What’s A Trim Tab?
Trim tabs are a pair of metal plates fitted on the starboard and port side of the transom of a boat. Their purpose is to let you manipulate the hydro-dynamic angles that a watercraft rides. Imagine the ailerons on an airplane, which help provide lift at take off and lower the plane down on landing. Trim tabs do the same thing, except under water and raise or lower via a hydraulic or electric ram mount.
Parts & Types
Each trim tab is a hinged metal plate made of either stainless steel or aluminum. The vertical section of this plate attaches to the vertical surface of the transom. Placed right above the bottom interface. The main section of the plate sits horizontal to the running surface when not in use. Between each plate is the actual hinge. Which depending on the type is an electric actuator or hydraulic cylinder/piston.
There are two common types of trim tab systems; both of which are electric powered. Either a mixed electric-hydraulic version or an all-electric system.
Hydraulic Trim Tabs
This type uses a motor in a power unit to pump fluid from a reservoir to actuate hydraulic rams up and down. This system moves the tabs akin to mechanical water pedals. Keep in mind, using this version you need room to run hoses from the tabs to the hydraulic reservoir. Which complicates the installation.
In this system, each tab uses a motor in the hinge actuator that runs a geared system to move the ram up and down. This version is much easier to install, due to not needing extra room to install hydraulic hoses.
Next, both types have a wiring harness that runs from each of the tabs to the helm. To control the independent operation of each trim tab, you use a 12v (volt) switch at the helm.
Why Not Use My Engine Trim Instead?
One error I see is boaters believing if they already have engine trim there’s no need for an added trim tab set. Yes, the engine trim can adjust the fore-to-aft attitude. However, it doesn’t fix side-to-side listing situations.
It’s preferable to use both systems as a partnership for increased speed and power. Use the engine trim to position the prop to deliver optimal performance. Then use the trim kit to set ideal hull position and planing efficiency. Here is an example to attain the best performance from a stand still:
- Adjust the trim tabs down to deflect water flow to create a planing attitude.
- Next, use the power engine trim to position the prop parallel to the water flow.
- Finish by making short burst adjustments to the trim tabs which will “fine tune” your ride.
How Do Marine Trim Tabs Work?
The basic operation of a trim tab is to change the pitch of the watercraft when in motion. They are like having two angled paddles at each corner of the transom. Each side deflects water flow under the hull to lift either the starboard or port stern sections. Moving the pitch helps compensate for speed changes, weight distribution, and water conditions.
If both tabs move down at the same time, the change in water flow creates upward pressure under the tabs. The pressure raises the stern and in turn makes the bow move down.
Using the correct sized trim tabs allows you to control both the side-to-side and back to front pitch of a vessel.
To control side-to-side trim, you would deflect one tab more than the other. Using trim tabs for this helps fix listing or leaning of either the port or starboard sides. For example:
At the helm rocker switch control, you press the starboard “bow down” button. This will engage the port side trim tab to tilt downward, which forces the port side stern upward. As the stern rises, the starboard bow will lower down completing the movement.
Once you engage the starboard “bow up” the following happens: First, it raises the port side tab which lowers the port stern. As the stern lowers, the starboard bow rises upward.
For the opposite side, engaging the port “bow down” deflects the starboard side tab downward. Thus, raising the starboard stern section and lowering the port bow. As expected, engaging the port “bow up” has a reverse effect.
Keep in mind, if you experience “bow steering” due to poor loading. Or if the watercraft has a “heavy bow” because of bad design. Don’t expect aftermarket trim tabs to be able to solve listing.
What’s The Right Sized Trim Tab?
Installing undersized trim tab kits is a common mistake. If you undersize, you need to deflect more to create lift. Whereas if you oversize, it causes speed-robbing drag behind a watercraft. It’s important to understand that the actual surface area of the trim plate is what’s critical to efficient performance. So having the proper sized planes on your vessel’s transom is important. You will base the size of your trim tabs on boat length, speed, and transom shape.
Start by considering vessel length. Most tab manufacturers suggest an inch of tab width (span) for every foot of boat length. The span is a side-to-side measurement, while the chord is a fore-to-aft measurement. Below shows the general tab sizing guidelines depending on the boat length.
General Chord Depths
In general, it’s best to choose a 9″ chord trim tab, and gain lift from a longer span. Combining the chord depth with the widest span you can fit delivers the most amount of lift. Yet there are some exceptions for moving up to a 12″ chord.
When To Use 12″ Chords
- Limited Space On Transom. Such as for boats with twin outboards or twin Inboard/Outboards (I/0). Another reason is for boats with a planing hulls design. In these instances a 12″ chord tabs can achieve the greatest lift. But like I said, doing so could lead to drag issues.
- For Extra Lift. This is for boats which fall into the following categories:
- Slow boats, with max throttle of less than 15 mph (24.14 km/h).
- Vessels over 50 feet (15.24 m) in length
- Heavy boats which need the largest amount of lift.
- Boats with semi-displacement hulls.
