prop question
 

I recently removed my old mercruiser 470 and replaced it with a v 8 305 chevy engine and outdrive. All Mercruiser 220 hp. I'm doing 23 mph @ 5100 rpm with the old prop. Don't know the prop pitch offhand. Does that sound like a good match? It's a step v20 cuddy 1983.
Corrections fellas: I'm actually doing 32 knots at 4300 rpm with full tanks and three people on board. Not too bad but can it be better. Don't have the prop numbers handy. Boats in the water.

Well something aint right. I cant imagine that no matter what prop you have on there you would only go 23 at 5100 rpm. check that you're tachometer and your speed measurement is accurate. And then check somebody didnt load your boat up with a ton of bricks... probably can rule that out. Check the pitch on your prop and report back....some pictures of the prop might help too.

I gotta agree with RW... Something just ain't right. 220 HP you should be skeedaddleing across the water close to 50mph.
Some things you can do...
First, get a hand held GPS... the one from a car will do fine, like a Tom-
Tom or a Garmin. This way you'll get an accurate measurement of your speed.
Second, look at your prop.. any dings on the edges of the blades with take MPH off your top end. Take the prop off the outdrive and look inside the prop on the hub. Mercury usually puts their prop part number and the prop pitch both on the blade and on the inside hub. The pitch will be a number like 17p or 21p or something like that.
Third, check your tach with another tach if you have one. Make sure you are really turning 5100 RPM.
Forth.. like RW said, Pics are always a good idea. One picture is indeed worth a thousand words. :head:

Also it couldd be possible the prop is slipping on the hub? Put a line on the metal part of the prop and on the end of the shaft or on the castle nut in line with each other. If after running it they are not in line the prop is slipping on the hub

did you change the setting on the tach, if its still set up for a four cylinder, your eight cylinder engine will be running half of whats shown. Check the back of the tach, it should have a dial with setting for 4 cyl, 6 cyl. 8 cyl, 6 poll, 12 poll. you need 8 cyl, check the accuracy with a shop tach, after that you will probably still need to reprop, I'd guess a 19p, you want that motor to turn around 4400-4800