Interestin' question, Rene. And one, from a technical point of view, to which I am not qualified to answer. From a practical point of view it would seem sense to keep the engine level with the bottom of the hull and the forward engine mounts as close to the longtitudinal stringers on which they will sit.
This will then determine the height of the backplate because the rear engine mounts are attached to it. The hole through the transom is, obviously, a cut-out within the backplate.
There are a couple of very practical complications to this that we discovered in changing the short Yamaha for the longer Yanmar. You'll see that when I post Part 4 of the New Guts thread.
The keys to the alignment of the engine is the angle of the transom and whether or not it has been fitted at 90 degrees to the keel. It is also important to get the hole dead in the middle of the centreline.
It cost us half a day trying to sort that little lot out. We discovered ....
1 The transom is inclined at the very bottom of the 10 to 16 degree range which is the norm. It is at 10 degrees and this has meant, because the new engine mounts are further forward than the old ones, that we had to grind down the bearers to lower the front of the engine, which is slightly nose down in the boat. That was not obvious with the short square Yamaha.
2 When the hull was originally built the transom was not set at a perfect 90 degrees to the fore and aft line This has now meant that the engine is slightly skewed as well. Again, this is more obvious than it was before.
3 And finally, as we discovered when we accurately located the centre line of the back plate, the original engine was not mounted dead in the middle of the transom.
None of these errors has caused the boat any obvious problems in the past, so presumably it should be OK when we have changed the engine.
But it just goes to show that my Ribtec hull, strong as it appears to be, is no piece of Hitec hardware.
One final thing, Rene. You may want to get in touch with Stuart "THE WAVEHUMPER" Hopkins. He's buying my old engine and is planning to do the same thing as you.