A Comprehensive Guide to Boat Trim Tabs
To counter the challenges of water-bound navigation, boat engineers developed a device – called the trim tab – that can be fitted at the trailing edge of the boat hull where it allows for adjustment of the pitch attitude and consequently enables the boat to be driven on a leveled plane.
What is a Trim Tab?
To be specific, a trim tab is a small articulated metal plate/sheet that is designed to be installed at the trailing edge of the boat control surface, and it can be raised or lowered into the water by an actuated mechanism.
The plate/sheet (called the tab) serves to extend the projection of the boat frame into the water, and in effect alter the shape of the frame, while its (tab) ability to be elevated or lowered on the hinge allowing the shape of the boat to be changed to create better hydrodynamics depending on prevailing conditions.
What this means is that the trim tab gives the boat adjustable buoyancy that allows the pilot to bring the boat to an even keel that allows the pilot to have a clear view over the bow.
In terms of dynamic stability, the trim tab serves to reduce the trim angle towards zero. To best understand the fundamental role of the trim tab, there is a need to discuss fluid dynamics and boat hydrostatics.
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Fluid Dynamics and Boat Hydrostatics
When on a still water body, and the captain/pilot in the cockpit (deck) and no additional weight on the non-moving boat; the center of gravity (CoG) of the boat is usually well-centered and it floats flat (or is longitudinally-leveled) on the water.
This boat position is called the even keel, and the boat is described as having zero trim, that is, there is no inclination of the hull along the longitudinal plane of the water surface.
The trim angle is measured as the angle of inclination of the (boat) longitudinal axis away from the zero-trim axis.
The stable equilibrium of a boat is defined by its shape, momentum, weight distribution, and force distribution.
If all the external forces acting on the boat cancel each other (for example bow lift is canceled by stern lift), and there is no change in shape, momentum, and weight distribution, then the boat achieves a stable equilibrium.
When these external forces are eliminated and only internal forces act on the boat, then it achieves a neutral equilibrium.
This equilibrium determines how stable a boat can be when propelled on the water while carrying loads that are not evenly spaced out on the deck.
The stability of the boat allows it to restore its equilibrium after it (the equilibrium) has been disturbed or altered.
This is achieved by the disequilibrium causing the boat to develop forces and moments that act on the hull to re-establish a stable equilibrium.
Water in flowing and relatively still water bodies such as dams, lakes, and seas are able to carry a vessel, and this is determined by the laws of hydrodynamics which govern how floating water vessels will move in different circumstances, for instance, during turbulent flows and steady laminar flow, as well as when the water is still.
Minor turbulence does cause a slow-moving boat to experience small trim angle changes (small disequilibrium), and how the boat regains its equilibrium from these small perturbations describes its intrinsic mechanical stability.
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Limits of the trim tab
The trim tab cannot significantly change the intrinsic mechanical stability of a boat, though expert placement on the trailing edge of the stern, keel, and/or outboard engine can compensate for some of the inherent mechanical instability; and make the boat towable or sailable till it reaches the beach or repair shop.
As alluded to earlier, the buoyancy of a boat allows it to be sailed on water, which is a fluid medium whose turbulence and flow create a (water) load that acts on the boat.
In fluid dynamics, water is incompressible and lacks a defined shape, and can be deformed easily hence allowing a floating boat to rotate in three axes – the longitudinal axis, the vertical axis, and the transverse axis. Each of these axes is perpendicular to the CoG.
The longitudinal plane bisects the boat from the stern to the bow into equal right and left parts, and expected rotation on the longitudinal axis causes the left side of the boat to be lifted up, while the right side is submerged into the water (the converse is also true).
This form of rotation is called listing or roll, while the angular displacement of either side of the boat from their neutral plane is described as the bank.
Generally, trim tabs – on their own – poorly compensate for banks. So how can a pilot stabilize a boat that is rolling? The answer lies in turning the boat so that it is parallel to the direction of the water waves (also called the water load).
This is because a roll is caused by water load (or sea load) acting more on one side of the boat than the other, and this necessitates the pilot to engage the trim tabs to allow the boat shape to be extended and streamlined so that it can be turned.
