With an aluminium prop I'm almost sure you want:
- anti cavitation plate in-line with the sibs bottom line
- trim set to 3rd, 4th or 5th position (you will probably have 5 trim positions on a 40hp outboard, could be 4 but than again you get my idea), but that's something you can easily test by comparing with the gps in your hand. This is when you see the 1st as the lowest trim position and the 5th as the highest.
With a SST prop I'm almost sure you want:
- anti cavitation plate slightly above the sibs bottem line
- trim set to 3rd, 4th or 5th position, but that's something you can easily test by comparing with the gps in your hand.
Engine height to deep will cause (in general):
- excessive spray
- lots of drag = harder steering = most important, far less speed
Trimming low will do the following in general:
- press the bow down = less speed but easier planing
Trimming up will do the following in general:
- push the bow up = far more speed but the bow will rise sky high when going on the plane.
Now what you also want to know about this whole story is that because of a sib has flexibal contacts between the transom and the rest of the boat, trimming isn't as effective as it is with hard bottomed boats, because of the flexing around the transom that could occure and that will result in trimming won't have much effect. But with 40hp you have enough hp I think to overcome that problem.
I would say try it and you will be pleased about the results, I'm sure about that. BTW the trim-position can be easily adjusted when driving the boat, just set your engine to swallow water position and you can change the trim-position if you haven't got powertrim. If you have powertrim you can play with it when driving te boat.