This month's article covers the often overlooked, misused, and misunderstood trim tab. Buckminster Fuller, the American architect, has these words etched on his tombstone, "CALL ME TRIMTAB". He likened the power that a small trim tab has on a large ship with the power one small individual can have on an entire society. Leaving the inspirational metaphor to stand on its own, lets explore the value of these neglected mechanical devices. Perhaps I should first explain that trim tabs are generally not mounted on boats under 20 feet or so in length, so if you can't find them on your small boat, don't panic.

Let's examine the three axes of motion around which our boats travel and that we control. They are: yaw, pitch, and roll. In each case there is a pivot line (axis) about which our boat rotates. Yaw is the right and left motion that controls our general direction of motion. Yaw moves our boat left and right across a horizontal plane, rotating about the vertical axis located somewhere on our boat. (The pivot point is not always in the same place, depending on what we are doing.) Yaw is the only motion we control when driving a car, so many new boaters falsely assume the same to be true with boating. They soon find out otherwise.

We control yaw on most motor boats with the steering wheel or tiller. In the case of inboard engines, the steering wheel is connected to a rudder. In the case of outboard engines, I/O drives, and personal water craft, the steering directs the angle of the discharge current. In either event, the stern is forced to move in one direction and the bow then pivots in the opposite direction. Trim tabs have little affect on controlling the yawing motion (other than sometimes creating drag, which may require some compensation with the wheel).

Pitch is the up and down motion we feel when passing under a wave. Our boat rocks up and down about a transverse axis at the balance point of our boat. We can create the same motion when abruptly accelerating or decelerating. Trim tabs play a big roll in controlling how much our boat pitches when we encounter waves and when we try to come up on a plane. Trim tabs are mounted at the stern of the boat, on both sides, and are controlled independently, generally with rocker switches on the dash panel. The rocker switches are usually labeled "Bow Up" and "Bow Down." Do I need to explain? Well, yes, I probably do; but not just yet.

The third axis of motion is roll - motion about the longitudinal axis. Align your boat perpendicular to the wave pattern and you will get all of this motion that you can stand. It is not comfortable and in the extreme, unsafe. In a small boat, it can place the gunwales dangerously close to the water surface. Unfortunately, a captain can't operate trim tabs quickly enough to stabilize the rolling motion of being caught sideways in the waves.

But when a boat rolls to one side and remains tilted, due to weight imbalance, it is said to "list." A boat that is listing creates an uncomfortable ride and can be dangerous to maneuver. A captain can and should adjust the trim tabs to compensate for an imbalanced condition that is uncomfortable or unsafe. Since they adjust independently, pushing the tab down (bow down) on the low side will raise that side. This will right the boat so that it will ride level in the water. It will be necessary to readjust when persons or items shift from side to side.

Counterintuitive? Yes, I know. Why push down to go up? Pushing the tab down, lifts the stern and pivots (lowers) the bow. Pushing both tabs down or up together lowers or raises the bow uniformly. This is what you do to control pitch. Adjusting them independently is what is needed to level a listing boat, or in other words, to control roll. For them to be effective, you must be moving through the water. The faster the speed, the more the effect.

When making adjustments to the trim tabs, they should be done slowly and in one second increments to determine the effect. There is a lag before corrections take hold. Done too quickly, an adjustment can create a dangerously unstable helm. If this happens, slow the boat down and regain control. It will become obvious that trim tabs have less effect at slower speeds.

To come up to full speed in a planing-hull boat, first place the bow in the down position, then accelerate. Doing otherwise will force the bow up in a very high position, obstructing your view, and prolonging the time it takes to get on-plane. Once on-plane, bring the throttle back to where you can still maintain the boat on a plane. Then adjust both trim tabs, pressing "bow up" simultaneously, until the ride becomes slightly uncomfortable due to the "porpoise effect" of the bow bouncing up and down. Trim it back down until that goes away and you have a stable ride. The more the bow is elevated, the less water will contact the hull. This yields better speed at the same rpm, therefore better fuel efficiency, but less steering control and a choppier ride. Find the best balance for your current conditions.

Just to complicate things a bit, in addition to the trim tabs, there is another way to trim the bow on outboards and I/Os. This only works to control pitch, not roll. Many manufacturers place a small rocker switch on or near the throttle. This lifts the engine up or down, causing the bow to respond. Pushing the engine down lowers the bow in the water. Lifting the engine forces the discharge toward the surface, which forces the stern down and pivots the bow up. Lifting it too near the surface will draw air into the props, thus revving the engine while dropping the speed. So again, it is a balancing act that you will get feel for over time. On some boats, the same switch will lift the engine all the way up in order to trailer the boat. Other vessels require that a second switch be used to lift the engine to the trailer position. Do I need to remind you not to lift the engine out of the water while underway? Practice balancing the trim tabs and engine trim for maximum control and efficiency.

Learning to properly use trim tabs and engine trim controls will not only make your ride more comfortable and safer, but will stretch your fuel dollars farther as well. Trim up. Be safe. Come join us for class and learn more. (Please notice the new class location and times on our website.)