Proper Sizing of Trim Tabs

Many factors affect the choice of properly sized trim tabs, and the ultimate responsibility for selection is up to the boat owner. These sizing recommendations are based on average performance. Your choice may vary based on power, engine configuration, weight distribution, type of boat, and use. 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.

General Guidelines

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

Measuring
After choosing a general trim tab size (see chart below), double check the size by measuring the transom using the diagrams. When measuring, disregard the strakes and follow the Vee of the hull.

12" Chord

The 9" chord is used in most applications. However, in certain applications 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 in minimum area. Using the measuring guideline above, fit the maximum span trim tab and use the 12" chord. Extra Lift Slower boats (less than 15 mph), semi-displacement hulls, boats over 50 feet, outboards on 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.

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 the 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.

Performance Boats

Many factors determine the appropriate size and type trim tabs for faster boats. Special consideration is required when selecting trim tabs for performance applications.

You may wish to contact Bennett Marine to discuss your particular application. Bennett Sport Tabs are available for boats in the 40-60 mph range.