mercruiser 5.7 magnum prop selection

[lilcane88]

I have a 1989 Century 4500 XL equipped with a 5.7 magnum Mercruiser 270hp engine and alpha outdrive. The boat is 23' long and it weighs roughly 4500 lbs. When loaded. The prop currently on it is an aluminum Michigan wheel 14.5" diameter 19 pitch. The boat accelerates well but is a little slow to get on plane. (I have planing tabs to help) I cruise at 3200 rpm at 29.6mph according to the GPS. However, at WOT I only pick up about 3-4 more mph. Is this an ideal prop for this boat or what other diameter/pitch should I try? All help is appreciated and thanks in advance!

 

[floater212]

what's your wot rpm? what gear ratio? you might look into a bigger diameter but you can only go up to 15" to a 15 1/4" on an alpha. a lower pitch is what makes it move quicker but slower top end so if you feel it's slow by speed and rpm is lower than the top rpm range you can go down in pitch for better hole shot and then you may get some more top end. the more info you put out here the better answers you'll get. then you can go from a 3 blade prop to a 4 blade then there's the big dog 5 blade, but they play like the other props. too much to figure out with minimal info.

 

[lilcane88]

what's your wot rpm? what gear ratio? you might look into a bigger diameter but you can only go up to 15" to a 15 1/4" on an alpha. a lower pitch is what makes it move quicker but slower top end so if you feel it's slow by speed and rpm is lower than the top rpm range you can go down in pitch for better hole shot and then you may get some more top end. the more info you put out here the better answers you'll get. then you can go from a 3 blade prop to a 4 blade then there's the big dog 5 blade, but they play like the other props. too much to figure out with minimal info.

1.47 gear ratio and 4,100 rpm at WOT

 

[MikDee]

Is that fully trimmed out? If not, then you need a 17" pitch prop.

 

[lilcane88]

Is that fully trimmed out? If not, then you need a 17" pitch prop.

Yes that is fully trimmed. What diameter should I try with a 17 pitch?

 

[jestor68]

Yes that is fully trimmed. What diameter should I try with a 17 pitch?

14.5 or 15" diameter in a 17 pitch.

A 14.5 X 17 Mercury Vengeance (SS) prop would be a good choice.

Your 1.47 gear ratio favors less diameter due to it's higher shaft speed.