OK this is a question that will probably make most people thing "doh what is he like"
Not yet having done the RYA powerboat courses (not available here) I am learning things mostly by trial and error at the moment, but one thing is puzzling me.
I have found on my RIB that unless you have the engine trimmed quite a long way up, at any sort of speed you get a huge rooster-tail of spray from the stern coming up at an angle of about 60 degrees, at anything over about 15 knots, so much that it comes up the side of the tubes and a certain amount comes into the boat.
This happens with the engine trimmed in the horizontal position or anything below that, but it doesn't make sense to me - it should (I would have thought) be the opposite, in other words it would spray water everywhere if the engine was too far up and the prop was near the surface. In fact several people asked me in the first few days if I was running with the engine trimmed too far up, because of the spray. I wasn't, in fact it was about in the horizontal position but since then I have been running it with the engine trimmed a couple of notches up on the gauge, to avoid this happening. I have never noticed it on any other boats.
I was under the impression that for best speed you would normally trim the engine down a bit, to lift the stern up out of the water a bit more and get it more up onto the plane, but you just can't do this on mine as it sprays too much. I have done some experimentation on mine and in fact there is a slight maximum speed advantage if you trim the engine down a bit (i.e. forcing the stern into the water), only about 2kt or so though.
The only thing I can think of is that it is some relationship between the height of the cavitation plate on the engine, and the bottom of the transom. If you can imagine taking something flat like a garden spade and drawing it across the surface of a pond at high speed, the way the water would spray out the sides of the spade, that is the best description I can think of. I can't check the height of the cavitation plate against the keel until I take the boat out of the water, but having done a bit of digging on Google it looks like the cavitation plate should be set at the same level as the keel?
I had 4 other people in the boat the other weekend, all were sitting on the tubes behind me, and it seemed to reduce this a lot having the weight at the stern, I tried it trimmed down a bit and it didn't do this. Most of the time I have been running solo, with just me (drivers seat is just forward of centre and of course the weight of the console is in front of that). No extra fuel cans or anything.
Any thoughts, or suggestions? It seems that the motor should perhaps be set a little lower on the transom, but there is no space to do this.
Or should I just carry some fat people round in the back?