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RPM troubles, need some input. 1982 470

P

pugetsounder

Advanced Contributor
1982 19' cuddy MCM 470

As some of you might remember I replaced my upper this past winter with a different one thinking that my current upper had the wrong gear ratio (1:1.98). The stock came with a 1:1.84. My rpm range is 3800 - 4200 but I can reach way higher rpm than that. And cruising in that rpm range my top speed is only 28 mph. I know the boat should go way faster than that and it does but at dangerously high rpms. I took the boat out yesterday for the first time since the new upper ratio and there is no difference! I expected a higher top speed. And I can throttle up to 5000 if I choose to.

My props are 3 blade alum MCM oem 14" 21P and a 4 blade SS 14" 21P and I get no difference between the two. I noticed in my 470 installation manual that some 470s came with a 1:1.65 ratio drive. But I don't know if this boat came with that one or not. the 1.84 seems pretty standard. And the prop is standard as well. I know with the proper ratios and prop I should be at about 40+ mph at 4200 rpms on this particular boat. I am stumped at this point. Any advice is appreciated.
 
Re: RPM troubles, need some input.

P.S.: If you can get 5000 RPM w/a 14" x 21P prop then you should be able to use a 14.5" x 25P prop which would drop the RPMs to a safer range.
 
Re: RPM troubles, need some input.

WOw 25P is that a common pitch? I thought I could go 23P but worry about fitment with the zinc.

Chief yes the speedo is accurate ( 3 of them!) but wondering about the tach now.
 
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