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March 05, 2004

Trim Tabs - Attitude Adjustment

Trim tabs help your boat find its footing.

Tired of playing the role of onboard choreographer, telling your chubby fishing buddies to move from one side of the boat to the other in an effort to keep things on an even keel? Rather than asking your friends to trim poundage from their waistlines, try adding trim tabs to your hull. You'll get faster, more reliable results without jeopardizing any personal relationships.

Manufacturers of production and custom boats have included trim tabs as standard equipment on larger vessels for years, but these useful accessories are not exclusively for the 40-foot-and-over crowd. "Many people think of trim tabs as a big-boat item, but small boats need them even more," observes Tom McGow, director of client services at Bennett Marine. "It takes more than a few people changing places to make much of a difference on a 45-foot boat that comes with factory trim tabs. But if a couple guys start moving around in a small boat, their weight distribution really affects the ride."

The most obvious function of trim tabs involves adjusting a boat's running attitude on both fore and aft and athwartships axes. So anglers who always fish alone in a small boat don't need tabs; they can carefully distribute side-to-side weight and simply use the outboard motor's power trim to hold down the bow. Right? Wrong! Power-trim controls propeller angle for peak efficiency. The prop should drive squarely through the water and generate thrust straight back. "As you tuck the engine in with power trim to hold the bow down at slower speeds, thrust angles downward. Cranking through water at this angle increases prop slippage. It's better to bring the bow down with tabs while adjusting motor trim for optimum rpm. Using these capabilities together helps the boat run much more efficiently at slower speeds," says McGow.

Comfort and safety also appear high on the list of trim-tab benefits. Say you're in choppy seas that force you to run slowly enough to reduce pounding - but then the boat falls off plane. Trim tabs offer a solution. "I have a 20-foot walkaround with a narrow beam," McGow says. "Before I put trim tabs on it, there was a speed range that would knock my teeth out. Now I can set the tabs and engine trim to run at those same speeds with less throttle. This lets the bow do its job better: cleave oncoming waves and smooth the ride."

McGow shares a trick learned from flats-boat fanatics, whom he calls the "gurus of trim-tab use." When running into a wind that kicks bothersome spray over the bow, adjust the tabs to tilt the boat slightly. "Raise one side of the bow just enough to block the spray. It looks funny because you're using tabs to intentionally put the boat off level, but this way you don't have to alter your course due to the wind," he says.

Properly deployed tabs allow a boat to stay on plane at slower speed. Running at a lower velocity, with the bow down for unhindered visibility, adds a greater dimension of safety whether crossing skinny water in a flats boat or cruising the Intracoastal in a center console.

Size Matters
Choosing the right trim tabs for your vessel and placing them correctly on the hull assures they'll deliver maximum performance. An industry rule of thumb calls for 1 inch of tab for each foot of hull length. For example, a 36-foot boat typically sports tabs 36 inches wide by 9 inches deep. However, many other factors enter the equation when figuring which tabs to install. "Size definitely matters in trim tabs," affirms Bruce Angel, vice president of product development and design at Hatteras Yachts. "But tab size is driven by a function of boat speed. You can get by with a smaller amount of trim tab on faster boats."

The 1-inch-per-foot rule establishes a trustworthy guideline, but boat owners shouldn't feel obligated to abide in all situations. "If in doubt, go up in size. You can't go wrong picking a bigger trim tab," McGow advises. "Larger tabs are more efficient because they generate greater lift at a given angle of deflection." In other words, if two different-size tabs are lowered the same amount, the larger tab exercises more influence on the boat's running attitude. The larger tab also enjoys a more favorable lift-to-drag ratio. Deflecting a small tab sharply (lowering it farther) to achieve a desired leveling effect also creates a fair amount of drag because the tab ends up acting as a brake.

On the other hand, smaller is sometimes better. "When installing trim tabs on small boats, we're often fighting for real estate among transducers and speedometer pickups on the transom," says McGow. Recognizing that space comes at a premium, Bennett Marine developed the M80 and M120 trim tabs for boats in the 17- to 24-foot range. These models feature folded-down edges called drop fins. "Regular trim tabs let water pressure escape off the flat edges. By folding the edges down 90 degrees, we capture pressure that would be lost out the side and channel it to create more lift. We make a small footprint but generate the lift of a larger tab, which allows us to maintain effectiveness while sneaking a tab in beside a speedo pickup, for instance. Drop fins also prevent spray from shooting out the side, so we can snuggle a transducer within a couple inches of the tab with no worries of interfering turbulence," McGow says.

In a clever effort to economize transom space, Lenco Marine offers Troll'n Tabs - trim tabs with electric trolling motors attached. This setup makes silent running quite convenient and, with a motor on each tab, affords a high degree of maneuverability while stalking the flats.

Evolving Technology
Imagine a typical Northeast canyon trip for tuna or billfish. While making an hours-long run to the fishing grounds, the boat consumes significant quantities of fuel, and people move about the salon and belowdecks. To compensate for shifting weights, the helmsman must constantly monitor and adjust the trim tabs. Or does he?

Bennett Marine's Automatic Tab Control brings hands-free simplicity to maintaining proper trim. "ATC consists of a very precise attitude sensor that sends information to a computer, which interfaces with the trim-tab system and makes necessary adjustments to keep the boat at the best running attitude. This alone will help save fuel," says McGow. "ATC also comes in very handy when the boat's on autopilot or has somebody at the helm who doesn't fully understand how to use trim tabs. ATC is easy to retrofit with Bennett trim tabs or those of other manufacturers, and it's also quite effective on small boats."

If you happen to be poking around the Hatteras boatyard in New Bern, North Carolina, and check out the new 68 Convertible, you won't see trim-tab pockets recessed in the hull as expected. The tunnel-hull boat incorporates trim tabs in the tunnel itself. This engineering innovation evolved from a technique that Hatteras has been using for years: fine-tuning running attitude by carefully shaping the tunnel. "To bring the bow down, we add a small, permanent wedge to the tunnel's roof. To raise the bow, we remove material," says Angel. "These small alterations have a significant effect on a very large boat's running attitude."

The discharge velocity from the propellers provides so much extra push that a little bit of tunnel tab goes quite a long way. One inch of stroke (downward deflection) on tunnel tabs produces the same result as 5 inches of stroke on tabs mounted to conventional deadrise hulls. The unprecedented design will get the job done with considerably less range of movement as well as reduced surface area. "The new tab has a stroke of just 1Ú/@ inches, whereas tabs on our other boats have 3Ú/$ inches of stroke," Angel says. "And it measures approximately 8 inches wide by 12 long, while most 50-foot-and-larger boats use tabs that are at least 24 by 12."

Trim tabs can't do much to directly improve your fishing because they're rendered ineffectual at trolling speeds. Used properly, however, they contribute to a more comfortable and fuel-efficient ride to your favorite spot.


Bennett Marine

Bluefin Trim Tabs

Duraplane Marine Systems

Lenco Marine

Trim Master Marine