Impact on the bow
Now, this turning causes the water load to act unequally on the bow and stern. If it causes the bow to rise, and the stern to sink, then the boat is rotating along its transverse axis that goes through the CoG.
This transverse rotation is called a pitch and it causes the bow to rise – which implies that the bow is raised higher than the stern.
Bow rise also occurs when the boat is driven at high speed as the streamlined bow hull cuts through the water hence reducing ‘preload’ (the resistance the boat must cruise through) while creating eddy currents that accentuate the sea load that the broad stern must handle, and this creates a drag, that mimics load placed at the rear of the boat.
The result is the bow rise and stern drop cause the bow to obstruct the view of the pilot in the cabin.
Relatedly, extreme bow rise during high-speed racing can cause the mid-section of the boat to be lifted off the water surface, and this reduces the drag on the stern hence causing it– by the sheer power of the high-speed momentum – to momentarily attempt to fly out of the water (stern lift) before falling back into the water, and this creates a rhythmic bouncing pattern seen in high-speed boat rides.
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This pattern in called porpoising; and it creates a challenge when trying to yaw the boat. Trim tabs play a fundamental role in pitch correction and minimizing (or eliminating) porpoising.
As mentioned earlier, rolling is corrected by changing the direction of travel, and this involves pivoting the boat along a vertical axis.
This pivoting along the vertical axis that passes through the CoG is allows the boat to yaw (that is, turn the bow to the left or right) which allows for change of direction.
Sometimes, only the bow yaws (wobblingly) without the boat changing its direction, hence making alternate movements favoring the port bow and starboard bow. These movements can be corrected by trim tabs.
Instability during boat rides
Instability during boat rides occurs when there is loss of equilibrium because the external moments and forces acting on the boat cannot be counteracted by the intrinsic (mechanical) stabilizing forces, thus causing these external forces to overturn the boat.
This instability can be reduced by expert placement of trim tabs, and knowing how to manipulate the engaged trim tabs so as to minimize listing, bow rise, extreme yaws, and porpoising so as to augment the efforts of the inherent stabilizing forces in restoring the equilibrium.
Euler theorem has been used to predict boat behavior in different (water) turbulent and draught conditions, and its calculations allow ship- and boat-builders to impart their products with a high inherent mechanical stability.
Nonetheless, boat users can add a trim tab to improve build efficiency, especially by allowing a stable hull to rapidly achieve an equilibrated attitude (also called trim) when carrying unevenly-spaced out loads on turbulent waves in windy conditions.
This attitude (or how the boat lies on the water) must allow for elimination of porpoising and listing, so as to enable the boat to be driven on a plane at a comfortable running angle (the angle between the water surface and the bow).
Therefore, the boat is driven smoothly with an appropriate bow height that minimizes drag, hence improving boat performance and fuel efficiency.
Still, pilot experience contributes to this efficiency as an expert pilot can adjust the running angle depending on sea conditions so that the front hull maintains a sharp forward-V that can knife easily through the water waves ahead.
It is well-known that shifting loads on the boat can adjust the attitude and running angle of the boat. Can these changes be corrected/adjusted by trim tabs? Also, there is a lingering question that some boat owners ask: Do trim tabs really work in the practical sense?
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Practically, do trim tabs work on boats?
Trim tabs are designed to allow for pitch correction, as well as for correcting porpoising and listing, so that the boat gets on plane as quickly as possible with a bow rise that allows for a low, yet stable, trim with the view of the pilot unobstructed.
Also, these tabs allow for adjustment of boat attitude depending on sea conditions, or following speed changes and load addition and/or distribution.
This section considers the most basic design of a trim tab – that is a hinged flat plate connected to an actuator assembly.
Another unique trim tab design is the one where a vertical blade is hinged on the stern, and it is commonly fitted in a boat whose hard two-chine hull creates a flat bottom that is unsuited for fitting a flat plate.
The placement of the trim tab is determined by the shape of the lower edge of the boat stern, which is basically an extension of the hull shape.
The hull of most boats features a composite shape with the fine entry forward section smoothly broadening out into an inverted-bell-shaped aft.
Most sterns follow the shape of the chined hulls, and since most hulls feature a hard single-chine, an S-bottom soft-chine, and a hard three-chine; then most stern profiles are either V-shaped or U-shaped.
