What is the best setback for a 2005 Tr21X with a 05 Mercury 225 Optimax?

I have the same set up you have except I'm running a CMC plate that is 51/2" setback. I installed as set of 21/2" spacers for a total of 8" setback and it made a big difference in handling. Didn't see a large increase in speed but the handling was much improved.

Not sure what 10" set back would do. I think the recommended setback on your boat is 8". I'm running 72.5 with 2 people and full tournament load and 1/2 tank of gas.

Have not run it by myself since I changed my prop so I don't know what it will run with a light load. More.

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