RPM over rev
 

I have a 92' z2000 with a 5.7 bored .60 over ( not sure if all boat 5.7 engines of this size are bored like this or not) has an alpha one gen 2 outdrive with 1.50 gear ratio. I just bought a new 14.25 x 21 aluminum prop because the S.S prop that came with the boat when i purchased it was REALLY beat up. I took the boat out this past weekend and it ran great except at WOT with 4 passengers and 2 dogs I could easily hit about 5200RPM at a little over 40 mph. Is it the gear ratio , engine power, or not enough pitch creating this over rev? I didn't run it long at that RPM I cruised around at 4500 at about 38 mph. Maybe my Speedo is reading slower? Any help would be greatly appreciated :D

Re: RPM over rev
 

Is it a Merc prop or other brand? Does it have any cup in the blades trailing edges? I tied a 19 pitch non-merc prop last year and it over rev and didn't really go any where just plain sucked, had no cup in the blades and was no where near the 19 pitch aluminum mercury prop in rpm and speed.

I always get props thru a dealership that lets you test them before you buy!! Most have demo props to let you test it, However there also is that one rule, if you ding it you just bought it!!!

Re: RPM over rev
 

It is a Mercury Black Max Aluminum Prop. It works great out of the holeshot and gets on plane fairly decent. Gets up to 40mph about 3/4 throttle around 4500 and then if i go WOT it jumps up to around 5200 rpm and only reaches about 42-43 on the speedometer. I live in the Phoenix area I'll have to look for a dealership that will allow me to test out props that would be way better than guessing and throwing money down the drain.

Re: RPM over rev
 

Are you sure you aren’t trimmed up to high? If not, you might need to go up in pitch. I put the Mercury Spitfire 4 blade on last year and it is amazing! I highly recommend it.

Re: RPM over rev
 

Bigred I usually trim up to the 3/4 mark on my gauge after I get on plane and up to speed. That's where it's stops gaining speed at and then I go down just a tad. What's your max speed with the 4 blade and what boat do you have?

Re: RPM over rev
 

Your RPM should change about 200 RPM per inch of pitch changed. Example a 22p should get you to 5000 RPM. Now going to a 4 blade in the same pitch should net you the same 200 RPM. Just food for thought.

Re: RPM over rev
 

Did you mean a 4 blade with the same pitch I currently have which is a 21?

Re: RPM over rev
 

You are correct, it sounded right in my head but is confusing when I re read it. My apologies.

The difference in RPM between a 3 blade 21p and a 4 blade 21p should be around -200 RPM.

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Re: RPM over rev
 

I'm sorry if I can't see it, but did you say the pitch of the old prop? I have a 23" prop on my Z202 and it works fine, 55mph at about 4800 rpm with 1-3 people aboard. If your boat was set up for water sports, that might explain the low pitch prop, so if you want to do water sports, keep the one you have and just watch the tach. 5200 is too high.

Oops, I just noticed the part about having a wake tower, so that explains it. And I guess the same description tells me both props are 21"