When you’re building an inboard boat, getting the engine and propeller shaft in perfect alignment is an interesting process. It’s a logical process that involves aligning the engine and transmission, shaft, shaft log, and strut. It starts with the strut (the piece that holds the shaft in place just forward of the propeller). There is a tube the shaft passes through that gets glued into a hole bored through the keel. Getting that tube, called the shaft log, glued in line with the shaft and engine is the aim of this process.
So I started by mounting the strut with a piece of shaft in it to make sure it lay along the centerline of the boat. Everything that follows depends on the position of the strut. Once the strut is firmly mounted, you pass the shaft through the strut, then through the shaft log, and attach it to the transmission on the back of the engine. So, get everything hooked up and that will hold the shaft log in position. Then just pore epoxy around it and let it cure. Pretty straightforward, huh? Well, there is a bit of cleanup afterwards. My tape job was not leak proof, so some of the epoxy leaked out under the boat. I had to do a little grinding, but no big deal.
All of this should be aligned within .003 inch in a good installation. If a piece is out of alignment, it can cause unnecessary vibration, noise, wear, etc.