Page 70
September 1996
Powerboat Magazine

Dockside Mechanic
Attitude Adjusters

Proper trim is important to any boat. Heavy gear loads and fidgety passengers can shift balance. A crosswind might cause the hull to list. And many of today's modified deep-V pleasure-craft can benefit from some amount of down-trim at cruising speeds. Trim tabs can compensate for or solve these problems, and more.

MULTIPLE BENEFITS
Trim tabs allow the hull's shape to be dynamically tuned for optimum performance in a variety of water, load and speed conditions. Tabs help get the hull on plane more quickly, keep the boat on plane at slower speeds and provide a softer, drier ride. Add seaworthiness in rough water, increased speed and improved fuel economy.

LEVERAGE IT
Trim-tab systems are largely electro-hydraulic. Controlled by relays and solenoid valves, activated by electrical up-down switches at the helm, hydraulic fluid is pumped from a reservoir to actuate rams to move the hinged tabs mounted along the transom bottom. The tabs are fabricated of stainless-steel plate, or cast of aluminum or some other alloy. Some models are treated with corrosion resistant processes or finished with a baked-on powder coating. Heavy-duty, double-ram systems are the choice for high-performance applications.

PHYSICS 101
Pressing the port bow-down switch extends the starboard actuator to create starboard lift; pushing the starboard bow-down switch does the opposite. By deflecting water flow downward, tabs produce lift and force the stern up while underway. If the stern goes up, the bow must come down.

JUST HOW BIG?
Lift is generated in proportion to hull length and weight, angle of tab deflection, and size of the tabs. A general rule of thumb in tab selection is 20 square inches per foot of boat length. However, follow the trim-tab manufacturer's recommendations in choosing span (athwartship) and chord (longitudinal width) of tabs. Size depends on how much effect is needed. Faster boats call for greater tab surface with longer chord and less width. If the tabs are large enough, they extend the hull length and the boat will feel and act longer.

INSTALLATION TIPS
Typically, tabs are placed a minimum of 3" from the chines, and at least 10" from the centerline to avoid interfering with steering. The farther outboard the tabs are mounted, the greater lateral control is over longitudinal control. To prevent tabs from dragging against the water when in the full-up position, trailing edges should be about 1/2" above the plane of the hull's bottom.

TRIM TIPS
Using tabs is largely a seat-of-the-pants situation. Electro-mechanical indicators are helpful, but should be regarded as just indicators - not gospel. A small change in trim angle often translates to a larger reaction from the hull. The boat should plane quickly without high bow rise. A wide difference in settings between the port and starboard tabs can cause quirky hull reactions. Excessively high trim brings the propeller so close to the water's surface that it ventilates, losing thrust and perhaps causing potentially dangerous chine-walking.

FOLLOWING SEAS
This condition is a problem for planing hulls. To improve handling and safety when climbing up waves, retract tabs all the way to keep the bow high. When rolling over the backside of waves, move the drive out to keep the bow higher. Tucking down trim too far causes a boat to plow on its bow, hampering maneuverability.

WORKING TOGETHER
Outdrives also provide power-assisted trim control by swinging the propeller forward or backward a few degrees from vertical. Use the drive and trim tabs in concert to keep maximum prop thrust parallel to the water surface. To reach top speed, you want to trim the drives until the cavitation plates are parallel with the bottom of the boat and work the throttle up as you move the tabs up. In effect, full-up tabs create an artificial rocker in the boat's bottom near the transom. The faster the boat goes, the greater the dynamic pressure at the rocker, and the higher the stern rises.