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Discussion Starter · #1 ·
I am currently running a 14.5, 20 pitch prop and it's running at the high end of the rpms. So, I am wanting to step-up to a 20 pitch. However, why all the differ diameters for the same pitch props? What size would be best for my boat? 19.2 with a 350.
 

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Discussion Starter · #3 ·
I have an Alpha 1, 225hp and at times rpms will go as far as 5k and redline is 4700 I believe. I am mainly curious as to why the differ diameter hubs. Which would be the best for my setup?
 

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Prop diameter is just as important as the pitch of the prop. You can take a 21P three blade say 14.5" diameter and spin say 4800 rpms. Take the same prop thats 14.75"diameter and you might only be able to spin it at 4500 rpms. The diameter makes a big diffenence. Are you running a stainless prop now?
 

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Your #'s don't make much sense to me, 225HP sounds low for a 350 cid, what model year is it ?? . lets say you have a standard merc of 260 hp, the red line on that is 4600 rpm's, if you have a newer efi you could go to 5K+,
So I cant really help you with this info you gave.

But here's some basic info for you, and you can take it from there, for every inch you increase the pitch, your rpm's will decrease by about 200, and vise versa.

The different diameters your seeing is to fit an assortment of drives, ie;, bravo1 can fit a larger Dia. prop than the Alpha1 , and so on. And.....you can use the different dia prop's for tweaking,(fine tuning) to get your rpm's on the mark.

If you have a 14.5 dia now, your with-in the correct size range for the alpha, and you can go 14.75 with no problem, or you can go smaller.. so besides a larger pitch, a larger dia will also decrease your rpm's., but not as much as pitch does.
But there are other considerations to choosing the right prop too, so depending on your boat and what your doing with it all comes into play..
Most common are 19 & 21 pitch for a healthy 350 I/O in a runabout.
 

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Discussion Starter · #6 ·
Yes, I have stainless prop now. I may be wrong on the hp rating but I just checked the top of the motor and the tag does read 4600 as redline. The boat is a 96 with only 220 hours on it and like I said at times it may peak into 5k at only about 53mph. So, I would like to step up in pitch to get the rpms down and poss. get the mph's up. Any suggestions on what diameter with a 21 pitch? Thanks and sorry for all the confusion
 

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I would suggest you stay in the 14-1/2 dia ballpark, and you should be fine, 1/4" + or - should be fine too, and the engine should be able to handle 4800 rpm's with no problems...
Currently I run an Apollo SS, 3 blade, 14-3/4 x 21 on mine, with a 350 mag./alpha1...




And if your reading the tag on the valve cover, same as where you saw the 4600 rpm's, it should also tell you the HP.. :)
 
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