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October 26, 2001

Attitude Adjustment

Don't think of trim tabs as a panacea for a poor boat design, improper load distribution or a bad engine mount.

One attribute most fishing boats share with their captains is the need for an occasional attitude adjustment. Shifting winds, rough seas and changing load conditions can all play havoc with a vessel's entry angle while under way, making for a potentially rough, wet and fuel-hogging journey to and/or from the fishing grounds.
The prescription for dealing with such maladies? A minor trim-tab adjustment can often go a long way toward improving the efficiency and comfort of your vessel's ride. Tabs operate by lifting a vessel's stern, affording the operator the ability to keep the chines level (side to side) in a crosswind to reduce spray and pounding, or to keep the bow down in a tight chop.
The sharpest area of any hull's point of entry occurs at the stem - where the left and right sides of the boat meet roughly between the bow and the waterline. That entry sharpness blunts and deadrise decreases as you move aft along the hull. A planing hull under way meets or cuts into the water about one-third of the way back from the bow.
The stem's sharp edge slices through oncoming waves more gently than the blunter bottom farther aft. So logically, adjusting trim tabs to reposition the boat's bow lower to cut through the water will reduce pounding, hull drag and engine laboring. Reduced pounding means greater comfort for you and your crew. Lowering the bow also creates improved visibility forward, which increases safety while under way.
Smart fishing captains also know tabs can create more white water in your vessel's wake when trolling, helping to bring more fish into your spread of baits or lures.
But don't think of trim tabs as a panacea for a poor boat design, improper load distribution or a bad engine mount. Trim tabs won't lift the bow of a vessel that runs inherently "bow heavy," and trim tabs will only treat the symptoms of a vessel that constantly lists to one side; they won't cure the disease.
Trim tabs haven't changed much since Charles Bennett invented them back in 1959. But whether you run a 17-footer or 70-footer, they still represent one of the best investments you can make to improve your sport-fishing rig's riding characteristics. Here's some more information about those magical, rectangular transom plates to help you determine if they're the right choice for your favorite fishing chariot.
How They Work
Bennett Marine pioneered trim-tab manufacturing. Director of marketing Nelson Wilner offers this explanation: "Trim tabs are two independently operated, adjustable afterplanes attached to the bottom edge of your vessel's transom corners. As the helm control is pressed, a hydraulic pump moves the actuating cylinders to deflect the trim tabs either downward or upward via hydraulic pressure, redirecting water flow at the transom corner junction. When a tab is moved downward, the water flow creates an upward pressure under the plane, raising that corner of the stern. As the stern rises, the bow on the opposite side is lowered. This opposing, or 'crossed-X,' effect is a normal trim-tab operating procedure."
In simpler terms, on most boats the starboard control button activates the port trim tab, which raises or lowers the port stern corner, subsequently moving the starboard bow in the opposite direction. Conversely, the port tab button controls the angle of the starboard trim tab, causing the pressure of redirected water flow at the starboard transom corner to similarly raise or lower the port bow. With properly wired trim tabs, the operation of the helm control is usually based on the direction you wish to move the bow; i.e., press up to raise the bow.

Matching Tabs to Your Boat
Because trim-tab surface area is critical to efficient performance, having the proper size planes on your vessel's transom is important. Bennett Marine suggests an inch of tab width per side for every foot of boat length on boats over 30 feet in length, with a 9-inch chord (depth) for fast boats and a 12-inch chord for slower, heavier craft. For example, outfitting a 50-foot inboard planing hull for a set of trim tabs would dictate a pair of 50- by 9-inch tabs as optimum.
Obviously, some narrow 24-footers with 8- and 8 1/2-foot beams and twin outboard installations will prevent you from installing a set of 24-inch-wide tabs per side. But at the speed these vessels travel, you can get away with a smaller plane area.
However, keep in mind that relying on the depth of the trim planes to make up for insufficient width can have a downside. Wilner says smaller, undersized tabs must angle much farther down into the water to have the same trim impact as a wider, more efficient trim plane. Of course, tabs too far down in the water act as speed brakes, or worse, as rudders, with neither result being desirable.
So what's the ideal solution for a transom sporting a pair of outboards or stern-drive units that limit the amount of tab-mountable transom space? Bennett Marine makes a trim plane configuration for just this situation that they call a "drop fin." The drop-fin tab, as the name implies, features a lip or edge that extends down a few inches on each side of the stainless-steel plate. By capturing the water flow that would ordinarily escape off to the sides of the tab, drop fins convert the trim tab's outflow of water into upward lift. Although this configuration helps to maximize the efficiency of any given tab size, the efficiency gain decreases as the tab size increases. The 12-by-12 tab size is the most efficient in the drop-fin format, and Bennett engineers stress that drop fin tabs should only be used in limited-transom-space situations.
Various trim tab manufacturers offer tab planes in 12-inch, 18-inch, 24-inch, 30-inch, 36-inch - all the way up to a whopping 72-inch width. Although many boatbuilders offer trim tabs as an extra-cost option on their fishing craft, some enlightened builders like Grady-White, Pursuit, Mako, Hydra-Sports and a few others include them as standard equipment. But beware: Be sure to get the largest tab that's right for your boat if the builder or the dealer give you this option. A properly sized tab creates the required amount of water pressure deflection in each corner of the transom to control the bow's attitude, but will accomplish this with a minimum of drag and without frighteningly dramatic steering or boat attitude reactions.