During installation, the plate of the trim tab is inclined parallel to the bottom beam of the stern (the stern transom), or directly installed on the stern transom; and this allows the plate to extend the hull without distorting its hydrodynamic profile.
To further preserve this hydrodynamic profile, the trim tab is designed as an articulated device with its plate hinged on the stern with an actuator arm raising, leveling, or lowering the plate. This allows the trim tab to act as a water pedal (analogous to that found on swimming suits).
Some Fluid Mechanics
The surface of the hull that is underneath the waterline form the bottom base of the boat, and is responsible – alongside the boat weight – for creating buoyancy.
As expected, as more load is added to the boat, its bottom base area increases while the waterline rises; and this increase the water volume displaced by the boat.
According to Archimedes’ principle, the weight of this displaced water volume creates an upward buoyant force of equal magnitude that keeps the boat afloat.
How Trim Tabs Work
The water stream flowing under the hull surface carries the boat and governs boat attitude. In fact, this water motion generates force that contributes to the upward buoyant force.
As mentioned earlier, water is a non-compressible fluid, and therefore, when flowing water meets an obstacle, such as a trim tab plate protruding underneath the hull, then the flow is forced to change direction and flow downwards.
This is described as flow deflection, and it creates a reactive force called a positive lift that raises the stern, and this automatically reduces the bow rise. Also, the tab plate increases the bottom base area which increases the upward buoyant force.
Therefore, the positive lift and the enhanced upward buoyant force act simultaneously to provide a strong and sustained stern lift, that can lower the waterline and reduce hull resistance, hence allowing for throttling while enhancing fuel efficiency.
When the actuator mechanism of the trim tab levels the plate (to the same level as the stern transom), then the surface area of the bottom base is increased, and this increases the upward buoyant force, which can provide adequate stern lift to level the boat to a plane in still water.
On the other hand, if the plate is pulled towards the posterior surface of the stern wall, then the boat shape is relatively unchanged, and the trim tab does not contribute to, nor change, the hydrodynamic profile of the boat. In this position, the trim tab serves the passive role of being available for use by the boater.
The amount of buoyant lift generated by a trim tab is determined by its angle of deflection on the stern (how much it inclines away from a horizontal stern bar), surface area of each tab plate, degree of downward adjustment (lowering of plate underneath the hull), and boat speed.
Evidently, fitting two trim tabs to the boat allows for equalization of stern lift on either side, and this effectively reduces bow rise and corrects porpoising; while operating only a single tab deflects water stream downward on one side hence correcting listing by causing stern lift on this side of the boat.
Therefore, trim tabs do work, and their practical benefits are confirmed by boat attitude correction, faster planing, and efficient inline power generation (by the boat).
There is also a unique related device called the interceptor, which has a nearly equivalent design as the trim tab, and performs the same function but its downward displacement is faster and deeper than that of a conventional trim tab.
It also generates considerably greater water stream deflection, yet lesser drag as compared to large trim tabs. Therefore, the interceptor generates more powerful lift than trim tabs, and for this reason, interceptors are mainly fitted in boats whose length exceeds 39 feet.
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The Trim Tab Unit
This unit is the packaged product, and it contains a pair of trim tabs, 2 actuator systems, the controller system (or control system/unit), mounting bolts and screws, and mounting hardware. Other packages come with installation accessories, mainly installation tools.
Generally, trim tabs come as a pair with each placed on either end of the stern transom, and to a lesser degree, the keel or the most posterior beam of the hull.
Evidently, its placement at the trailing edge of the hull or keel means that one cannot physically access the tabs and their actuators systems when piloting the boat. So how does the boater operate and control the trim tabs when steering the boat from the helmsman cabin?
The answer is that a controller is provided that links the actuator systems to the user panel fitted in the pilot dashboard.
This controller allows the user to operate and guide the tabs by actuating them. In other words, this controller allows the boater to operate the actuator system that elevates or lowers the trim tab. Some controllers allow for independent tab operation, while others only support joint/simultaneous operation of both tabs.
There are two main types of actuator systems as defined by the operating mechanism of the actuator arm – the electric actuator and the piston-operated hydraulic system.
