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#1
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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. Last edited by Linsey T; 06-04-2012 at 08:31 AM. Reason: updated info 
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#2
05-23-2012, 03:10 PM
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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.
__________________
1986 V-20 1986 Yamaha 150 HP
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#3
05-23-2012, 04:54 PM
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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. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer 1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer 1996 V21 w/1993 200HP Mercury on a Shoreline Trailer All towed by a 5.7L Hemi Durango. If God didn't have a purpose for us we wouldn't be here, so Live simply, Love generously, Care deeply, Speak kindly. (Leave the rest to God)  Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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#4
05-23-2012, 05:46 PM
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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
__________________
1986 V-20 1986 Yamaha 150 HP
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#5
05-23-2012, 07:00 PM
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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
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