Rpm vs pitch on a prop

You know the drill..

Moderators: Redneck_Randy, badmoonrising, lakerunner

Post Reply
Posts: 2
Joined: Thu May 31, 2018 8:58 pm

Rpm vs pitch on a prop

#1 Post by braindead89 » Thu Aug 23, 2018 6:36 pm

I damaged my prop yesterday. Went to the marina and they told me the pitch was too much for what I had. My boat/motor is so old, I can't find any info on it. It has no tach so I dont know what it was doing for rpm

Its 1996 20 ft harris 90 hp Evinrude (1983)
Seemed to run great, didnt seem to redline, had good take off and wide open with 4-5 people is 25 mph
prop is 13.75 x 15

Marina said use a 13.75 x 13

My question is should I change it or stay the same?

Posts: 150
Joined: Tue Jul 17, 2018 8:00 am
Location: Jax Florida

Re: Rpm vs pitch on a prop

#2 Post by riplipper » Thu Aug 23, 2018 11:38 pm

My understanding is that you should be at the top of your rpm range at full throttle and a light load if you are propped correctly. Without a tach , you really don't know if you were at the top of your range. Maybe someone with same size boat and motor can chime in and help you.
I am the guy at the boat ramp that gives you dirty looks when you are a complete idiot and too stupid to know it.

User avatar
Posts: 1386
Joined: Tue Jan 13, 2009 10:03 am
Location: Lake Livingston Tx

Re: Rpm vs pitch on a prop

#3 Post by wwind3 » Fri Aug 24, 2018 2:21 am

If speedo is correct--leave it alone.. Especially if performance is good--out of the hole, no cavitating, etc. I get 27 out of mine by myself -20 ft Lowe 2009 90 Merc Optimax-I'm turning a 16 pitch at 5800 if my tach is correct. Red line is 5850 I believe and I will overrev if I trim up too much. Good speed for an engine that old. Mercs tend to run at higher rpm's than Johnrudes.
2009 LOWE SS204 25in logs
90 Merc Optimax
Motorguide W55 Wireless
28 GPS

User avatar
Posts: 74
Joined: Wed Jun 22, 2016 12:32 pm
Location: Merritt Island, FL

Re: Rpm vs pitch on a prop

#4 Post by Jake_Daddy » Fri Aug 24, 2018 5:33 am

I would agree that your right where you want to be. I have a 20' with a 1991 Merc 90hp and if your numbers are good your right where you want to be. When I got my boat a 16P would keep me about 200-300RPM out of my WOT range then I did a 13P and could get about 100 over I have settled on a 14P four blade that keeps me in the middle with our standard load and kept the 13P for if I need to swap it out.
2001-20' Godfrey Sweetwater
1991 Mercury 90hp
Merritt Island, FL

User avatar
Posts: 197
Joined: Tue Aug 22, 2017 6:33 am

Re: Rpm vs pitch on a prop

#5 Post by steve1313 » Sat Sep 01, 2018 3:47 am

So you've got a 35 year old motor that runs fine and your getting decent performance with the current prop...………..

IF IT AIN'T BROKE, DON'T FIX IT! :biggrin2 :biggrin2
2017 Sweetwater 2286 WB Tritoon w/ 150HP Yamaha "Serendipity"
Slipped at Lake Anna, Virginia
2018 Tesla Model X

Post Reply