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
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
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.
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.
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
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.
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.