I purchased an old bass boat last year and I've been fixing it up.  It had sat for 15 years, so I spent this summer getting it working right mechanically.  Now it's running great.  I had it out on the water today and I'm noticing that I have to keep left turning pressure on the steering wheel to keep it from turning right when I haven't trimmed it up.  This is the time when I'm just planing out and starting to trim up.

 

The strange  part is that once I trim it up, it doesn't require any left turning pressure on the wheel.  If I keep trimming it up, it actually starts requiring right turning pressure to keep it going straight.  I'm not sure what's going on here.  Can somebody explain why this is happening?  Do I need to adjust the trim tab at the back of the outboard that is right above the propeller?

 

In case it helps with the question, the boat I have is a 1989 Astroglass 16.0 with a 100 HP Mariner 2 stroke.

There is a torque tab located on the underside of the cavitation plate. The tab needs to be adjusted to neutralize the prop torque. If the torque pressure is toward the right turn it about 10 degrees left or until the steering pressure is even.

Tom

 

If the boat is not up on plane to the point the hull spray is not back to or behind the drivers seat and properly trimmed out, the hull torque is usually going to be too much for the trim tab under the anti-cav plate to do any good.  It usually works when you are up at a good cruising speed with the motor trimmed out, that's where you adjust it for neutral steering.

Coming off plane or slowing way down the increased drag on the hull is going to make the boat want to turn right and there is not a big enough trim tab made to compensate for that.

 

One other note. If the motor is on a jack plate and you have it raise up a lot, you usually have to put a trim tab on the skeg because the one under the anti-cav plate is not in enough water to do anything.

If I am interpreting the issue properly it is that the engine is leading/turning the boat differently at different trim positons.  If this is happening I think you have to make a decision as to which position you want the effect to be neutral, or straight.  I would do this at the throttle and trim positon I use the most for long distance running.  

It could be the single cable steering, which by its design requires gripping the wheel when underway.  I’m not sure if there is a way to eliminate that. 

1 hour ago, papajoe222 said:

It could be the single cable steering, which by its design requires gripping the wheel when underway.  I’m not sure if there is a way to eliminate that. 

My Monark had  that tendency, but my Lund Rebel 165 does not.  With the Monark there was nothing that could eliminate that tendency.  

The torque tab is designed to neutralize prop torque.

Trim is another issue, that is the angle of the prop thrust. Trim affects the boats hull bow lift. The wetter the hull is on plane affects speed and steering.

Tom