Today, almost every planing and semi-displacement vessel is equipped with some form of transom mounted trim tabs. They assist vessels to get onto plane sooner, they give a more level ride and they allow vessels to be trimmed for roll to correct weight differences and side swells. What they really do is make boating safer, more comfortable and more fun.
There are currently three types of trim tabs. Traditional tabs are characterized by flat plates, or flaps, with cylindrical actuators to deploy and retract them. Interceptor tabs use flat blades that move vertically. The third and latest system is Seakeeper’s innovative and lightning fast “Ride” system, which the company doesn’t refer to as trim tabs; instead they call it their Vessel Attitude Control System and it is perhaps unique enough to warrant its own separate category.
While traditionally manually controlled, most tabs are now offered with automatic trim systems that control pitch, roll and turning. These use sensors, GPS and other inputs tied to software algorithms to control and adjust trim tabs based on boat speed, weight distribution and sea conditions.
From a design aspect, manufacturers are focusing on creating trim tabs that provide a better ride with little or no operator input. They are also using more advanced composites and alloys to reduce weight while maintaining strength and durability.
TRADITIONAL TRIM TABS
Adjustable trim tabs were first developed for recreational vessels by Charles Bennett in 1959. Today, Bennett trim tabs remain one of the most popular brands of “traditional” trim tabs worldwide.
Traditional trim tabs consist of adjustable plates mounted on the transom of a vessel, typically just below the waterline. They operate using hydraulic or electrical actuators to raise and lower the plates. When lowered, the tabs create drag and generate lift on the stern of the vessel, effectively pushing the stern up and the bow down. This adjustment helps to improve the vessel’s planing ability, reduce bow rise and optimize its running attitude in various sea conditions and speeds. By independently controlling the port and starboard tabs, the operator (or automatic controls) can also correct listing and improve the boat’s overall stability and fuel efficiency.
There is also a type of traditional tab known as passive, or self-levelling, designed for smaller vessels. They incorporate pre-loaded gas (or spring) actuators, like automobile shock absorbers. At slow speeds, the springs force the tabs into the down position to lift the stern. As the vessel speeds up onto the plane, the force of the water causes the tabs to retract automatically. They also act independently to help control listing.
Developed in 1959, Bennett remains one of the most popular brands of “traditional” trim tabs worldwide.THE COMPONENTS
Traditional trim tabs consist of the metal trim tabs themselves, a transom plate, a hinge system, actuators and a control system.
HINGES
Traditional trim tabs are mounted either on the transom or the hull bottom. Several hinge types are used, typically designed to deploy within a range of 20 to 25 degrees.
The most common trim tabs use piano type hinges that consist of an interlocking hinge and pin. Another system joins the transom plate (with a curled edge) to the actual tab, which also has a curled edge. Together they lock into place. Imagine cupping the fingers from each of your hands together and pulling. This is the favoured hinge system used by Bennett. A unique “no break polypropylene” hinge is offered by Lectrotab. Their transom plates are joined to their tabs using a flexible polypropylene hinge. Their system uses epoxy coated aluminum and is sold primarily to aluminum boat builders, which minimizes galvanic action.
TAB DESIGN
While trim tabs are basically flat plates, the sides can be bent up or down and the trailing edge of the tabs can be bent up or down to add reinforcement and/or lifting capacity and/or to serve as the actuator attachment point. Bennett’s rule of thumb for tab width is one inch per tab for every foot of boat length (for vessels that run at speeds less than 35 knots). Most lift is generated by span—the wider the better for efficiency. A wider tab doesn’t need to be deflected down as far to gain the required lift. While increasing the fore and aft length of the tab (the cord) does increase the capability of a trim tab system, according to Bennett, it is not nearly as efficient as span when you’re at speeds less than 35 knots.
ACTUATORS
Electro-mechanical trim tab systems are the most common system for controlling tab movement, though some companies offer both electro-mechanical and electro-hydraulic actuators. Electromechanical actuators convert electrical energy into mechanical motion, utilizing separate motors for each actuator with deployment times of four to five seconds.
