I have a 15 ft boat with a 20 hp mercury, and when I run wide open I can hardly hold it stright, it pulls really hard to the left. The littlle trim tab on the motor is all the way to the right and works a little better. Could it be the prop?
What works for me is to trim motor until pressure on the steering wheel lets off. Much beyond that, the prop starts to ventilate.
However, your 20 hp Merc may just be a fixed trim that you adjust amont 4 or 5 holed on the motor mount like my old 40 hp merc, 1980 vitage did. Experiment raising it up from first hole to next until steering feels right. If she still pulls, keep asking for alternate solutions.
Trailing edge of tab needs to be turned left. Aluminum or fiberglass boat?
There may be a tension adjustment near the pivot that will help resist the torque.If the antivent(cavitation) plate just above the prop is below the bottom of the boat try raising the motor on the transom by adding 1/4" sticks
to the top of the transom.These will hold the height till you tighten the clamps.Just be sure it has a good grip and pumps water.
This will also improve performance slightly.
