Is something missing here near the prop where this open hole is at? I'm on using my cell phone so please look at the 2 photos I just uploaded if I can't get them attached to this post.
If you're holding the steering wheel straight, at speed, and the boat is drifting either left or right, the trimtab can compensate for that. It's like a little rudder. If you don't have a problem, don't worry about it.
I think the reason for what Carl mentioned is prop walk. Right? So the trim tab compensates for the torque the motor has, even when going straight. Ever held a spinning gyroscope? Ha ha ha. I haven't thought about one of those in a while.
My optimax 150 and my cousins 200 optimax don't have them. I never noticed it until this thread was brought up. Wonder why? My last outboards all had them.