Originally Posted by SHAZAM!
I own a 65' Donzi and just finished up rebuilding the entire machinery space. Based on my experience with my Donzi as well as others that I'm familiar with, I thought 2.5" seemed a bit undersized. Regardless, congrats on the purchase.
It all depends. The smaller the shaft, the less drag and theoretically more speed. I worked on a 75' Jim Smith with 2000hp 16v2000's that had 2 1/2" shafts, but they were a special shaft that was splined like an outboard and didn't have a keyway, making it stronger for it's diameter.