Is two inches enough?

tunes

Seaman Apprentice
Joined
Jul 26, 2008
Messages
34
I have a 24 foot Harris Floteboat with a 1995 Evinrude 90HP. Guessing boat around 3000 lbs. Prop on it now is a 14x13 aluminum. WOT on the tach is only 4000 rpm. GPS speed showed 19 to 20 mph. Lakes here are all 6000 to 7500 feet above sea level. From what I've read two inches is the recommended change so that would be a 14x11. My question is that enough? The next thing I see is a 14x9 is this one too much. Thanks in advance.
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
i think i would go with the 9 pitch if it were me - you need to get your RPM up quite a bit. It's gonna slow you down a few mph though
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Are you saying rpm is 4000 at 6000 ft? If that is so i think a 9" would be better than 11.However the 11 would be a little faster.
but may lug your motor.I think your rpm range is 4500-5500. So a 11 might put you at the bottom of the range.If you often run a heavy load the 9 may be better.
 

tunes

Seaman Apprentice
Joined
Jul 26, 2008
Messages
34
Thanks for the input sounds like a 10 would be the best compromise but doesn't look like I can go there. Will check availability of both pitches and see what I can find.
Thanks Mark
 

crb478

Lieutenant Junior Grade
Joined
Dec 6, 2006
Messages
1,036
I would go with the 9, I would also look into rejeting the carb as you are probably running rich in the thinner air. Get the motor running right for your altitude the address the prop.
 

tunes

Seaman Apprentice
Joined
Jul 26, 2008
Messages
34
Have been instructed by several people that unless you consistently operate at 8000 feet or above to leave the jetting alone. The local repair folks feel that the small gain in performance isn't worth the risk of running to lean. Within a few hours driving I can be at lakes that range Lake Powell at 3500 feet to Vega Dam at 8000 feet. Anyway I am sure need to do something with the prop. May need to get some other opinions on the rejet as well.
Mark
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
I too agree with the 9 considering everything else is in order.

2 strokes have to have rpms to develop HP as they usually are shy on cubes and cubes (torque) and rpms are what makes HP. Also the bang in the buck poops out at about half travel of the piston down the cylinder since the exhaust port gets uncovered, unlike a lugger 4 stroke low rpm diesel for example with a long stroke getting power down the whole thing. So like a prize fighter: Lots of little jabs. I run mine at the top of the range or a little over. Never have engine issues. Lugging builds up carbon that makes rings stick, helps to foul plugs, goos up exhaust ports and makes for smoky exhaust.

HTH,
Mark
 
Top