Although it was nearly 50 years ago, I still clearly remember my first trip offshore. I don’t remember much about the day, or whether or not we caught any fish. But I vividly remember the ride out to our spot, with the boat careening from wave to wave like some sort of crazy fibreglass dolphin porpoising from crest to crest, bouncing and crashing for what seemed like an eternity before we finally arrived at our destination. Worse still was the realisation we would have to do it all over again to get back home, and the question of what would give out first – my kidneys, or my knees?
I don’t get seasick, and I wasn’t fearful for my life, but the ride was just plain uncomfortable – to the point I didn’t know if I wanted to ever do that again.
Sound familiar? If it doesn’t then I guarantee it will to someone in your crew. Because the reality is that a rough ride can’t help but suck the fun out of boating for many people, including lifers like me who just prefer a smooth ride over bouncing around like a tennis ball.
Thankfully, it doesn’t have to be that way. Thanks to some brilliant and innovative engineering, it’s easy to smooth out that choppy ride on almost any boat by installing a set of interceptor tabs.
Interceptor tabs burst onto the boating market about 10 years ago, but many people still don’t quite understand what they are or what kind of change they can make to a boat’s ride. Like the trim ailerons on an airplane’s wing, interceptor tabs are small devices that make an outsized impact, so perhaps we should take
a closer look at what they’re all about.
Not your father’s trim tabs
Interceptor tabs —old under various trade names like Zipwake, Humphree, Hydrotab, and Seakeeper Ride—are compact, trim tab-like devices that mount along the bottom of the transom. They intercept water flow, hence their name. They’re sold in models suitable for an enormous range of boats up to 30m in length.
Although both interceptor tabs and traditional trim tabs mount to the transom and re-direct water flow to level the hull and improve a given boat’s ride, there are significant differences in how they work.
For starters, their physical construction is quite different. Traditional trim tabs extend from the bottom of the transom on a hinge, with the tab angling aft and down as an extension of the boat’s running surface. A hydraulic
or electric actuator either lowers or raises the tab as desired, with inputs controlled by the captain using dashboard-mounted rocker switches or toggles. They’re fairly substantial devices, with a trim tab that’s 30cm wide typically measuring a further 20 to 30cm in depth.
By comparison, a 30cm wide interceptor tab will typically use a blade that’s only two or three centimetres deep, and which extends vertically below the transom rather than trailing behind it. While much smaller in area, the interceptor’s sharp blade angle and position against the hull generates essentially the same amount of lift.
The advantage to the smaller blade used in interceptor tabs is reflected in the great speed at which it can be raised or lowered. Where a typical trim tab takes from six to eight seconds to fully raise or lower, interceptor tabs can adjust at speeds of up to 100 times per second, allowing virtually continuous adjustment to the boat’s running attitude for a much smoother and level ride. These adjustments are computer controlled and based on a variety of sensor inputs, allowing faster and more measured tab response to wave motions than any human skipper could ever provide.
Both trim tabs and interceptor tabs rely on connected onboard equipment to control them. In the case of trim tabs, that additional hardware consists of dash-mounted switches to operate them, and either a hydraulic pump or electrical connection to actually power the devices. Interceptor tabs also require a power connection and most include a dash gauge or MFD connection to display their operation, as well as a black box control unit where the digital brains of the device reside.
With their digital controls and multiple sensors, interceptor tabs can control more than the up-and-down pitch of a boat running headfirst into swells. They can also effectively reduce roll (rocking from side to side) and yaw (rocking from bow to stern) by as much as 50% to further calm a bouncy ride. AI-enabled software can even allow some of these devices to actually predict hull motion based on recent inputs, and proactively adjust to further smooth a bumpy sea.
With their active ride management capability, interceptor tabs do more than guarantee a smooth ride. By improving the boat’s running attitude, they can measurably improve both top speed and fuel economy, while simultaneously reducing the volume of GHG emissions generated in a typical day on the water. Now those are benefits everyone can appreciate.
In for the long haul
There are further differences between interceptor tabs and trim tabs when it comes to service life and everyday maintenance. With their extensive flat surface, trim tabs face significant pressure when deployed, and particularly along their outer edges, away from the support of the actuator. Over time, the tab can warp or deform, reducing its effectiveness and ultimately contributing to new stability issues.
The smaller exposed surface of an interceptor tab, by comparison, is far less liable to deform from extended use, allowing it to render a longer service life.
If you don’t remember to lift them first, traditional trim tabs make it more difficult to control a boat when reversing, such as when backing down on a hooked fish. They’re also vulnerable to breaking due to the reversed water flow putting pressure on the actuator mounts. What’s more, by extending aft of the transom in their retracted position, traditional trim tabs are vulnerable to impact damage should they come into contact with pilings or docks – or damage by swimmers using them as steps to clamber back on board.
Not so for interceptor tabs. With their reduced profile, reversing without first withdrawing the tab won’t lead to damage, nor are you likely to hit the interceptor against the dock with its snug fit against the transom. Being more compact, they’re far less likely to be damaged in everyday use, or to snag fishing lines when the action comes close to the boat.
Further, where trim tabs can be magnets for marine growths, barnacles and other marine organisms simply can’t cling to the retracted blades of interceptor tabs. With many designs using a composite polymer housing, they can even have a hard time clinging to that.
So what’s the catch?
So it sounds like interceptor tabs beat trim tabs in every possible way. Or do they?
One bugaboo with interceptor tabs is that they require water pressure to work, so their impact is substantially reduced on slow boats, or when operating at slower speeds. A large, heavy trawler cruising at six knots simply doesn’t generate enough water flow for an interceptor tab to function well. On slow boats with displacement hulls, traditional trim tabs – or better yet, a gyro stabiliser – tend to work better.
Interceptor tab systems require flat transom surfaces for mounting, which makes them difficult or impossible to fit to some boats. Results on stepped hulls seem to be mixed depending on the exact hull design, with some combinations of hulls and tabs working better than others.
Finally, interceptor tabs can cost twice as much as traditional trim tabs, reflecting the additional digital brains and hardware involved. But in the grand scheme of things, those are comparatively minor caveats.
With their ability to mitigate unpleasant ride characteristics associated with pitch, roll, and yaw issues, interceptor tabs are simple devices that work amazingly well. But don’t take my word for it. Go for a ride and experience it first-hand. Your kidneys and your knees will thank you.
