By: Bill Jennings
It is easy to underestimate the importance of that little trim button on your throttle. It’s not even painted red. Understanding why, when and how to use trim will help you improve the quality of your ride and the efficiency of your engine.
When the position of your outboard or stern drive's lower unit is in a vertical or neutral position, the cavitation plate above your propeller is parallel to your keel and the thrust of the propeller is directly astern (i.e., parallel to the surface of the water). By selecting up or down on your trim switch you change the angle of the outboard or sterndrive and this moves some of the thrust either down or up.
Trimming “down” directs the prop thrust slightly downwards. In response the stern of the boat pushes slightly upwards, which in turn prevents the bow from rising. Technically, you are moving the boat’s center of gravity forward. Every time you bring a boat onto plane, ensure that the trim is in the full down position. With the bow held down, the passing water has a lifting effect on the boat’s chines and strakes, efficiently raising it onto plane.
Once onto plane you can increase speed and fuel economy by using your trim button to find an optimum cruising attitude. Trimming “up” directs the prop thrust slightly upwards. This pushes the stern down lower into the water and moves the center of gravity aft, which in turn raises the bow. Because you are already on plane, this forward lift raises part of the bow out of the water. This reduces the wetted surface which of course means less drag. This reduction in drag will significantly improve your boat’s efficiency as measured in both speed and fuel economy. As your boat initially breaks free from a fully wetted attitude, you will feel a slight hull lift. This tells you drag has been reduced. Tapping trim up in gradual increments will raise your drive until you can feel the smoothest ride. A trim gauge will accurately indicate your trim position, but if you don’t have one, just look over the side of your boat and note the point where the water is breaking away from the hull. The further aft the water breaks, the higher you are trimmed. Some boats will trim so high that only a few inches at the back of the boat are touching the water. When trimming up, you may find that the bow begins to “porpoise” – or slowly move up and down. To correct this press down trim for one second and your boat will settle back into a smooth ride.
Have you ever experienced “chine walk” after trimming? Over-trim and speed on small boats can induce a situation called chine-walk. Your boat will start to ‘roll’ with increasing frequency, back and forth between port and starboard. Should you experience chine-walk there is only one fix - - immediately reduce throttle to regain directional stability.
Trim adjustments also become necessary in differing sea conditions. When you find yourself in a following sea, trim up. This reduces the chance of stuffing into an oncoming wave. Conversely, in a head sea, trim and speed should be lowered to prevent launching the boat into the air while crossing a wave.
If any of you find the theory side of this totally confusing, just remember this: To get onto plane, start with trim down. To go faster, trim up. While cruising between these two speeds, play with your trim until you find the most comfortable ride. Remember that the reason your trim switch is positioned close to your thumb is because of the need to always be making trim adjustments.
Oh – and disregard the boat racing creed, “When in Doubt - - Trim it Out”