The two surfaces would need to be parallel and all that seems too much of a hassle for what it is. I recently discovered clearance issues (everything is rather small and cramped) that would leave me with substantial additional modification to the stringers to make it fit.

The initial idea was to mount them on two steel plates welded in an L shape and bolt through the stringers horizontally. Therefore, I decided to fix the engine mounts on a solid steel plate (8mm) and mount that plate onto the stringers (not glassing them in as I then have to thread into the plate in situ after proper alignment of the engine and the engine room is way too small to get that job done properly). I did not want to mount the flexible engine mounts with lagbolts into the wooden core as a) it’s the wrong wood for that kind of job, b) I wanted to prevent rotting, c) the engine shall be removable for maintenance and to get to the fuel and water tanks behind the engine room if later require cleaning (I’m not a fan of removing lagbolts and then reusing the holes as is) and finally d) the rear mounts sit on only 2cm of stringer, no way a screw that short will do anything!

We had to drill out and replace the stern tube and other bits and pieces too and that gave me a good reference to align everything and mount the drilling jig. I retained the old structure as a reference. I will glass over it with epoxy when done. The structure is now longer and made out of a wooden core (fairly good quality European spruce, the only thing I could get hold of at short notice) that sits in one half of the old stringer (the other half was cut away to put in the core). We needed to modify and reinforce the previously hollow stringers on which the old engine sat. 120kg (before that 1 cylinder petrol engine, had 5hp when new 50 years ago, don’t know how many horses escaped). We will put in a 2 cylinder Diesel Engine with 16hp, weight approx. We are replacing the engine on our 27ft sailboat. I’ve read through several posts on this topic but I am still unsure what my best option is:

Inboard motor mounts