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Guide To Trim Tabs

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Trim Tab Sizing Guidelines

What size trim tab is right for me?
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. If you still have questions about the size you need, just contact us and we’ll be happy to walk you through it.

tpa_span_chord_smallGeneral 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. Click Here to See When 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.

The sizing guidelines below are intended to assist in the selection of tab sizes for vessels that do not currently have tabs installed by the boat manufacturer. For assistance with unique applications not outlined below, please call us.

Boat LengthTrim Tab Size (span x chord)
17’-19’ (5.2-5.8 m)8” x 10” (20 x 25 cm) M80
19’-23’ (5.8-7 m)10” x 12” (25 x 30 cm) M120
15'-19' (4.6-5.8 m)12” x 9” (30 x 23 cm)
19’-24’ (5.8-7.3 m)12” x 12” (30 x 30 cm)
20’-23’ (6.0-7.0 m)18” x 9” (46 x 23 cm)
22’-27’ (6.7-8.3 m)18” x 12” (46 x 30 cm)
22’-27’ (6.7-8.3 m)24” x 9” (61 x 23 cm)
25’-31’ (7.6-9.1 m)24” x 12” (61 x 30 cm)
25’-31’ (7.6-9.1 m)30” x 9” (76 x 23 cm)
25’-31’ (7.6-9.1 m)30” x 12” (76 x 30 cm)
32’-38’ (9.7-11.6 m)36” x 9” (91 x 23 cm)
32’-38’ (9.7-11.6 m)36” x 12” (91 x 30 cm)
36’-44’ (11.0-13.4 m)42” x 9” (107 x 20 cm)
36’-44’ (11.0-13.4 m)42” x 12” (107 x 30 cm)
42’-50’ (12.8-15.2 m)54” x 9” (137 x 23 cm)

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.

Trim Tab Mounting
Two types of mounting hinges are available:

  • Transom Mount – This hinge style fits to the boat’s transom and is used in the majority of applications.
  • Bottom Mount – Used only when transom mount is not practical; requires a flat bottom with no strakes or other protrusions.

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.

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