Handle with Care: Trim to perfection

By Dusty Miller

The balance between speed and hull attitude is a vital element of control. We have all seen boats racing down the lake at full throttle, throwing a huge bow wave with the hull flat on the water. At other times we have seen boats cruising with the bow way up and out of the water. The problem in both cases is improper hull attitude, which is a contributing factor to the loss of performance, fuel economy and handling ability. However, it’s a problem that quite easy to understand and fix by using proper trim and speed.

Trim tabs are large, flat pieces of stainless steel attached to the leading edge of the transom on a hinge and secured with a hydraulic ram. The tabs are controlled by a set of toggle switches at the helm. Toggling the tabs up brings the tab up flat against the hull, while toggling down pushes it away from the hull. The trim tabs act like elevators on an aircraft and tilt the bow up or down to correct attitude problems.

The trim adjustment for I/Os or outboard engines is on the throttle handgrip where your thumb rests. Moving the switch up swings the drive away from the hull, raising the prop higher in the water. Adjusting it down brings it closer to the transom. Remember the default position is all the way down for I/Os and outboards and all the way up for trim tabs. 

Correct trim helps the boat get on plane and remain there at lower speeds. When the boat rides low in the water at the stern, lowering the trim tabs will help raise the hull on plane and keep it there. You’ll feel the difference in the time and effort it takes to get on plane and the speed of the craft once it’s on plane. If you apply the tabs too much, they can force the bow down, slowing the boat and decreasing efficiency. Again you have to find the right balance.

One boat I was testing years ago — 34-foot direct-drive express cruiser — had oversized trim tabs. I just had to touch the toggle switch and the trim tab would heel the vessel way over to one side. When I adjusted the other trim tab it would roll the other way quite dramatically, indicating trim tabs can be adjusted to correct any port and starboard imbalance.

The trim tabs with an I/O are primarily used at speed to maintain cruising efficiency. The stern drive (or drives), on the other hand, can create a certain amount of drag. As you raise the drive, less of the unit will hang below the transom and reduce drag, causing faster speeds without adjusting the throttle. In addition, the angle of the prop changes, causing the power to push the stern down, which makes it difficult to get on plane since there’s not enough force propelling it forward. The props will also be spinning in the turbulence behind that transom and will not achieve an effective bite in the water.

In extreme cases, the downward force of the prop pushes the stern down and raises the bow. Since gravity drags the bow back down, the props force it back up and the boat starts to porpoise.

Start with the trim tabs all the way up and the I/O all the way down. Bring the boat on plane and trim the I/O up in small increments. The speed will increase and the bow will rise slightly. Continue to lift the trim until the hull speed starts to drop off or the boat begins to porpoise, Then bring the trim back down in small increments to find a comfortable cruising or top speed.

To adjust trim tabs, start from a fully disengaged position (all the way up). Toggle the tabs down little by little until you find the most efficient angle. Keep your eye on the tachometer and the RPM will increase until you get to the ideal speed. Then it will start to drop again. Applying the trim tabs too much will cause the bow to dig. In fact, it should be raised slightly above the waves.