Virtually any type of powerboat can be affected by adjusting the trim, either with tabs or with an outboard or stern drive. Manipulating the trim can improve top speed, efficiency and overall ride.
Trim 101. For powerboats there are two ways to adjust trim, either with the engines/drives or with trim tabs. Most outboard motors and stern drives have power trim that lets a user change the angle of the thrust by tilting the drive or motor in or out to change the running angle. A boat handles best when it’s running parallel with its at-rest waterline.
Tabs for All. In the past, trim tabs were most often found on cabin cruisers and sport-fishers over 25’ (7.62 m), but today they have become common on boats in the 17’ (5.18 m) to 25’ (7.62 m) range. They’re becoming more prevalent on center consoles, bay boats, even tow sports craft. On a smaller boat, tabs can have a big impact on speed, planing, fuel efficiency, and overall boat performance. Remember that tabs can only lower the bow and raise the stern. They cannot raise the bow.
Double Check for Accuracy. With the engine(s) in neutral, use the trim switch to tilt the stern drive or outboard down as far as it will go. Monitor the trim gauge to make sure that it is accurate. Note the change in sound when the engine is fully trimmed down. It goes from a louder whine to a quieter one. The gauge should also read all the way down.
Level Headed. Grab a level and place it against the underside of the anti-cavitation plate on the drive or outboard motor. Have someone at the helm adjust the trim until the instrument reads level and note the position on the indicator. Next, use the level to check the alignment of the tabs with the bottom of the boat. When the drive is level and the tabs are in line with the running surface, this is known as neutral trim.
Now Let’s Head Out. Start by putting the engine into forward gear and throttle up steadily to cruising speed, noting the reading on your speedometer, GPS, or tachometer. Using the trim switch, slowly tilt up the engine or drive. There should be a slight bow rise and the boat should feel like it’s riding more freely. A speed increase will also be noted.
The Sweet Spot. To find maximum efficiency, advance the throttle to wide open and continue to trim the outboard or drive until a drop in speed and a spike in rpm is noticed. When a drive or motor is over-trimmed, the propeller will cavitate or ventilate. The boat may also begin to hop or pound in a rhythmic pattern known as porpoising. Trim back down in small increments until the maximum speed and rpm are reached. This is the boat’s most efficient trim position for running top speed. Use this technique to find the most efficient trim at any rpm when the boat is on plane.
Wave Walking. Of course, a boat needs near-perfect conditions to run top speeds, and we rarely see that opportunity, so trim can also be used to adjust for waves, chop and other boat wakes. Trimming down the bow in chop should decrease pounding from waves, but can also increase the amount of spray. When running head-on into the waves, trim the tabs to where they are neutral with the bottom to extend the running surface.
Trim the drives up a little at a time to make the best forward headway while keeping the boat’s attitude level and the ride comfortable. Running in following seas usually requires a little more positive trim to keep the bow from stuffing into the back of the wave ahead. When the waves are coming at a boat from the side, lowering the tab or drive opposite of the wave direction will level the ride.
Trimming the Angle of the engine's thrust too far down (drive in) picks up the stern and forces down the bow. When the boat is running, the bow feels like it’s plowing through the water in a condition called "bow steering." The vee or entry of the bow digs into the water and makes the boat difficult to control. A boat that has its thrust angle trimmed too far up (drive out) will cause the stern to squat and the bow to rise. This creates problems with hull pounding, also known as porpoising, and visibility forward.
Inherent Elements. There are some traits in any boat that will affect trim. How much water and fuel does she carry? Where are the tanks located? On a smaller center console or bay boat, if extra batteries are added, should they be in the console to keep things balanced? For example, in performance boats, the ideal installation for fuel tanks and engines is to have them as low as possible in the hull because it has the minimal effect on the boat’s center of gravity and attitude.
Task Oriented. On single-outboard-powered fishing boats such as a bay model or skiff, the captain will use tabs to help the boat plane off and smooth out the ride in chop. Larger cuddy-cabin cruisers or center consoles have a deeper V design and could be more susceptible to listing because of weather conditions or unbalanced loading, so trim tabs can be used to level the boat. Tabs on a family runabout let the boat pull more or heavier skiers and can help the boat plane at lower speeds, which is good for wakeboarding and wake surfing. Lowering the tabs also brings down the bow, improving forward sightlines.
Improved Efficiency. A boat burns fuel more efficiently when it’s on plane and trim tabs can help a vessel maintain plane at a lower rpm. With trim tabs providing additional lift, the amount of throttle required to stay on plane is reduced, which saves fuel. Bennett Marine claims that some boaters have saved as much as 30% by adding its trim tabs.
Hydraulic vs. Electric. There are two basic types of trim-tab systems—hydraulic and electric. The best-known manufacturer of hydraulic systems is Bennett Marine (www.bennetttrimtabs.com) and the company now has an electric line as well. The system consists of an electric pump and fluid reservoir actuating a pair of independently operating hydraulic rams that move the tabs up and down. The captain controls the tabs with switches at the helm. Lenco Marine (www.lencomarine.com) offers electric trim tabs in a system that consists of two electric motors, one inside each of two independently operating rams powering screw jacks that move the tabs up and down, also controlled by switches at the helm. Both systems have proven themselves and can be added to most boats. Consult the manufacturer’s websites for sizes and pricing.