Just about anyone who's handy with common tools can install a set of aftermarket tabs. But if you're all thumbs, let your local marina's service yard do the job.
Either way, the operation should take 2 1/2 to three hours to complete, and the systems can be purchased through most marine wholesalers and discount catalogs.
Each installation will vary a bit, but generally you should mount the tabs at least 3 to 4 inches inboard from the outside edge of the chines and at least 18 inches outboard of the vessel's centerline.
Having gone the aftermarket tab route a few times, I would recommend purchasing a boat with factory-installed tabs whenever possible. The builder can accomplish a top-quality installation easily while the boat is being assembled.

Improve Your Fishing Too
Can trim tabs help you catch more fish? Absolutely. Some vessels, like 21- to 25-foot single-outboard and stern-drive-powered rigs, may not produce enough white water to attract pelagics like tuna and billfish into their boat's wake. This can really frustrate a captain and crew trying unsuccessfully to troll them up with lures or bait, while surrounding boats can't seem to keep these voracious game fish off their hooks. By dropping both aft trim planes into the extreme bow-down position, you increase the wetted surface of your boat, which will create additional turbulence and white water, as well as slow your boat speed a tad in the process. On the other hand, too much white water can be a big negative. Remember to leave a few clear water channels, so the fish you attract to your vessel's wake can find the lures and baits skipping along behind your boat.

A Boatbuilder's Lie
"A boat with power trim on its outboard or outdrive doesn't need trim tabs." That common misstatement comes from boatbuilders who don't offer tabs. Although power trim can be used to adjust the boat's attitude under certain conditions, it's a highly inefficient way of doing so. A propeller is designed to force the boat forward. When trimming the boat with the prop, the prop must not only push the boat forward, but raise the stern as well. In this situation, prop slippage is greatly increased, thereby compromising efficient engine operation. Additionally, power trim cannot compensate for listing (leaning over to one side) and is relatively ineffective at slow speeds.
But trim tabs, used in conjunction with power trim - well, there's the best of both worlds. This combo trims the hull while keeping the prop angle as perpendicular as possible to the flow of water for maximum thrust efficiency.
About the only negative associated with trim tabs has to be the fact that no matter how you look at it, they cost extra. Expect to factor in an extra $500 to $700 for a basic system whether aftermarket or from the factory.

Recessed or Transom Mount?
Boaters can choose from two styles of tab mounts: recessed or transom. While cruisers may find no particular advantage or disadvantage to either style, fishermen know from experience that if something sticks out of the hull anywhere, they'll eventually cut a fish off on it. Anglers with transom-mounted tabs must always remember those sharp-edged sheets of metal are there. Although Bennett, Boat Leveler and Teleflex all make trim planes with rolled edges (called upfins) to soften the potential impact of fighting fish too close to the transom tabs, recessed tabs offer a better solution.
First, since recessed tabs must have an indentation in the hull to accommodate the tabs, you can only get these as factory options. Rest assured, however, that everything in life is a compromise. Recessed tabs cost more, as the molding indentations require more time and effort.
According to several trim-tab engineers, recessed tabs also move the vessel's fulcrum to a point slightly forward of the transom edge, thus cutting down trim tab's efficiency by roughly 8 percent to 10 percent. And whether the tabs are transom-mounted or recessed, if your builder happens to cut costs by specifying smaller tab planes, the tabs might provide a significantly reduced degree of efficiency.
Some naval architects believe a properly designed hull shouldn't need trim tabs. Many manufacturers buy into that philosophy. But the ocean has many moods, and just as you can't design a car that runs as well on a smooth turnpike as it does off-road, neither can you design one boat that runs perfectly under all sea, wind and loading conditions. Trim tabs can help any boat meet the ocean's many challenges and make the ride more comfortable at the same time.

A licensed charter skipper, John Raguso of Bay Shore, Long Island keeps busy taking groups of up to four people far off the Northeast Coast in his 26-foot Grady TigerCat MarCeeJay for makos and other sharks, tuna and marlin. Raguso shares his wealth of experience with offshore fishing and boats, electronics and rigging as a contributor to several magazines. He can be reached at 516-499-8140.