Imtra’s Zipwake Product Manager Jamie Simmons explains the key differences between trim tabs and interceptors.
If you’ve ever owned or operated a powerboat, you’re likely familiar with trim tabs, used to lift the transom of the boat up, and push the bow down to help the boat plane and ride comfortably. However, in the last 5-10 years, manufacturers like Zipwake, Humphree, and Volvo QL, among others, have introduced interceptors to help better control the trim and roll of powerboats. Both systems have their advantages and disadvantages, so which one is best for your boat?
What is an Interceptor?
In the world of boats, an interceptor is usually a small vertical plate that’s set on the bottom edge of the boat’s stern or transom. The plate creates additional lifting pressure beneath the boat, and in the case of mechanical interceptors, the plate can be extended or retracted as needed to increase or reduce the amount of lift.
What Do Trim Tabs and Interceptors Have in Common?
Boat trim is the fore-and-aft attitude of the hull of a boat at different speeds. A primary purpose of trim tabs and interceptors is to modify the trim, or the water flow past the boat so the hull adopts the best attitude for speed and ride. Simply put, both interceptors and trim tabs aim to lift the transom and lower the bow while underway for a smoother, faster, and more fuel-efficient ride.
How Do Trim Tabs and Interceptors Work?
Trim tabs are a common device attached to the transom or within a transom pocket, usually in pairs, that can lift the transom and lower the bow under way by pressing down at an angle into the water flowing past the transom. The sharper the angle, the greater the effect of the trim tab.
Interceptors are mounted on the lowest surface of the transom, and like trim tabs, interceptors redirect the water flow to change the attitude of transom and bow. Where they differ is that interceptors enter the flow at approximately a right angle, or vertically. Also, interceptors extend only a small distance, just 30 millimeters – roughly 1 inch – in the case of Zipwake interceptors.
For example, a Zipwake interceptor that is 12” wide will have a blade that measures 12” x 1” and a trim tab that’s 12” wide will typically measure 12” x 9” or 12” x 12”.
Because of that size, trim tabs can foul lines in the water and easily be damaged against pilings or docks. As a result, the lower-profile interceptors are often preferred by fishing, lobster, pilot, and other commercial boats.
What are the Advantages & Disadvantages of Trim Tabs and Interceptors?
Bow rise occurs initially because most boats are heavier in the stern and lighter in the bow, and as a boat accelerates, the bow lifts up and the stern sinks, pushing a wall of water in front of it. As the boat accelerates, the engine must drive the boat uphill to get “over the hump” and onto a plane. Both interceptors and trim tabs assist the engine by creating upward pressure at the stern, reducing the steepness of the hill the boat must climb and helping it level out faster. While both can work well, where they differ is in their reaction time and efficiency in creating lift without lots of drag at the same time.
The main advantage of an interceptor system is the smaller blade and the speed at which it can deploy and retract. Zipwake interceptors move from fully retracted to fully extended in 1.5 seconds. That’s four to five times faster than trim tabs, which typically take 6 to 8 seconds. That means as you leave the harbor and accelerate up to cruising speed, your boat will get “out of the hole” and onto a plane significantly faster with interceptors.
That speed also comes into play with automatic roll control. If a boat is planing but listing perceptibly due to sea state or cargo, some interceptors and trim tabs can be used to manually counteract the list or roll. With a Zipwake 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 tim 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.
As illustrated in the video, the smaller blade of the interceptor also creates less drag than a conventional tab, improving speed and fuel efficiency.
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 0.5mm at a time, allowing the boat to be balanced in a fine-tuned manner. There is a physical gauge within the control-panel wheel that allows you to gauge each increment.
By comparison, trim tabs generally move at one speed, and cannot be customized to fit the specific pitch & roll needs of each different hull shape.
Boat Size & Speed
Because of the small blades, interceptor systems require strong waterflow to be effective. As a result, one of the main disadvantages to the interceptor system is that it does not perform well on slower boats. A heavy trawler that cruises at 8-10 mph does not provide enough waterflow over the interceptor blades for them to create lift. For a boat like this, trim tabs with a larger surface area are going to be a more effective system for trimming the hull.
At the other end of the spectrum, most interceptor blades can be damaged at speeds greater than 60mph, because the force on the blade perpendicular to the waterflow becomes too strong. Because tabs deploy at a very shallow angle at first, they can still be used at high speeds. That being said, there is very little need, if at all, for trim control on a boat that is already travelling 60 mph or more.
Similarly, most interceptor systems are going to be ineffective on boats of 65-feet or more in length because the small blades don’t generate enough lift to raise the transom of the larger, heavier hulls. To put an effective interceptor system on a boat of that size, you would have to get a custom system from a manufacturer like Humphree, which specializes in that sort of work.
Cost to Install
The install processes for trim tabs and interceptor systems are essentially the same, so the cost of the installation generally is similar as well. Both require mounting the tab or interceptor to the transom, mounting a control at the dash, and snaking cable to connect the two. That means the labor and materials cost of the install will be near identical.
The pricing of the actual equipment varies vastly, but typically an interceptor system is going to cost more. See a comparison below for Zipwake interceptor systems and Bennett trim tabs with the automatic control function added.
A pair of 12” x 9” Bennett BOLT electric trim tabs has a list price of $545.00, and the added AutoTrim Pro for automatic control lists for $650.00, bringing the total cost of the equipment to $1,195.00.
A comparable interceptor system from Zipwake, the 300S kit, lists for $2,490.00, and comes with everything needed for the system.
Of course, all of these items are available for less than the manufacturer printed list price, so always be sure to get a quote from your dealer or installer.
Written by John Burnham & Conrad Taylor