Electro-hydraulic trim tab systems incorporate either single acting hydraulic or dual acting hydraulic cylinders. Single acting actuators apply hydraulic pressure to only one side of the piston while a spring retracts the tab. Typically, one hydraulic power unit operates each actuator with eight to 10 second deployment times. Dualacting hydraulic systems apply hydraulic pressure to both sides of a piston with a dedicated hydraulic power unit for each tab. This reduces deployment to four or five seconds.
CONTROL SYSTEMS
Control systems range from simple up/down rocker switches, rocker switches with indicator lights to show the position of the tabs to complex auto tab controls with displays showing realtime pitch, heel and yaw in manual or automatic settings on remote units or on multifunction displays.
MAJOR TRADITIONAL TAB MANUFACTURERS
Lectrotab USA-based Lectrotab offers a wide range of electromechanical trim tabs, including custom options. Their actuators feature dual O-rings, check-valve and vent tube systems, and internal friction brakes. They offer both manual controls and Automatic Levelling Controls with various display options.
Lenco Marine owned by Brunswick Corporation’s Navico Group, has produced trim tabs for over 30 years. Their latest Pro Control Auto system is claimed to be 40 times faster than their previous system, with actuators 50 percent more load capable than competitors. The system offers direct connection to Mercury SmartCraft and includes a bank angle calculator for improved turning performance.
Bennett Marine remains one of the most popular brands worldwide. They offer both hydraulic and electric trim tabs, a variety of controls and displays, some with automatic trim controls. They also offer self-levelling trim tabs. The company is now owned by Yamaha Marine Systems.
INTERCEPTOR TABS
In the last decade, interceptor tabs have gained widespread use. While serving the same function as traditional tabs, interceptors work differently, using vertical blades that move up and down below the transom. Three major manufacturers dominate the market: Humphree, Zipwake and Volvo Penta. Some require factory installation while others can be installed by DIYers.
The modern interceptor concept is attributed to Niclas Olofsson, who in 1994 solved a high-speed ferry’s performance issues by welding a blade across the ship’s beam at the transom. He later co-founded Humphree in 2002 and Zipwake in 2015, significantly influencing all three current interceptor manufacturers.
HOW THEY WORK
Interceptor performance is based on blade width and deployment depth—typically between 1.2 inches to 2.4 inches (30 mm to 60 mm) for most yachts under 100 feet (30.5 metres) When deployed, interceptors create high pressure in front of the blades to create hull lift, forcing the bow down. This differs from traditional trim tabs, which create high pressure under the tab itself. Interceptors offer rapid deployment (about one second) and low drag when retracted.
INTERCEPTOR TYPES
Each manufacturer offers various interceptor designs for different boat configurations. Zipwake claims the largest standardized range, including Straight, chine, v-shaped, intermediate, tunnel-shaped and curved interceptors. Servos can be built into the interceptor unitor mounted inside the hull with shafts through the transom.
All manufacturers offer automatic control functions for pitch, roll, yaw and coordinated turning, using different algorithms and sensors such as GPS, 3D gyros and accelerometers. Most controls are displayed as dashboard units and most provide continuous feedback and can be integrated into multifunction displays and smart devices.
All three manufacturers have automatic blade cleaning functions to keep their interceptors from fouling.
SETUP AND CALIBRATION
As with the auto trim function of traditional trim tabs, most interceptor tabs must be set up and/or calibrated before use for the automatic function to operate properly for each individual vessel. Some systems are simpler to calibrate than others and some require a technician to do the setup, but all systems have menus on their control units where tweaks can be made to the automatic functions such as the pitch curve after initial setup.
However, when it comes to most production vessels, once the first hull is fitted with interceptors and set up, the settings can be cloned for each subsequent build of that model, eliminating the need for initial set up.
