How To Install Trim Tabs

By Roger Marshall

Is your boat slow to plane, and difficult to keep on plane? Adding trim tabs can solve that. Here's how to install them.

"How's your trim?" I was asked many years ago when setting up a racing boat.

"No idea," I replied, not quite sure if I was being asked whether it was myself or the boat whose trim was in question. "Was it level, down by the stern or down by the bow?" Now, I understood the question. Though at the time I'd no idea that trim affects performance.

Photo of boat floating stern downThe author's boat floats stern-down, as indicated by the white boot stripe,
after he mounted a heavy four-stroke outboard.

Trim is the fore-and-aft "attitude" of the boat. A boat that is down by the stern, meaning its transom is deep in the water, is slow to plane and difficult to keep on plane. It also causes a large wake when in displacement (non-planing) mode. For example, wake-boarding boats are deliberately trimmed down by the stern to generate a large wake. Stern-down trim is usually caused by too much weight aft. With the current generation of four-stroke outboard engines replacing lighter two-stroke engines, many older designs tend to trim down by the stern.

When a boat is down by the bow, it also has difficulty getting on plane. It tends to push water in front of it, and might have a very wet foredeck. In a seaway, trim down by the bow can be dangerous. Bow-down trim is often caused by installing anchor chain too far forward in a boat not designed to carry it. When you figure that an anchor for a 30-foot (9.1m) boat can weigh 25-30 pounds (11 kg to 13.6 kg), that a windlass weighs about 40 pounds (18 kg), and that 200 feet (61 kg) of 3/8" (9mm) chain weighs 340 pounds (154 kg), you can see how easily this problem arises.

Locating this much weight forward in the boat is equivalent to having two large adults permanently standing on your boat's bow. When such a boat gets onto a plane, the extra weight forward tips the bow down and the boat comes off plane, which brings the hull back into balance and lets it get onto a plane again. When this motion continues and results in a series of on-off plane performance, the boat is porpoising, which can only be eliminated by moving weight farther aft until the boat trims level, both in displacement and planing mode. This effect can be reduced by trim tabs and wedges, discussed below.

To maximize trim, consider where to stow all heavy objects, such as batteries and anchors. The best practice is to concentrate the heaviest weights near the hull centerline. Never try to balance a heavy weight in the bow with another heavy weight aft. Suppose you add anchor chain and a heavy anchor to the bow on a boat not designed for them, putting the bow down by a considerable amount. Then, you mount the dinghy aft on the swim platform to balance the increased weight forward. Not a good idea. You've now hung heavy weights off both ends, resulting in a see-saw effect, producing another unpleasant and potentially dangerous motion that lessens your enjoyment of the boat.

On The Level

So, how do you ensure that your boat will float level? The easiest way is to put it in the water and adjust gear until you get level trim. What happens when a new boat is launched and it floats out of trim? In the old days, builders would surreptitiously launch the boat before the official launching date, check the trim and boot stripe level, haul the boat, correct any trim problems by putting strategically located lumps of lead in the bilge, repaint the boot stripe in the right place and then launch the boat again for the official launching. Today, designers use sophisticated computer programs to determine exactly where the boat should float. Still, builders surreptitiously launch the boat, check the trim, and correct any trim problems.

Entering Wedges

If you have an older planing boat and stick a heavy four-stroke outboard motor on the stern, your boat will probably float like mine does, stern down. While this is not a major disadvantage on a small boat, which tends to have more horsepower than needed, it's a slight disadvantage when it comes to getting a larger boat on plane.

The best approach is to adjust the trim angle. According to Lindsey Lord's book, Naval Architecture of Planing Hulls, the ideal planing angle is around 3-5 degrees of bow-up trim. There are several ways to obtain this angle. The easiest is to move crew around until the boat is trimmed properly. Another way to get the ideal planing angle is to install a wedge at the stern. This is a transverse wedge running across the boat bottom at the transom. Its size depends on the size of the boat. For a 24-footer (7.3 m), it might be only a 1/2-inch (12 mm) deep at its thickest (trailing) end and 18-inches (46 cm) long. There are no hard-and-fast rules for wedge size. You need to sand or shave thickness off the wedge to get it to work at its optimum potential.

Illustration of how to determine the length and depth of the transverse wedgeIt takes trial and error to accurately determine the length and depth of a transverse wedge.

One story tells about a powerboat around 90 feet (27.4 m) that had a wedge fitted. It was hauled and launched several times while the architect measured trim at acceleration. Once the job was done, the boat ran for years at a perfect trim angle.

Wedges work best when the boat runs at a constant speed. However, most powerboats don't run at a constant speed, so most people resort to installing trim tabs, which allow the planing attitude to be adjusted while the boat is underway. For example, if the boat trims up by the bow as it transitions onto a plane, the tab is cranked down a little more to adjust the trim angle until the boat is at its optimum trim attitude for the speed run. If there is a crosswind, the tab on one side can be lowered slightly to compensate for the slight heel that the crosswind creates.