- If the transom uses outboard brackets, like an inflatable dinghy.
Trim Tab Installation Requirements
If you install the tabs correctly, they have no effect on a boat’s performance when retracted. Trim tabs require a run of constant dead-rise angle at the junction of the transom and the bottom of the boat. Generally, mounting is best on the outboard, usually starting 3 to 4″ (inches) from the chine. Running towards the keel. The further outboard the tabs are the more force they can exert during side-to-side lifts.
For example, one installation mistake is to mount the surface area of the tabs too close to the centerline of the boat. The side-to-side corrective movement, relative to the fore to aft trim, is less.
In case of inboards, the complete run from chine to keel is available for mounting. As long as this is an unbroken span, of the same angle. Bumps, such as strakes can have a bridge over them, provided you don’t change in angle on both sides.
On boats powered by inboard/outboards (I/O) or outboards, keep trim tabs 7-8″ away from lower unit(s). This is to avoid disturbing any of the water flow to the propeller.
When To Use Trim Tabs
You can use the trim tabs in a variety of different boat positional changes. Some of which takes place during adverse wind and sea conditions. So, when and how you use them depends on your intended movements and conditions. Below are explanations for when you will engage corrective trimming:
The right sized trim tabs reduce the time needed for your boat to get up on plane. Likewise, when the boat is moving at low speed they can keep the bow down and stay on plane.
When you accelerate the throttle, the stern of your boat squats downward and lifts the bow up. As boat speed grows, engage the bow down button at the helm in short bursts. Using this method provides smooth movement. As the stern raises, the bow will lower while also making the boat speed up and make the engine more efficient.
Listing is when your boat leans or rolls to one side either in motion or not. It occurs due to uneven weight distribution, improper prop torque or wind. This is a particular problem for deep-vee hulls in strong cross winds. Running a boat that lists to one side is uncomfortable, and unsafe. To bring the boat to a level attitude you will use your twin tab controls at the helm.
- If the starboard bow is high, push the starboard side “Bow Down” button which lowers the trim tab on the port side.
- Otherwise, if the port bow is too high, push the port side “Bow Down” button to lower the trim tab on starboard side.
Fix A Boat Porpoising:
Porpoising is a rhythmic bouncing effect of the bow or a bobbing up and down. It’s a condition common in performance boats. What happens is as speed increases, the bow rises out of the water until gravity overcomes lift. Then the bow bounces down, this repeat over and over.
To fix this condition, you need to press both “Bow Down” buttons together in half second bursts. This make both lower to deflect the undulating water flow beneath the hull. In turn the porpoising affect will subside, as it does keep your speed to constant to regain balance.
Reversing Or Backing Off:
For reversing or backing off your boat, ensure to keep both trim flaps is the retracted position. If left down in while in reverse they’ll create drag which puts strain on the system. Plus they will affect the boat’s handling.
Imagine if one is down and deflecting more than the other while in reverse. The boat will pivot around the deflected tab, causing an unintended accident hazard.
Wind & Sea Conditions
Using In Choppy Sea With Head Winds:
When running into a head wind with a choppy sea you want to trim the bow down. This is so the front “vee” of the hull will cleave the waves. Rather than having the waves pound the side aft parts of the hull. Producing this cleaving of the waves is how a watercraft creates an enjoyable ride for the occupants.
Maneuvering In A Beam Sea With Side Winds:
In this instance imagine you’re operating a vessel with swells coming from the side. Often these waves will throw spray up on the boat from this windward side. This is particularly a problem for smaller vessels.
To correct this by trimming, you would raise the bow of the windward side of the boat. Resulting in blocking any spray that could blow into the vessel. Doing this will deliver a smoother and drier ride.
Cruising In A “Following” (Diagonal) Sea:
In waters where waves come toward you at a diagonal direction, it’s best to keep the system retracted. An example of this is moving in an inlet. In this type of water you want the greatest steering control. So the current or tides won’t push the vessel’s stern from side to side.
You also want to prevent the bow from dipping into an oncoming wave in a following sea. So, keep the underneath of the hull free from deflection to let the vessel ride over the swells.
How To Prevent Over-Trimming
Fixing over-trimming is all about driver feel when engaging the system. To get the best results from your trim tab kit, you should operate them in short bursts (0.5 seconds per press). Wait until the boat reacts before pressing the button again. The faster your vessel is the lighter the reactions will feel. Done well, you should feel the bow start lifting and the wake spray out the sides start moving back along the hull. Next, beneath you the hull should feel like it’s skipping along on a light chop.
Am I Over-Trimming My Boat?
To tell if you are over-trimming, pay attention to the feel of the steering wheel. If you get an increase in rpm and not in speed, you over trimmed. A steering wheel that feels less sensitive, means the boat is over-trimmed. Another issues that will arise is if the vessel begins to “bow steer” or “plow” while planning.
To fix, slow down the throttle and press the “bow up” button to raise the bow to balance the attitude. As always, short burst adjustments to the trim tab kit will help avoid over-trimming.
