I don't think there is a 'lock' as it were.. you can only adjust the turn friction. On some models, it's just a bolt on the side of the sleeve portion (behind the mounting clamps), and I've seen some that have a lever on front (between the mounting clamps) that you slide left or right to adjust. Check your manual (or find one online for your model).
As for reversing and docking.. I never had any problems with the inboard diesel in our H27, even though it was a bit underpowered (original 8HP yanmar). Maybe there is a problem you are overlooking..
Cheers,
Brad