Trim tabs come in a number of different sizes depending on the boat size, type, and speed capability. Smaller, slower boats tend to have wider and shorter tabs. The way to get the best size tab for your boat is to talk to the tab manufacturer. Most tabs are operated by means of an electro-hydraulic control. When you press the control switch or toggle a joystick, an electrical pulse is created that causes a hydraulic pump to turn. This, in turn, causes the hydraulic actuators to raise or lower the tab. I predict that one of the next great steps we'll see in the electronic-hydraulic control systems is an automatically adjusted trim tab. Just dial in the optimum trim angle for your boat and the tabs will automatically sense the boat's attitude and set the tabs accordingly.

Long narrow tabsLong narrow tabs used on high-speed power boats.

Installing Trim Tabs

Trim tabs need to be located fairly precisely — about 3-4 inches (7.6 cm to 10 cm) inboard from the edge of the hull chine and around 1/4 - 3/8 inch (6 mm to 9 mm) up from the bottom of the hull. The farther out the tabs are from the hull centerline, the more effectively they operate. Before installing tabs, make sure that the outboard turns fully without hitting them. I installed Bennett trim tabs on my SeaCraft and the manufacturer recommends a minimum of 8 inches (20 cm) from tab edge to the motor centerline.

Bennett tab kits contain the hydraulic fluid and everything you need to complete the installation. Installed first is the backing plate. Using the backing plate as a template, mark the hole locations and check the hull inside for any obstructions. (Alternatively, drill a small pilot hole.) If you are drilling into solid fiberglass (uncored), drill the correctly sized hole. If you're drilling into core material, I'd advise drilling an oversized hole, filling it with epoxy resin, then drilling the correct-sized hole when the epoxy has set. [Ed: This procedure is known as the potting technique.] This forms a barrier between the fastener and the core to prevent water from migrating into the core. All screw holes were touched up with the countersink bit that makes a small recess in the gelcoat. This prevents splitting of the fiberglass and allows caulking to form a small doughnut that helps to prevent water ingress.

Mounting the actuator to the tab plate is a simple bolt-on job. Install the backing plate, tab, and hinge plate with the actuator. Now, you have to make some careful measurements. The best way to do this is to set a straight edge against the hull bottom running fore and aft. Measure the distance from the straight edge to the backing plate at the transom. It should be about 1/4 inch (6 mm) to 3/8 inch (9 mm). Set the aft end of the tab at 5/8 inch (15 mm) to 3/4 inch (19 mm) above the straight edge.

The easiest way to get the tab correctly located is to cut a 5/8-inch (15-mm) block and position it at the aft end of the tab on the straight edge. Now, place the actuator against the hull. Bennett recommends that you slide the template sheet behind the actuator mounting and tape it to the hull. I didn't. I marked the three holes in the top pad of the actuator and drilled them out very gently. I hate drilling holes in my boat.

Locating proper trim tab placement, per the Bennett installation guideLocating proper trim tab placement, per the Bennett installation guide.

The next job is to drill the center hole for the hydraulic tubing that connects to the actuator. Bennett recommends drilling a 3/4-inch (19-mm) hole in the transom and filling with sealant. I drilled a 5/16-inch (8-mm) hole first and then made a 3/4-inch (19-mm) hole deep enough to hold the 7/16-inch (11 mm) locking nut that holds the hydraulic tubing in place. I just didn't like the idea of drilling a 3/4-inch (19-mm) hole all the way through the 2-inch (5-cm) thick transom. At this point, I ran the hydraulic tubing from the center console to the actuator and connected it. All that remained was to screw the actuator to the transom with the #14 x 1 1/2-inch (38-mm) screws provided. The hydraulic tubing cannot be bent at 90 degrees. It has to be formed very carefully around corners to avoid kinking.

Next, install the hydraulic power unit. It comes with a bracket and all you do is screw the bracket to the hull or console and slide the unit into the bracket. You'll need about 3 inches (7.6 cm) above the bracket to slide the unit into it. Place this unit in a dry location where it won't get drenched or submerged. Run the ground wire to the nearest grounding point and connect the hydraulic tubing by tightening the 1/2-inch (12-mm) nuts. Use a wrench to torque them up snugly. Fill the reservoir with the automotive transmission fluid contained in the kit.

Now install the rocker switch by drilling a 1-inch (25-mm) diameter hole and cutting it square to fit the switch. It was wired so that the bow-up and bow-down modes worked properly according to the manual. Finally, purge the hydraulics by setting the tabs to the maximum down angle, letting them stay down for 15 seconds, then raising them to the maximum up position for 15 seconds. Once you've done that a couple of times you should have purged all the air out of the system. Now refill the fluid level in the reservoir.

That's it. Put the boat in the water and enjoy your new tabs. When accelerating onto a plane, bring the tabs down a little to help trim the bow down and get the boat on plane faster. Trial-and-error operation will help you find the optimum trim angle to get your boat onto a plane quickly. When running at speed, the only reason to use tabs is to keep the boat's bow a little lower so the trim angle stays around 4 or 5 degrees. If your boat naturally trims at that angle, you won't need much in the way of tab angle. If you're in a heavy crosswind, lower the tab on that side of the hull and bring the boat back to level. 

This article was previously published in BoatUS Magazine in January 2009 as "The Trim Truth."

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