1992 mercury 25 hp pulls badly

We have a 1992 mercury 25 hp tiller steer that pulls badly to the right at full speed. It is much easier to steer at a slow speed. I have googled the problem but it is a little confusing. I am not much of a mechanic. Any thoughts or suggestions would be greatly appreciated. Thank you.

All is good with the outboard. A new prop and some adjusting of that fin piece. It runs much better. Thanks everyone.

I am going to check and repair it on Tuesday 8-21 . I won't be back until Fri. 8-24. I will post then if we were able to fix the problem. Thanks to everyone for the ideas and tips. It is most appreciated.

Assuming you got this resolved? That little trim tab can sure apply a lot of Torque to the steering of your outboard.

they're right, ...you can Google trim tab adjustment... here's and example on YouTube 

youtu.be/NA-_cq5ah9I

1UP is right, I had the same problem on my 40HP Mariner, solved it with some trial and error.