There’s a ton of tech you can add to a boat to improve its ride, ranging from shock-mitigating seats to gyroscopic stabilizers, but few accessories have as dramatic an effect as plain old trim tabs. In fact, a huge proportion of the powerboats on the water today depend upon trim tabs to deliver acceptable seakeeping, efficiency, and performance attributes. But trim tabs are often misunderstood, and many boaters don’t use them even though their boat is equipped with them. Are you guilty of gazing at those little switches on the dash, and wondering if you should press one or the other? Do you own a boat without tabs, and wonder if it would be a better boat if you added them? We’ll answer these questions and many more in our definitive guide to trim tabs, by examining:
What are Trim Tabs
Trim tabs are small surfaces added to larger surfaces in order to control trim (running attitude). They aren’t unique to boats and are also found on other vehicles that travel without sitting level on the ground, including submarines, airplanes, and helicopters. In the case of boats, they usually appear in the form of metal plates extending back from the transom at the lowest point of the hull, on either side of the propulsion system.
Demystifying trim tabs can be difficult in part because the term “trim” is used in so many different ways. Trimmable drives such as outboards and stern drives also get “trimmed” and effect the running angle of a boat, but they aren’t tabs. The small fin attached to the lower unit’s anti-ventilation plate, behind the propeller, is also called a trim tab even though its effect is merely to counter propeller torque. And on many wake boats purpose-built trim tabs may be utilized not to trim the boat, but instead to increase or shape the type of wake it makes. So, unsurprisingly, there’s a lot of confusion surrounding just what trim tabs are and what they do. For the vast majority of the boats out there, however, it’s safe to limit the discussion to the metal plates protruding from the transom on either side of a boat’s propulsion system.
In some cases trim tabs are fixed, but the majority are adjustable with the flip of a switch. If the captain lowers the trim tabs the flow of water against them pushes the stern of the boat upwards, which in turn forces the bow of the boat down. This can be especially helpful on boats that tend to plow through the water with an excessive bow-up attitude, and/or when a boat is carrying a heavy load far aft that causes the stern to dig in and the bow to rise up. In some cases when a boat is heavily loaded (or underpowered) deploying trim tabs may be necessary to get onto plane.
In addition to adjusting fore and aft (longitudinal) trim, trim tabs can also be used to regulate side to side (lateral) trim when the port and starboard tabs are adjusted independently. Let’s say, for example, that three passengers are seated on the port side of your boat and none are on the starboard side. On most boats the uneven weight distribution will cause a list to port as you run. However, if you deploy the starboard-side trim tab slightly more than the port tab, the extra force generated by the water pushing against it can eliminate the list and get your boat running on an even keel.
Different Types of Trim Tabs
There are actually quite a few different categories of trim tabs. As stated earlier, there are fixed tabs (some of which can be adjusted through a range of settings but can’t be adjusted while the boat is in the water and running) and adjustable tabs, and the vast majority you’re likely to encounter are adjustable. These can be broken down into hydraulic trim tabs and electric trim tabs.
Hydraulic tabs generally get the nod for reliability and longevity. Since the hydraulic power units are located inside the boat, they live in a relatively dry environment, whereas the electric motor on electric trim tabs are in actuators, which sit partially or entirely beneath the waterline. Marine growth or sand can destroy the O-ring sealing water out of the actuator, which can put the trim tab out of operation. However, electric tabs are much easier to install since there’s no hydraulic system. Plus, they tend to be noticeably faster than hydraulic tabs.
In recent years manufacturers have also developed some rather tricked-out automatic trim tab systems. These incorporate gyroscopes, accelerometers, and other sensors with a control unit, helm controls, and the tabs. The control unit constantly senses the attitude of the boat, and adjusts the tabs accordingly all on its own.
Although technically speaking they arguably aren’t trim tabs, no conversation on this topic would be complete without also talking about interceptors. Interceptors serve the exact same function as tabs, but instead of using those big metal plates they have blades enclosed in housings on the transom. The blades slide down below the bottom of the hull to create the same sort of upward force. They have a significant advantage over trim tabs in that they only need to go down a tiny bit to create the same amount of force, so they can move through their entire range of motion in a fraction of the time it takes for trim tabs.
That fast speed means automatic trimming systems incorporating interceptors are fast enough to make their adjustments — multiple times per second — to vastly reduce pitch and roll while underway. In fact, some of the latest systems incorporating interceptors or interceptor-like blades claim be able to make up to a 70-percent reduction in the motion of the boat. The downside? As one might expect, it’s cost. Generally speaking a system like this will cost three to five times what a set of trim tabs would, or possibly even more.
How to Use Trim Tabs
When first learning how to use trim tabs remember to distinguish between longitudinal trim versus lateral trim, and deal with them individually. It’s usually best to address the fore and aft attitude of the boat first. Unless an application of tabs is necessary to get the boat onto plane, leave them in the up position as you bring the boat up to speed. Then adjust both tabs simultaneously in very small increments until the boat reaches optimal performance. Usually this means making continual adjustments until you find you’re applying a bit too much tab, then backing them off a bit. Remember to make the adjustments small — hitting the buttons for just a moment five times in succession is a lot better than holding them down for several seconds at a time, which often leads to over-trimming.
Once you have the boat running at optimal longitudinal trim, it’s time to pay attention to lateral trim. Again, remember that minor adjustments can have major effects, and making numerous quick clicks on the button will prevent you from accidentally going right past the even-keel adjustment and causing the boat to list the other way. Also, remember to think of the tab’s effect as pushing the bow down as it’s deployed. So, if the port side of the bow is lower than the starboard side, it means you’ll want to apply additional starboard-side tab to push that side of the bow down and even the boat out.
Just what is the optimal trim on any given boat? It will change, not only from boat to boat but also on the very same boat depending on variables like sea conditions and load. Finding it on any given day is as much an art as a science, but there are a few key items to keep in mind as you search for it. First, keep an eye on your speedometer. Either over- or under-trimming will reduce speed, and when you have the boat trimmed ideally, you’ll be going as fast as possible for the given rpm. Second, keep an eye on fuel burn. Trim has a similar effect on efficiency as it does on speed. How the hull handles waves is another factor, but this is one you have to feel for yourself. In some conditions some boats ride better with the bow a bit higher, but in different conditions they may ride better with the bow a bit lower. A following sea versus a head sea may call for completely different trim. Also note that in some conditions trimming too much can be dangerous, as it may hold the bow down too low as it meets waves. So, this part of the equation is a judgement call and you have to constantly assess how your boat is handling the current conditions.
The most critical thing to remember? You won’t learn where the optimal setting is if you don’t try different adjustments. Many people figure their boat is pounding because it’s rough out, and never play with the tabs to see if they can improve the ride. Others find a setting that works out pretty well most of the time in most conditions, and always set the tabs there. In either case, doing so simply means you’ll never know just how big a difference adjusting those tabs may have made. Savvy captains will adjust and re-adjust the tabs multiple times through the course of a day, as conditions or the direction of travel changes.
Should You Add Trim Tabs to Your Boat
Although trim tabs aren’t found on all boats, generally speaking this is more a matter of cost and complexity than it is of effectiveness. Just about every planing powerboat from an 18’ runabout to a 60’ cabin cruiser will enjoy a performance benefit to some degree from having trim tabs. Whether or not the cost and labor of adding them to your boat is worth it or not is up to the individual. Check out Adding Trim Tabs to get some additional insight into just how big a deal it may be in terms of both the effort it takes and the benefits it might deliver.