Trimming From A Tower Or Bridge:
The problem with steering from a tower or bridge is lack of sight of the fore sections of the hull. To remedy this try to watch either the stern wake, bow spray or the rooster tail. A boat that isn’t trimmed makes more spray aft of the bow and produces a bigger wake.
Whereas a proper trimmed vessel, produces a bow spray that’s farther forward. As well the wake is smaller, along with the rooster tail. Plus, the engine RPM will increase if you have the correct level of trim.
Automatic Trim Tab Kit:
To make trimming even easier some brands offer an “automatic” mode for their kits. You simply press a button and it trims itself for any activity you find yourself in. One such brand is Bennett Marine with their Auto Trim Pro.
The Bennett system makes automatic adjustments to the levels of pitch and roll of your boat. It will also reduce bow rise and corrects listing in the auto setting.
Otherwise, if you want a “real feel,” you can set it to manual mode at anytime. Plus the automatic retraction function protects tabs during storage or trailing. If you are looking for a sort of “set-and-forget” system the Bennett models are a good option.
We can all agree that marine environments are harsh and unforgiving. They are particularly damaging for ignored components. Such as trim systems, that are always contact with water.
For example the older Bennett and Lenco Marine tab kits didn’t feature an auto-retract option. This special detail moves the plates into a horizontal position at engine shutoff. By not having this option, the plates could fall into a vertical position. This exposed the water tight seals for actuator to potential damage (barnacles).
If you do own an older system, it’s best to upgrade to a Lenco replacement actuator kit with this feature. Not only do you get new actuators, but also a new control box and an LED rocker switch for control.
This is just one example for why it’s so vital to do regular maintenance to ensure a long, trouble-free life. Let’s look at some other ways to maintain your trim system.
Hydraulic Kit Maintenance
Make regular checks of the hydraulic fluid levels in the reservoirs. For example if you own a Bennett set do the following.
- Ensure to retract the trim tabs.
- Look for a fluid level of around 2 inches (5.08 cm) from bottom of reservoir.
- Refill: remove Lexan cover and filler plug at the front left hand corner of the reservoir.
- Using only Automatic Transmission Fluid (ATF) refill. Any brand is fine and you can mix different brands.
Another important part in the hydraulic trim kit is the pump and control unit. Make sure installation of these parts are in a dry location to avoid water damage and failure.
Electric Kit Checks
Completing electric maintenance is important for both performance and safety. Do periodic checks for clean electrical connections on all 12v components. Such as the helm-mounted rocker switch control, the ground wire and the unit’s quick-disconnect wiring harness. Also, look for deterioration of wiring such as bulging or hot spots, fix or replace as needed.
Stop Saltwater Deterioration
Two issues that are specific to saltwater are electrolysis and corrosion (rust). Thus, anglers who use and keep their vessels in saltwater need to add zinc anodes to the top of each trim tab.
Electrolysis Versus Galvanic Corrosion
This is an often misused term by boaters as a description for galvanic corrosion. It’s not, below is an explanation of each.
This is a reaction that happens when two dissimilar metals come into contact with each other. Next they get immersed in an electrolyte like saltwater. This contact and the electrolyte create a natural electrical current.
In simple terms, this is what happens when a wire touches something it shouldn’t. Like bad wiring in the actuator sending current into the saltwater (electrolyte). Causing one metal to give up electrons and corrode. Electrolysis is way more destructive than galvanic corrosion on your vessel. Which is why proper wiring and sacrificial anodes are critical.
To deter electrolysis, attach zinc anodes on the top of the stainless steel trim plate. Ensure you make direct attachment of a 2-inch round zinc anode tab and never paint over them! Doing so will defeat their purpose.
Avoid Barnacle Build-Up
It’s also a good idea to paint your stainless steel tabs, to avoid marine growth build-up. Both Bennett and Lenco kits that get caked in barnacles are going to be less effective.
How to paint my trim tab set?
This is a typical process for paint protection of your trim kit.
- Before applying any paint. Always start by wiping the metal plates clean with acetone to remove contaminants.
- Get some 100 grit sandpaper and gently sand the top and bottom of the plates. This will make the plate better accept the primer coat.
- Re-wipe the plates with acetone and then paint on 2-3 coats of epoxy primer. Make sure to allow for 10 hours of drying time between each coat.
- After you finish applying the last coat of primer, wait 24 hours for the primer to cure.
- Now, apply two coats of an ablative/polishing anti-fouling paint. Some brands like Bennett say to paint the actuator, including the piston too.
Some boaters skip the priming step, I recommend you don’t. No primed tabs tend to peel the paint within weeks due to high water pressure pounding against the tabs.
Finishing Up: Trim Tabs Guide
Whether you choose to buy a Lenco electro-hydraulic kit or a Bennett electric trim set, by now you know what to look for. How to operate them and the level of maintenance each type needs.
As always, thanks for letting Outdoors Informed help with your research. We do it so you spend less time indoors and more time keeping your vessel balanced.