Attitude Adjustment

If you're not sure what trim tabs do and why they're considered performance enhancers, we have the lowdown.

CaptureYou may have seen flap-like appendages hanging off a boat’s transom and been unsure of what those plates might be used for or why. Even if you are aware of what they are and what they do, how much do you really know about them? Let’s level the playing field here and explain what benefits they offer the boater.

The flaps we’re referring to are trim tabs, and they essentially do what their name implies: trim a boat to run at a preferred pitch and/or roll (terms you may have heard because they are also associated with airplanes). Pitch is the up/down angle of the boat, from bow to stern. Roll, or list, is the side-to-side movement, across the beam. (Yaw is another dynamic in flying and boating, and is the lateral movement controlled by the rudder or outboard angle, and we’ll save that for another time.)

Trim tabs are attached to a boat, flush to the hull’s running or wet surface, at the transom. They are actuated to deflect pressure off the water that is running under the hull. Ineffective when a boat is not moving or drifting with no power applied, they require a flow of water across the bottom surface to work. They’re different than stabilizers, which are part of a different discussion.

Along with a set of installed trim tabs is typically a set of rocker switches or buttons on the dash that, when pressed, will activate the trim tabs, lowering or raising them via a piston or actuator attached to the hull and tab. Adjusting them in unison or individually will have different effects on the way a boat responds. The benefit is that the operator can improve the efficiency of the running angle, fuel economy, balance and load, and the actual safety aspect of the operation.

You may have come across a boat that’s listing a little or even quite a bit, from passengers sitting to one side, gear or provisions that aren’t properly distributed, or seas taken on the beam or quarter. Such symptoms can make a boat run stern heavy or make the bow dig due to forward weight. Trim tabs will actually help reduce (not always eliminate) listing, if the driver adjusts one tab or the other until the boat is running level. Tabs, for the most part, can also raise the stern, giving an improved running attitude, decreasing drag and softening the wake.

There are varying types of trim tab systems, so let’s explore the options and their pros and cons.

Once you understand which system is best for your application, it’s time to figure out how they help. When a boat is listing, the hull doesn’t perform as designed.

Strakes and chines are too high on one side, too deep on the other. Lift may be compromised, and so may turning ability. When the tab is actuated on the side with the list, water pressure against the down-trimmed tab pushes against the tab, forcing it up. More tab actuation will provide more lift, so ease off when the boat is level again. If people are moving around in the boat, trim may be hard to maintain.

Proper trimming will assist a boat in getting out of the hole and on plane faster, which gets the boat up and on top of the water instead of pushing it. Not only is this effective for level cruising, but it also helps to navigate in shallow water. Getting the tabs all the way down lifts the stern, potentially gaining more clearance and lessening the draft.Capture

Varying sea conditions will dictate how tabs are used. A boat driving into a head sea needs a little tab to lift the stern and lower the bow, allowing the hull to slice into the swell or waves. Don’t give too much trim, though, or you might bury the bow.

In a following sea, trim tabs should be off and fully retracted, which will help to maintain steering. If the tabs were fully extended, the force of the sea could make the stern hard to control.

When running a quartering sea, determine which side of the boat is lower as you come off a swell or wave. You don’t want to be jockeying the tab switch every time you crest a wave, so I prefer to set the tabs for the downside or back of the wave, to compensate for the dip.

Installing trim tabs is not difficult, but care must be taken, especially when drilling through the transom. The first order of business is to make sure the tabs are properly sized for your boat and for how you boat. The folks at Bennett Marine understand that varying engine configurations, the amount of weight and its distribution in the boat, the type of boat and other variables will dictate the size of tab used. But as a general recommendation, Bennett promotes at least an inch of tab span (side-to-side measurement) for every foot of length. Keep in mind that the largest trim tabs that will fit will be the most efficient.

On multiple-engine boats, there may not be sufficient space to install a tab with sufficient span. Not to worry. There are tabs that have very narrow span but are much longer. They may require more hardware, possibly a second actuator per tab due to the added force of a longer tab.

During installation, tabs are generally mounted three to four inches from the chine toward the keel. On inboard boats, the entire run can be used even if strakes are built into the hull. For outboards and I/Os, the tabs need to be kept clear of the drives, so turn the outboard or I/O fully left and right to ensure there is clearance.

The standard configuration for mounting tabs includes a mounting hinge that is fastened to the transom. If tabs are being mounted through the hull, use a backing plate and a sufficient sealant. Some tabs are mounted to the boat bottom, when there is no transom space. They require a flat bottom with no strakes or protrusions.

Boatyard-Bottom Paint pics 094TABS VS. POWER TRIM
Your boat may have power trim on the outboard, giving an operator the ability to trim the engine — that is, to trim the direction of the propeller. And many folks think that if they have power trim they don’t need trim tabs. Understand that a propeller’s job is to force the boat forward; trim tabs trim the hull’s attitude. Trimming the hull with the power trim is highly inefficient, as it can cause prop slippage and reduce fuel economy and performance. Power trim also can’t do anything to correct listing.

The proper way to use both systems, and to maximize economy and performance, is to adjust the trim tabs to level the boat to a planing attitude and then adjust the power trim to maximize the efforts of the prop by trimming it parallel to the water.

On most any boat, trim tabs can be a beneficial system to have in order to maximize the vessel’s efficiency and performance. They can also make the ride more enjoyable and safe. But do your homework to determine what is best for your vessel — hydraulic or electric, the proper size and the final cost.

Much more information regarding trim tabs and interceptors can be found on these major manufacturers’ websites.

• Bennett Marine,
• Humphree,
• Hydrotab Marine Engineering,
• Lenco Marine,


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