Without even running the numbers I can see that you are already killing that engine. Revs should be 4400-4800, so you are throwing horsepower away by being overpropped (as well as damaging it!). And if you're looking for speed, a 4 blade prop is another way to chop 3 or 4 knots off the top. 4 blade props are great for a lot of things, but top speed is not one of them.
So, let's start with those figures. From those I have calculated that you have a 1.84:1 drive. (for those who use 'slip' as a performance guide, it's at 18.5%. Ok for a boat that size, and an engine so overpropped.)
My first recommendation would be to go to a 15" 3 blade prop, but since the current engine is cracked and you are looking at replacing, I would definitely go to the 5.7.
Using Merc's prop calculator, it suggests that with the current V6, you could get your top speed up to about 35 with a change to a 15" 3 blade prop. But when I put the numbers in for a 5.7 engine, a 21" prop should put you up in the 42mph area.
So that pretty much clears up the question of whether you're current drive will work.
(yes it will)
As for fuel consumption. Up to the point you reach 30mph, both engines will use pretty much the same amount of fuel. Once you start pushing into the 30s, obviously you're going to use more, but at the same time, you'll be putting more distance behind you. My best estimate would be that you'll use around 2 to 2.5 litres/nautical mile up to about 35mph with the V8. Any faster than 35 and I think you'll be looking at around 2.8 to 3 L/Nm.
Chris......
