennett Trim Tabs (Model
M80) are used in this example on a 17-foot walk-thru with an inboard/outboard engine. Position
the trim tabs against the transom, and check to see that the upper mounts of the hydraulic
actuators don't center on an inside obstruction. The optimal location of the tabs on the transom is
as far outboard as possible, leaving at least three to four inches from the chine (edge of the
transom). Because of the shape of the transom and several inside obstructions, one of them being
the gas tank, the boat prohibits you from putting them too far outboard. If you cannot position the
tabs wide on the transom (as in this case), at least be sure that they're a minimum of eight inches
from the center of the outboard or stern-drive unit. Once the position has been determined, attach
the mounting plates and trim plane with the supplied stainless-steel screws, making sure to leave
one inch between the mounting plate and the bottom of the hull. Don't forget to use a marine-grade
silicon sealant to waterproof the screw holes.
Step 2 - Once the trim tabs are in place, you'll use the supplied hardware to secure the
lower hinge of the hydraulic actuator to the tabs. Using a straight edge under the trim tab and the
hull bottom, set the center of the tab trailing edge to a 5/8-inch negative angle. Position the
upper mount of the actuator against the transom. With a pencil or a drill bit, mark the holes while
holding the upper mount in the proper place. Drill a 1 1/8-inch hole centered equally between the
three screw holes that you'll drill to secure the upper mount. This hole needs to go completely
through the hull to feed the hydraulic fluid tubing through it to a hydraulic power unit (HPU). The
hydraulic tubing has a brass screw fitting on one end. This needs to be attached to the threaded
hole on the mounting side of the upper mount of the actuator. Make sure it's snug, but do not
over-tighten, as you'll strip the plastic threads on the actuator. Feed the other end of the tube
through the 1 1/8-inch hole that you drilled earlier, apply sealant on the mounting surface of each
actuator, and then secure to the transom with the hardware.
Step 3 - Next, install the HPU in a convenient location with a dry environment. It's
important that it be mounted in a dry enough location to avoid submersion and drenching. Because of
the limited space in the stern storage area of the boat, the supplied mounting brackets did not
work in this example. As an alternative, some smaller brass pipe brackets and Velcro are used to
easily secure the HPU in place. Before securing into place, though, be sure to fill the reservoir
on the unit using any type of automatic transmission fluid (ATF). Once in place, attach both the
hydraulic tubes to the nipples on the HPU, which are clearly marked for both port & starboard.
When connecting these tubes, a supplied brass nut with a ferrule fitting is used. First, slide the
nut onto the tube, then push the tube by hand onto the nipple until it bottoms out. While holding
the tube in place, slide the nut up to the threads and finger tighten, and then use a 1/4-inch
wrench and tighten additionally, but with only one turn, no more.
Step 4 - Installing the control switch is the last step. The electrical wires coming from
the HPU are bundled into a plug that could be connected to an extension, and then run from the
stern of the boat, under the gunwale and behind the control panel/steering assembly. There's a
template provided to cut the hole in the control panel to mount the switch. There are directions
provided for making the right size hole by using a 1- and 1/8-inch drill bit and drilling four
holes close together to create one large hole. File off rough edges, attach the wires to the back
of the switch, and then mount the switch with the supplied hardware. The exterior snap-on plate for
the switch covers up any imperfections. Last, but not least, test the system. Using the switch
control, press the "bow down" position for 15 to 20 seconds, then the "bow up" position for 15 to
20 seconds. Repeat this process three times. This will purge any air from the system. No bleeding
is necessary. Next, place both trim tabs in the "full down" position, and check the hydraulic
system for leaks. Then, bring both tabs to the "full up" position, and check the fluid level. Add
ATF if necessary.