An improved version of the hydraulic actuator is the spring-loaded hydraulic system that incorporates the hydraulic system with a spring that serves to dampen shock.
There is also a special type of actuator system called the self-adjustable automatic trim tab. The two actuators are connected to a pair of tabs, and they form the entire marine trim tab unit (that is, it lacks a controller unit).
This system is fitted in small boats such as inflatables that lack space for fitting outboard trim tabs. This automatic unit comes with a nitrogen gas cylinder and operates like a suspension system.
Nitrogen gas is compressible and expansible, and this allows for the tab to be adjusted downward or lifted up automatically (without human intervention) by the gas pressure in the cylinder.
Even so, it generates little downward displacement force which makes it unsuitable for racing boats, yachts, and other boats whose length exceeds 20 feet. For these boats, one needs to install user-adjustable hydraulic or electric actuators.
All actuator systems share the same basic design which is a composite unit made up of three parts, the actuator upper and lower hinges, the cylinder, and the actuator arm.
The actuator upper hinge is mounted to the stern, and allows the cylinder to be pivot up and down when the actuator arm is pushed out or pulled into the cylinder. This arm is mounted to the trim tab by the lower hinge.
In the hydraulic model, this arm terminates in the cylinder as a piston which can be moved up and down by hydraulic fluid. For the electric model, there is an electric motor whose rotation powers a geared mechanism that changes circular motion into up-and-down vertical motion that lowers or raises the actuator arm within the cylinder.
Electric models provide faster response (downward adjustment), but generate less torque as compared to hydraulic actuators. Nonetheless, in high-quality electric models, the motor powers a gear set that drives down a threaded ballscrew actuator arm hence generating torque equivalent to that generated by hydraulic actuators.
As expected, the maximum downward displacement of a tab is always less than the length of the cylinder that houses the actuator arm. Even so, how is the hydraulic fluid pumped into and out of the cylinder? This is handled by the controller system.
The controller system allows the boater to operate the actuating mechanism. This system, or unit, has three main parts: an easily accessible user module (mostly a rocker switch) with a control display that is installed in the dashboard, electric cables or fluid hoses, and the control mechanism.
Electric controller systems
In most electric controller systems, the user module is an auto tab control (ATC) unit that is connected to a power source (usually a battery and/or back-up generator), and it integrates rocker switch functions with smart controls of the display, besides connecting them to the relay unit and cables that run to the control mechanism.
In electric actuators, the cables terminate in the motor unit which is the control mechanism that drives the actuator arm. In hydraulic actuators, the cables terminate in the hydraulic power unit of the fluid reservoir.
This hydraulic power unit contains a motor that uses a geared mechanism to covert circular motion into linear motion that drives a piston in the fluid reservoir, hence causing fluid to be pushed into the actuator cylinder where it displaces the actuator piston.
Conversely, when the piston in the fluid reservoir is drawn back, a suction effect is created which causes emptying of fluid from the actuator cylinder, and this allows the spring-loaded actuator arm to retract back into the cylinder while pulling the tab.
The variance in piston size between the larger piston in the reservoir and the actuator arm piston creates pressure amplification during fluid transmission, hence allowing low pressure of the reservoir piston to generator high pressure in the actuator.
Basically, maximum amplification is achieved by making the reservoir piston as large as possible while minimizing the surface area of the actuator piston.
Relatedly, the time lapse caused by fluid transmission through the hydraulic lines from the reservoir to the (actuator) cylinder is what accounts for the delayed action of electrically-controlled hydraulic actuators, and also explains why such a system is slower than an electric actuator.
Hydraulic controller system
In hydraulic controller system, the push buttons of the user module and the control display are connected to hydraulic lines that terminate in a hydraulic pump fitted on the side of the boat, or at any convenient outboard position.
The small pressure transmitted from the user module to the hydraulic pump, moves a large piston that displaces hydraulic fluid into the actuator cylinder where it pushes down a much smaller actuator piston.
This occurs as long as the push buttons are pressed, and when the finger is lifted, then the actuator arm slowly retracts into its cylinder. In quality models, a retract push button is provided so as to allow for quick retraction of a trim tab which quickly adjusts the tab position to suit the needs of the boater.
