Trim Tabs or Interceptors, Part 2: Interceptors — Fast, Efficient Alternative for Boat Trim

In the last decade or so, interceptors have become popular to control boat trim. Interceptor manufacturers say their systems require less machinery, are faster and more durable than traditional trim tabs.

Humphree and Naiad were among the first interceptor manufacturers and Humphree is still one of the best-known for larger yachts.

Interceptors are movable composite plates that mount on the lower edge of the transom and can be lowered vertically to intercept the flow of water under the hull.

Interceptors can be installed in locations where trim tabs don’t always work, such as at the aft end of a propeller tunnel, between outboard motors and on boats with unusual bottom shapes.

Some interceptor users with automatic control report reduction in pitch angle of as much as 70% and more than 50% reduction in roll, the side-to-side motion created by beam waves. The systems can also automatically optimize running trim and balance turns regardless of the boat’s natural tendency to lean while turning. 

Improved Fuel Economy or Speed

Some manufacturers say any drag created by the blades is offset by the improvement in running angle, resulting in frequent overall improvements in fuel economy and/or speed.

Interceptors can create more lift with less surface area and they can also be extended and retracted much faster than trim tabs.

There are now a number of other companies producing quality interceptors, including IMTRA’s Zipwake, one of the most widely distributed for recreational boaters, the commercial fishing industry and other users where a compact, fast-acting trim system is needed.

Interceptors are notable for their quick response time, useful when the crew is moving around a lot on a small boat or when sea conditions are changing rapidly. They don’t have the brief lag seen when adjusting a conventional trim tab—the boat’s response seems almost immediate in most cases. (In fact, some first-time users caution that the very quick response requires a bit of mental adjustment to be prepared for the boat’s rapid change in running attitude.)

Many interceptors move from fully retracted to fully extended in under 2 seconds. That’s considerably faster than trim tabs, which typically take 6 to 8 seconds. In addition, the lift characteristics are non-linear — meaning the small blade extension generates more lift at higher speeds, making the interceptors even faster to react as they don’t need to use the full extension of the blade.  

That means as you leave the harbor and accelerate up to cruising speed, your boat will get “out of the hole” and onto plane significantly faster with interceptors. When decelerating, Zipwake interceptors automatically retract as the boat decelerates to allow the boat to “go off plane” without much trim change, making the landing more controlled and safer. 

And with a fully automated system, the computer takes over to keep the boat level as sea conditions and load changes, greatly smoothing the ride without any input on the operator’s part except to put the interceptors on “auto.” 

Innovative Technology from Greece

HydroTab has innovative technology that promises reliable response and fewer mechanical issues. Its interceptors are activated by tough pneumatic tubes that are rapidly inflated to push the blades down and deflated to bring them back up. The company says that since the movable parts do not have to be sealed against saltwater intrusion, the system is designed to be one of the most reliable and trouble-free on the market.

It also offers the Moby joystick auto-control system for easy, intuitive operation of the tabs. The system readily links to MFDs and includes a built-in gyroscope and accelerometer for full auto-control. This Greek company has been in business since 2010 and makes stainless steel and composite frame interceptors. It also offers custom-built interceptors to fit unusual transom shapes or boater needs.

Advantages of Automatic Control

With an interceptor automatic system, a gyro sensor in the control head can balance the boat automatically when planing, by extending or retracting the port or starboard units separately, as needed.

Given that an interceptor retracts and deploys faster than a trim tab, and a small increment of interceptor will generate a large amount of lift, the system can react quickly enough to offset the varying period of different sizes and shapes of waves. 

While some trim-tab systems attempt to automatically control roll as well, tests conducted by Imtra showed that the trim-tab roll-control systems may work in a long swell but simply can’t keep up in a shorter, steeper chop.

Interceptors Can Be More Trouble-Free Than Tabs

Interceptor companies also point out that their systems don’t stick out from the transom like trim tabs do and so are less likely susceptible to damage when putting a boat on a trailer. 

They also won’t create obstructions for swimmers getting back aboard with a stern ladder. 

Sensitivity & Control

Interceptors travel relatively small distances and can be adjusted in fine increments. For example, a Zipwake unit has a stroke distance of only 30mm and can be adjusted within that range by 0.33mm at a time, allowing the boat to be fine-tuned as speed and sea conditions change. 

Most trim tabs move at one speed and can’t be customized to fit the specific pitch & roll needs of each different hull shape.

Interceptors can be added in pairs across the transom of most boats to provide an appropriate degree of control for the weight, power and design of the hull. All can be controlled from a single control head. 

Boat Speed

Because of the small blades, interceptor systems require strong water flow to be most effective. 

As a result, one of the main disadvantages to the interceptor system is that it does not perform as well on slower boats without adding additional interceptors to the transom. A standard Zipwake system can operate up to 6 interceptors. 

A heavy trawler that cruises at 8-10 knots does not provide enough water flow over the smaller interceptor blades for them to create lift. For a boat like this, trim tabs with a larger surface area or more interceptors with longer blades, like Zipwake’s Series E, are going to be a more effective system for trimming the hull.

At the other end of the spectrum, many interceptor blades can be damaged at speeds greater than 60 knots, because the force on the blade perpendicular to the water flow becomes too strong.

Because tabs deploy at a very shallow angle at first, they can still be used at high speeds, and special high performance, extra strong tabs are available from companies like Livorsi and Mercury Racing. 

Cost to Install

IMTRA notes the install processes for trim tabs and interceptor systems are essentially the same, so the cost is similar as well. Both require mounting the tab or interceptor to the transom, installing a control at the dash and snaking cable to connect the two. Interceptors are generally more expensive to produce and cost more upfront. If your boat needs four or five interceptors across the transom, the cost goes up proportionately.

Whether you chose conventional tabs or interceptors, given the improvement in performance and comfort and the potential fuel savings on many boats, the cost is well amortized over the years you own the boat.

Special Trim Tabs for Special Applications

There are many manufacturers who make special-purpose tabs or interceptors for the top and the bottom of the market. Here's a look at several:

Quantum’s Archer — (Advanced Ride Control High-Speed Extendable Retractable) — provides pitch, roll and trim control for high-speed yachts using retractable winglets built into active trim tabs.

The design incorporates a hollow section trim tab with a hydraulically actuated and electronically controlled extendable wing that extends from inside the trim tab and retracts inside the tab when not in use. Archer is intended for vessels in the 79’ to 197’ (24 m to 60 m) range with speeds of 25 to 50 knots. Units for larger yachts and commercial vessels up to 249’ (76 m) long are in development.

When sea conditions demand enhanced roll and pitch control, selecting “full deployment mode” extends the winglets to increase the control forces. This activates the acceleration sensor and control for pitch, the angle and velocity sensor and control for roll and the trim tab attitude control for trim.

Ultraflex’s tab system comes with a joystick that makes it easy to trim the boat regardless of the conditions. Aptly named EasyJoy, the joystick automatically operates the tabs together and returns the boat to a level attitude. Ultraflex tabs include an electromechanical ram with an integrated wiring system that is sealed with thermoplastic resin, dual seals and the added protection of rubber bellows. They are always protected and have automatic retraction, according to the company.

Volvo Penta Boat Trim System (BTS) interceptors offer an Automatic Boat Trim option that adjusts trim automatically. An integrated GPS receiver senses change in course to keep the ride smooth whatever the heading. The trim system is recommended for speeds up to 45 knots and because it's fully electric, there’s no concern for leaking hydraulic fluid.