Many factors affect the choice of properly sized trim tabs, and the ultimate responsibility for selection is up to you, the boat owner. To help you decide, Bennett Marine offers you a few guidelines below.

General Guidelines
These sizing recommendations are based on average performance. Your choice may vary based on power, engine configuration, weight distribution, type of boat, and use. The greater the surface area, angle of deflection and/or speed of the water flowing under the trim tab, the greater the lift.

When making a choice between trim tab sizes, remember that the largest trim tabs that will comfortably fit on the transom will be the most efficient.

As a rule, choose at least one inch of trim tab span (per side) for every foot of boat length. (Examples: 22-foot boat = no less than 24" x 9", 36-foot boat = no less than 36" x 9".

Span = side to side measurement
Chord = fore to aft measurement

Whenever possible, choose a 9" chord trim tab, and gain lift through a longer span. The span of the trim tab has more of an effect on the amount of lift. However, a longer chord can be used effectively, and there are situations where you may need to use a 12" chord.

Measuring
Choose the approximate tab size for your boat using this sizing chart. Make sure the tabs will fit your transom using the diagrams below as a guideline. When measuring, disregard the strakes and follow the Vee of the hull.
 

Installation Requirements
Trim tabs should follow the Vee 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.

In case of 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 provided there is no change in angle on both sides.

On boats powered by inboard/outboards (I/O) or 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.


When to Use a 12" Chord
The 9" chord is used in most applications. However, in certain circumstances the 12" chord may be more effective:

Limited Transom Space - Boats with twin outboards or twin I/Os, or boats with transom configurations that limit trim tab span, can use 12" chord tabs to achieve maximum lift. Using the measuring guideline above, fit the maximum span tab and use the 12" chord.

Extra Lift - Slower boats (less than 15 mph), semi-displacement hulls, boats over 50 feet, outboard brackets or boats with any other feature that increases the need for lift aft benefit from the 12" chord. The 12" chord provides greater surface area, thereby utilizing more water flow and providing more lift.