Started this response on another thread but thought more relevant here.
Always suffered the dreaded 'splash' to a degree although never into the boat so more of an annoyance than the real problem many suffer. To try and bottom this out I knocked up a couple of temporary transom raisers at 50mm and 65mm.
Only tested solo this weekend so findings not fully conclusive yet, though I fear the underlying cause is not just engine height. Splash is worse on a lightly loaded boat and on mine only at near max speed.
Have been running 25mm riser as stock, now tried 50 and 65mm. No top speed gain (21knots) with any, splash is getting less with more height but not yet fully eliminated. Not suffering any significant ventilation even at 65mm so may be room to go higher though reluctant to do this. Also trimming either side of optimum makes little difference to splash, but significant to performance.
In my opinion it's the disturbed water exiting off the concave transom/v-floor edge curling up and inward hitting the lower leg. The AV plate sits nicely at the surface of the main water flow with 50mm raiser, but the intermittent secondary flow hits the leg randomly and above the splash plate. This is what causes the splashing. Photo below indicates this soft corner between the transom and floor. Note: 25mm riser in pic.
Below are a couple of crude sketches to try and indicate water flow off the Aerotec and a typical SIB.
Have tried a splash plate in the past, which helps, but would rather eliminate splash at source rather than mask the problem. However, now I know the splashing is not significantly affecting performance, then a combination of raising the engine to optimum and then a splash plate to deflect the pesky secondary intermittent water flow, may be the way to go.
The quest continues.