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Bennett Trim Tabs Information & Sizing Chart

What Trim Tabs Do
Bennett Trim TabsMost boats are designed to plane at a particular speed and weight distribution. However, as weight increases (due to additional fuel, passengers, or gear) and/or speed decreases, the stern settles down creating an inefficient, untrimmed condition. As the boat pushes forward, it creates a "hill of water." In this bow-high position visibility is limited and the hull bottom is pounded. In addition, due to significant hull drag and extreme prop angle, fuel economy is poor. Properly sized Bennett Trim Tabs enable your boat to overcome this "hill" and plane at speeds slower than designed planing speed.

As the helm control is pressed, the stern rises, lowering the bow. Without touching the throttle, speed increases and optimum attitude is achieved regardless of speed or weight distribution.

Bennett Trim Tabs reposition the boat's bow to cut through the water reducing pounding, drag, and engine laboring. Repositioning the bow creates greater visibility which increases safety. Reduced pounding means greater comfort. Less drag and engine laboring translate into increased performance, greater speed, and reduced fuel consumption.

With all the advantages and efficiencies created, Bennett Trim Tabs virtually pay for themselves.

Without Trim Tabs:

With Trim Tabs:

The Principle of Bennett Trim Tabs
Bennett Trim TabsBennett Trim Tabs are two independent, stainless steel, adjustable afterplanes attached to the bottom edge of the transom. As the helm control is pressed, the trim tabs move into position.

Water-force on the trim tab surface creates upward pressure, thereby raising the stern and lowering the bow. The principle is simple. The results are impressive.

The Bennett Advantages

Increase Speed • Reduce Pounding • Correct Listing • Eliminate Porpoising • Offset Prop Torque

Reduce Fuel Consumption • Reduce Engine Laboring • Eliminate Squatting

Improve Visibility • Reduce Wake • Improve Handling • Reduce Hull Stress

How to Use Trim Tabs

Getting Started
The key to obtaining optimal results from trim tabs is to operate them in short "bursts" and let the boat react before making another adjustment. The amount of time between corrections is influenced by the size of the trim tabs and the boat's speed. This will help avoid overtrimming or ending up with one tab too far down when correcting lateral trim. You will quickly become acquainted with a boat's particular traits.

Take Off
Properly sized trim tabs can significantly reduce the time needed to get up on plane. They also allow a boat to keep its bow down and stay on plane at lower speeds. As the throttle is advanced the stern of the boat begins to squat, lifting the bow. As the boat accelerates, push the bow down position of the helm control in short bursts. The boat reacts by the stern lifting, the bow coming down, speed increasing, and reduced engine laboring. If you over do it and deflect the tabs too far the boat will end up overtrimmed. When over trimmed, the steering becomes "over sensitive" and wants to pull off course to port or starboard. If this occurs, operate the control "bow up" until the desired attitude is established.

Correcting a Listing Condition
As a result of uneven weight distribution, prop torque or wind, a boat runs with a list. Deep "V" hulls are particularly vulnerable to this condition. Running with a list is uncomfortable, as well as unsafe. Bennett Trim Tabs operate independently for effective list correction. To correct for list, lower the trim tab on the side of the boat that you are listing to. This will bring the boat level.

Trim Tabs and Power Trim
It is a common misconception that if a boat has power trim on the outboard or outdrive it does not need trim tabs. Power trim can be used to adjust the boat's attitude, but it is highly inefficient. A propeller is designed to force the boat forward. When trimming the boat with the prop, the prop must not only push the boat forward but raise the stern as well. In this situation, prop slippage is greatly increased thereby wasting RPM's. Power trim cannot correct listing, and is ineffective at slower speeds. Bennett Trim Tabs, in combination with power trim, enable both the hull and prop to be trimmed independently. The trim tabs trim the hull, while the power trim adjusts the prop. The result is optimum performance and efficiency not attainable by the use of power trim alone.

To achieve maximum performance, first adjust the trim tabs to achieve the desired running attitude. Next, use the power trim to position the propeller thrust parallel to the water flow. If necessary, re-adjust the trim tabs to fine tune the attitude. By observing the boat's speed and engine RPM's the best combination of trim tabs and power trim will be apparent. Trim tab angle indicators and a power trim angle indicator are particularly useful in duplicating effective settings.

Trimming to Sea Conditions
When running into a head sea you want to trim the bow down so the sharp forward sections of the boat do their work cleaving the waves. This provides the most comfortable ride and minimizes stress on the boat (and passengers). In a following sea the tabs should be fully retracted for maximum steering response.

Correction of Porpoising
Operate the tabs in very short bursts of about half a second. Continue until porpoising subsides. The objective is to have only a very slight amount of tab deflection, just the amount needed to cure the up and down motion of the bow.

Trim Tab Sizing Guidelines

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

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.

Boat Length Configuration Trim Tab Size

(span x chord)

15' - 19' Single I/O, Single Outboard, or Inboard 12" x 9"
17' - 20' M80 Sport Tab for Trailored Boats 8" x 10"
19' - 24' Single I/O, Single Outboard, or Inboard 18" x 9"
20' - 23' M120 Sport Tab for Trailered Boats 10" x 12"
19' - 24' Limited Transom Space or Extra Lift 12" x 12"
22' - 27' Single I/O or Single Outboard 24" x 9"
22' - 27' Twin I/O or Twin Outboard 18" x 12"
25' - 30' Single I/O or Single Outboard 30" x 9"
25' - 30' Limited Transom Space or Extra Lift 30" x 12"
25' - 30' Twin I/O or Twin Outboard 24" x 12"
28' - 34' Inboard 36" x 9"
28' - 34' Limited Transom Space or Extra Lift 36" x 12"
28' - 34' Twin I/O or Twin Outboard 24" x 12"
32' - 38' Inboard 42" x 9"
32' - 38' Limited Transom Space or Extra Lift 42" x 12"
36' - 44' Inboard 48" x 9"
36' - 44' Limited Transom Space or Extra Lift 48" x 12"
42' - 50' Inboard 54" x 9"
50' - 60' Inboard 54" x 12"
60' - 65' Inboard 60" x 12"
65' - 70' Inboard 66" x 12"
70' - 80' Inboard 72" x 12"

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