SEAKEEPER STABILIZATION
US-based Seakeeper, known for gyro-stabilizing systems, has introduced their Vessel Attitude Control System called Seakeeper Ride. This unique system claims to eliminate up to 70 percent of underway pitch and roll. Unlike traditional or interceptor tabs, Seakeeper Ride uses short linear hinged blades incorporated into horizontal electrical rotary actuators that “roll” the blades up and down.
Each unit consist of a glued transom plate, wedge pack (used to angle the unit to meet the shape of the hull bottom), actuator plate (connects the moving parts to the transom plate), seal plate (to support the actuator) and the rotary blade itself, which is shaped similar to an oval but with a flat bottom.
The cylindrical actuator rotates the curved blade up and down to create lift. Seakeeper says it takes less energy to use rotation to drive the blade, than it does to move the blade vertically. In addition, unlike most interceptors, Seakeeper blades only deploy a maximum of one inch (25.4 mm) which results in less energy use and less drag.
The primary benefit of their system, according to Seakeeper, is the fast speed at which the blades can operate. The system takes 1,000 measurements and can make up to 100 adjustments of the blade in one second. Seakeeper says they need to move that fast to counter how fast boat roll and pitch motions change at speed, and that any slower and the system would not be able to get these reductions in roll and pitch rate.
Seakeeper claims their system can adjust faster than motion can be felt by passengers, effectively damping the effects of waves and vessel movement. Their control system requires GPS input and can automatically adjust to different hull types and sea conditions without complex setup procedures.
The system is controlled via compatible multifunction displays or keypads and requires minimal setup beyond sensor zeroing. It’s available for vessels from 26 to 42 feet (eight to 13 metres) and is used by both OEM builders and for aftermarket installation.
MAJOR INTERCEPTOR MANUFACTURERS
Zipwake Founded in 2015, Sweden-based Zipwake claims to be the world’s largest interceptor-based trim control system manufacturer by volume (50,000+ units to date). They offer two series for vessels up to 50 feet and 100 feet (15 to 30.5 metres), with 25 different models both for OEMs and DIYers. Their system features integrated electrical servo motors and 1.5-second blade deployment.
In addition to manual controls, Zipwake has introduced their fully automatic PRO, an upgraded version of their Dynamic Trim Control. It offers an interceptor blade deployment rate said to be five times faster, plus active pitch control, active roll control, a wake-shaping feature and a new adaptive interface. Their DIY kits contain all necessary components and are said to be relatively easy to install.
Volvo Penta Interceptors, manufactured by Humphree, integrate with their Electronic Vessel Control system. Unlike other manufacturers, their tabs are part of complete propulsion packages and not available separately. They work with boat builders from initial design stages to develop optimized systems that can be cloned for production runs, eliminating the need for individual calibration.
Jens Bering, Volvo Penta’s VP of Marine Sales, says that in some ways, their complete system is similar to purchasing a vehicle that comes as a complete package—one that doesn’t need toying with.
Humphree Established in 2002 and acquired by Volvo Penta in 2016, this Swedish company pioneered interceptor tabs for both commercial and recreational vessels and now also manufactures interceptors for Volvo Penta. They offer the most customizable options, with blade deployments from two inches to 4.3 inches (50 to 110 mm) and widths from 12 inches to 60 inches (300 to 1,500 mm). Their Lightning range serves boats 20 to 45 feet (under 14 metres). Their Active Ride Control system is said to be a market leading anti-roll torque system that provides true stabilization in reducing constant roll and pitch motions. Humphree says their system also detects driver intentions, such as turning, and adjusts their list control function accordingly together with coordinated turn to improve performance when driving in waves, regardless of their direction. Humphree is also the only manufacturer to use the rudder input for its co-ordinated turn function. Humphree also produces interceptors designed for steering waterjet propelled vessels, typically catamarans such as high-speed ferries. They do this by utilizing the drag created by the deployed blades and this is said to be more efficient than turning the waterjets to steer.