Revised by BoatUS editors in June 2012
Planing boats normally achieve their best possible running attitude at or near full speed and lightly loaded. As the boat slows from maximum speed it will begin to settle at the stern or "squat". And weight can vary extremes by adding passengers, fuel, and gear. Uneven weight distribution makes the boat harder to handle, difficult to get up on plane, and usually results in some listing to port or starboard.
Large and small boats benefit from trim tabs for the same reasons—they get the boat out of the hole and on plane quickly, they correct for uneven weight distribution (although the skipper should strive for even weight distribution at all times), improve speed, safety, and overall boat performance. A common complaint is "I only have two speeds—trolling and full throttle." With trim tabs a boat can often perform better at intermediate speeds while staying on plane.
This is a very general discussion and your boat, motor and other gear may require different methods, product types and use. Always follow the instructions and recommendations of the manufacturer of your boat and equipment and defer to them if there is anything inconsistent below.
The Benefits of Adding Trim Tabs
- Performance — Trim tabs increase speed, reduce pounding, some may correct listing (although the proper method to correct listing is by proper weight distributiona), eliminate porpoising and offset prop torque.
- Efficiency — Reduce fuel consumption, reduce engine laboring, and eliminate squatting.
- Safety — Improve visibility, reduce wake size, improve handling, and reduce hull stress.
How They Work and What They Do
Trim tabs usually consist of two adjustable stainless steel planes mounted at the transom of the boat. Controlled by a hydraulic power unit, the tabs can move up and down when activated by the boater.
Think ailerons and elevators on an airplane. Trim tabs do much the same thing for a boat. They provide lift in order to compensate for changes in speed, weight distribution, and water conditions. When they are deflected downward, the water force on the trim tab creates upward pressure, raising the stern and reducing hull resistance. The surface area of the tab, the angle of deflection, and the speed of the boat all contribute to greater lift.
Getting on Plane
In order to get on plane faster, boaters often have to ask passengers to move forward. With the additional lift from the trim tabs, the boat will spend less time operating in the inefficient transition period before planing. The engine labors less, the boat gets better fuel economy, and passengers can sit where they should for even loading.
Because trim tabs are mounted on both sides of the vessel's stern and can be operated independently, the vessel’s side to side trim may be controlled by deflecting one side more than the other. This independent control of the tabs is essential in correcting for port or starboard lists, although it can be dangerous if not properly used. Correcting for list should be achieved by proper weight distribution.
Trim Tabs and Trailered Boats
Smaller boats will feel the benefits of trim tabs to an even greater extent than larger boats. Bass boats with high-pitch props can use the additional lift for an improved hole-shot. When pulling heavier skiers or multiple skiers, trim tabs provide the lift without the need for additional horsepower.
Things to Consider When Buying Trim Tabs
Installing undersized trim tabs is one of the more common mistakes. A tab that is too small will have to be deflected more in order to create sufficient lift. Just remember, within reason, the larger the trim tab, the more lift it will produce with the least amount of drag.
As a rule of thumb, the largest trim tab that will comfortably fit on the transom will be the most efficient. Some recommend that you choose about one inch of trim tab span for every foot of boat length. For example if your boat is 19-24" in length, you'll want a trim tab with an 18-24" span.
Installing Your Trim Tabs
Proper placement of trim tabs will ensure optimum performance. Trim tabs should follow the "V" at the junction of the transom and the bottom of the boat. For maximum side to side control, trim tabs are generally mounted 3-4" from the chine and run towards the keel. With inboards, the complete run from chine to keel may be utilized if it is an unbroken span of the same angle. Protrusions, such as strakes, may be bridged. On boats powered by inboard/outboards, it is necessary that the trim tabs not be placed too close (8" minimum) to the lower unit(s) to avoid disturbing the water flow to the propeller. Because of many potentially serious performance issues relating to improperly installed trim tabs, it's best to leave this job to a qualified professional. His advice, and that of the manufacturer, may differ from this information because of the particular boat involved, the product and other considerations. Defer to that advice.
Enhancing Trim Tab Performance
There are a number of trim tab accessories on the market that will enhance the performance of the system. While it may mean spending a few extra dollars, some of these devices will not only make them easier to use, they may save you money.
- Automatic tab retractors: If you forget to raise your trim tabs and trailer your boat, you run the risk of damaging your tabs. The same goes with boatyard hoists and front-end loaders, they can all cause damage if you leave your trim tabs in the down position. An inexpensive tab retractor automatically retracts the trim tabs when the ignition is turned off. This device installs easily and may pay for itself when you consider the cost of replacing a trim plane assembly.
- Trim Indicators: After you've used your tabs for a while, you'll want to know exactly where the tabs are at any given time. Are they up, down, or anywhere in between? Trim indicators are a great accessory for anyone with trim tabs. They're precise, relatively inexpensive, and the average boater can install them in about two hours.