Ask The Experts | What Everyone Should Know About Trim Tabs
Article courtesy of Bennett Marine
So you’re convinced of the benefits of trim tabs – a better ride, fuel savings, greater visibility – and now you just need to select the right tab for your boat. This decision will actually have a major effect on the effectiveness of your new tabs. When sizing for your tabs, “Bigger Is Better”.
Installing undersized trim tabs is one of the more common mistakes for new trim tab owners. A tab that is too small will have to be deflected more in order to create sufficient lift and could actually create more drag than the lifting benefit. The larger the trim tab, the more lift it will produce with the least amount of drag.
Remember this guideline: When making a choice between trim tab sizes, 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 22" x 9", 36-foot boat = no less than 36" x 9".)
To understand sizing, you have to know how trim tabs work. Trim tabs improve the performance of your boat in a much wider range of weight, weather and water conditions. The purpose of trim tabs is to recreate your ideal running attitude, even on a rough day. Much like ailerons and elevators on an airplane, trim tabs provide lift to compensate for changing conditions.
Three variables combine to affect lift:
- Size of the trim tab surface
- Angle of deflection
- Water-flow under the tab (caused by speed of the boat)
When they are deflected downward, the water force on the trim tab surface creates upward pressure, raising the stern and lowering the bow.
As you can see, surface area plays a big role.
Trim tabs are described by their span and chord measurements:
Span = side to side measurement
Chord = fore to aft measurement
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 longer chord. Such as for boats with limited transom space that limit trim tab span. And for slower boats (less than 15 mph), semi-displacement hulls, boats over 50 feet or boats with any other feature that increases the need for lift aft.
Make sure the trim 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.
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. The transom mount includes three parts - a backing plate mounted against the transom, a trim plane and a hinge plate.
Bottom Mount - For vessels that have curvature in the transom. It can only be used if a flat section of the hull bottom is available to accommodate the hinge.
After decades of use on the water, it's been proven that all boats, large and small, benefit from trim tabs. However, having the right size tab will make all the difference in just how much your boat’s performance improves.
Article courtesy of Bennett Marine, the largest manufacturer of trim tabs in the world. Take a look at the wide selection of Bennett trim tabs for your boat type, size and boating use at iboats.com.