cb, my first boat had a 3.0 and was a real dog. I put a 4 blade stainless on it and it kicked it up about 8-10 mph. yours could do the same or better depending on what boat.
Is your boat the Jet in your profile? If yes, you best bet is probably a custom trimmed impeller. Another option may be a whipple-supercharger but that's going to be much more work.
the prop on it has already been pitched down to 15 and top end at the moment is about 32mph. Ralph was just taking the piss as he knows I am going to sell it as an X2 arrives in the country tomorrow I was wanting a cheap fix really and a four blade Stainless at the right pitch would be over 1000 New Zealand Dollars Mike its not the boat in my profile ..its a 92 bayliner with a 3.0 l mercruiser. and the boat in my profile is a Direct Drive Skicraft Senator (aussie boat) that part at the back which made you think it was a jet is actually the exhaust..but many people previously have thought the same thing