On another boating forum there was a question about using the outdrive trim and trim tabs. The person asking the question wondered when you would use one or the other. Here's is the reply...
Generally speaking, your outdrive should be trimmed all the way down when you're starting from a low speed and getting up on plane. I might add here that it takes the pressure off the lift system.
As the boat transitions from a low displacement speed (moving through the water) to being on plane and the bow has come down, you should start raising the trim.
Keep coming up with the outdrive trim and when you have raised the trim a bit too much you will hear the prop start to cavitate. This sounds a little like a rumble and you'll hear a change in the sound of the engine. At that point just tap the "trim down" button a tiny bit until the noise goes away and you're in the best trim position. This optimum position will depend on the boat’s load (full or empty fuel tank, number of passengers, where they’re sitting, etc.).
After you get used to the boat a bit you'll learn to listen for the sound of the engine at your optimum cruising rpm. When it's properly trimmed you'll hear the rpm's increase a bit and the speed will increase without any change in the position of the throttle.
If the boat is porpoising (the bow will be going up and down like you're running in a chop but you're on calm waters) it's because your trim is up too far. Just ease it down a bit and the porpoising will stop and you will pick up some speed.
My boat has V-drives so I do all this with my trim tabs, and also use them to balance the boat for varying loads. On boats with an outdrive, you should mainly use the trim tabs for side to side balancing. As people move around in your boat and the load characteristics change you can adjust for the weight differences by tapping the tabs up or down.
In these drawings, the top one shows a boat properly trimmed. There is a pivot point slightly ahead of the outdrive (shown in red) where the boat will pivot as the outdrive trim is changed. If the trim is correct, the bow will be raised a bit above the water surface and the boat planes easily over the top of the water.
The second drawing shows a boat where the outdrive is trimmed too low. It's plowing through the water rather than riding over the surface and, it would be throwing water out to the sides. This wastes a lot of the boat's power because the thrust of the outdrive is trying to force the bow down and it's pushing the boat through the water rather than along the surface.
The bottom drawing shows a boat with the lower unit trimmed too high. The lower unit is trying to raise the bow out of the water. It will succeed in doing this for a few seconds but can't sustain that much weight out of the water so the weight of the bow overcomes the ability of the thrust to hold it up, and it falls back to the surface of the water. The thrust again tries to life the bow and the same thing happens over again. That's where the porpoising comes from...that repeated attempt of the thrust to lift the bow but the inability of the thrust to keep it up there.
Just thought I would pass this along!
Mike -