Evidently, hydraulic actuators can use either electric or hydraulic controller system.
Also, the user module can allow for each trim tab to be operated separately by having separate controls for each tab, or use shared controls to operate both tabs simultaneously. It is advisable that one acquires a marine trim tab unit that support independent operation of each tab.
The length of the tab is described as the span, while the width is called the chord. Trim tabs are hinged along their span.
On the tab plate, upfins and/or downfins can be fitted. Moreover, the tab plate can be specially-shaped – besides the normal rectangular shape – so as to increase size, depth of displacement, and waterflow deflection capabilities.
The mounting of the actuator arm on the tab plate does impact how much force the tab generates during downward displacement.
As expected, linking two actuators to a single tab allows this tab to generate more force than a tab linked to a single actuator.
Another factor that impacts force generation by the tab is the plate surface area, with large surface areas generating more force per unit of downward displacement as compared to small plates.
Regarding mounting, the lower hinge of the actuator arm can be mounted in one of three positions; edge-mounting at the middle section of the span edge, mid-mount at the center of the tab plate, and near-edge mounting which is half-way between edge-mount and mid-mount.
The mounting position is the secondary determinant (after actuator arm extension) that determines how much downward displacement the plate achieves, with mid-mount mounting providing the greatest downward displacement while the edge-mount provides the least downward displacement for tabs mounted to the same actuator.
Sizing the Trim Tab
As mentioned earlier, the tab extends the length of the hull, as well as streamlines its (hull) hydrodynamic profile. Therefore, tab surface area contributes majorly to creating the buoyant lift that reduces the bow rise, corrects listing, and allows for faster planing.
The other secondary contributors to this lift are the angle of deflection and depth of downward displacement under the hull.
This means that a smaller trim tab would provide inadequate upward lift even if its depth of downward displacement and angle of deflection are fitter than that of a correctly-sized tab.
Consequently, it is important that one knows how to size the tab based on the length and tare weight of the boat, with placement of outboard engine another consideration.
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How to Size the Tab
The first step involves measuring the span of the stern transom from either end to the centerline. This is important as the outboard engine, propeller drive unit, and the lower unit of the sterndrive are placed at the center of the stern, and the tab must not contact/touch them.
Secondly, in hard single-chine hulls, the stern is V-shaped, with the inclined bottom transoms meeting at the keel, and therefore knowing the distance between the keel (which is the centerline) to either end allows one to know where to place the tab, and set the most appropriate angle of deflection for maximum positive lift.
The next step is to measure for two clearances:
- Clearance between the tab and the outer edge.
- Clearance between the tab and the centerline.
These sum of these two clearances is deducted from length of the transom span so as to find out the space available for fitting a tab plate.
A simple mathematical formula is used for a boat whose length is less than 30 feet:
Distance between centerline to outer-edge – 12 inches = Tab Span.
Clearance measurements allow for proper placement of the tab on the stern transom, with the tab placed 8 inches away from the centerline, and 4 four inches away from the outer-edge.
In boats whose length exceeds 30 feet, the 12inches in the formula is replaced by 16inches as the tab needs to placed 12inches away from the centerline. This changes the formula to
Stern transom span – 16 inches = Tab span.
The alternative sizing option is to size the tab span based on the boat length. The rule of the thumb is that the length of the boat in feet should be almost equal to the tab span in inches, for example, for a 20-foot long boat, the tab span should be at least 20inches.
Now that the span is known, there is need to size the chord so as to get the right tab size. The chord contributes to the depth of displacement, and longer chords provide for greater downward displacement as compared to shorter chords.
Usually, chords come in two measurements, 9inches and 12inches. Even so, specially-designed high-performance models have chords that are sized to complement to buoyancy of the tab span.
As expected, the 12-inch chord generates greater positive lift than a 9-inch chord, as long as their span is the same.
Even so, most boaters are advised to acquire a 9-inch chord if their boats are less than 39-inches, uses a single outboard engine, and there is enough transom space to place a tab with a long span.
The 12-inch chord is used in boats whose double outboard engine reduces their transom space, or boats longer than 39inches which need a considerable lift.