RPM question

g1vetter

Seaman
Joined
Nov 1, 2004
Messages
69
I have a 15' Center Console, 1992 50hp Evinrude.

I have a standard Aluminum prop 11 3/4 x 17. Boat runs 31mph @ 5250 rpm.
I also have a Cupped Stainless prop 12 3/8 x 15. Boat runs 31mph @ 6000 rpm.

Book says I should be running 5500rpm at wot. Which prop should I run?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
In reality it's all about how light or heavy you like running your combo as weight affects max wot rpm in terms of : more weight, less wot rpm and viceverse.

Max wot rpm factory stated for any OB should be reached as usually loaded, have not specified if during wot test combo was lightly, medium or heavily loaded. For instance with 17 alum prop, if you reduce weight or even one less boater, wot rpm should increase towards max 5500 wot rpm.

OTOH, if going for the SS prop adding more weight or extra boaters to tested load/rpm achieved with 17 prop can decrease rpm excess for OB to rev around max 5500 wot revs.

Both cases applies if like running full hammer down on flat calm, no wind water cond as "loaded" and OB screaming happily inside its safe max wot range.

Happy Boating
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Get rid of unnecessary weight, Kids, Wives, Mother in laws, especially Mother in Laws, should never be allowed on the boat. Mistresses and Girlfriends can still come aboard.
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
g1vetter, if your are limited to just those 2 props then the 17p aluminum is the one to run.
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Well, meant to ask this first... is there a Range of rpm specified for WOT from the mfg? The recommended rpm is usually a range and not just one number. Example, a 65hp 'rude from 1984-2000 model year is 4500-5500. There was not a 50hp on the chart I found, but you get an idea of what the RPM specification is, it is a low and a high. So in your case if we overlay the 65's numbers then the 17p you have is near ideal.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
The 1992 Evinrude Brochure shows 4500 - 5500 as the Full Throttle range
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
OP has not specified current load, N? of passengers on wot test, if lightly or medium loaded the 17, if way heavy, over loaded the 15, that's if liking running at full hammer down at max wot range.

Happy Boating
 

b.gagnon

Master Chief Petty Officer
Joined
Apr 28, 2001
Messages
835
Your #'s don't make much sense..... If the weighs are close to the same, a larger diameter cupped stainless prop should not have anywhere near the rpm gain you are seeing... I would go with the stainless and use caution at WOT....
 

g1vetter

Seaman
Joined
Nov 1, 2004
Messages
69
I appreciate the responses. The 17p rpm of 5250 light/normal load on light chop water. several trips with this result sometimes I saw 5300 rpm.
I only ran the Stainless 15p prop one time and that day was very windy with heavy chop and I had one extra adult on board so some extra weight. 6000rpm.
 
Top