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.
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
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.
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.
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.
Del Mar, Ca 92014 United States Toll Free: (877) 406 4562 for USA (858) 925-7705 for International Customers