rpm choices

[jackBnimble]

I have a 1992 50 hp two-stroke motor. It is rated at a maximum rpm of 5000-5500.

This is a maximum rpm. But what does it mean and what other rpms might be chosen? Is the maximum rpm just that or does it also indicate where the engine develops its rated horsepower. Is there another rpm that would be useful - such as maximum torque or fuel efficiency?

Automobile engines have a torque-power curves. The maximum horsepower is almost always occurs at an rpm way less than the maximum rpm. And the maximum torque (where fuel efficiency occurs) is almost always less than the maximum horsepower. One never drives these engines at WOT.

I know marine engines are different, but the laws of power. torque, physics, and load dynamics are not. There has to be an rpm for maximum torque and power. If I can choose to run at a lower rpm than WOT, what would that rpm be ?

 

[Scott Danforth]

Your max RPM at WOT under load must fall between 5000 rpm and 5500 RPM. If your propped wrong, you can lug the motor (WOT RPM lower than 5000) or over-rev the motor (above 5500)

The RPM range is chosen for a variety of reasons, including bearing life, cylinder wear, piston speed, etc

 

[GA_Boater]

WOT in a boat motor is the redline or maximum RPM for safe and reliable operation.

When you drive your car or truck to work, do you drive at redline? Of course not. Same with a boat. Where you cruise and conditions determine, not the RPMs but the safe speed to operate your boat.

As far a what is the most efficient RPM, there are many variables. Prop size and pitch, motor mounting depth, motor condition, water/wind conditions and last but not least, the boat hull and condition.

 

[roscoe]

Somewhere between 50 % and 70% of WOT will usually give you the best compromise between speed and fuel economy long as this speed is sufficient to keep your boat up on plane.

 

[Scott Danforth]

if you want economy in a boat, get one with sails or oars.

 

[Silvertip]

Your other "rpm choices" are determined with your right hand on the throttle lever. Whatever setting you choose uo to red line is strictly up to you. Your wallet, gas gauge and calculator will help determine what works best for you.

 

[444]

That 5000-5500rpm is the rpm your engine makes peak HP at, not max rpm. Two strokes don't have a hard max rpm limit like a 4 stroke that will rev until it floats the valves. You want to prop your boat so you are making 5000-5500rpm at full throttle. Any less and you are lugging the engine below where it makes peak power and any higher and you are above the rpm in which you are making peak power.

FWIW in a car engine, max torque rpm doesn't equal peak fuel efficiency. Well unless for best mileage you drive full throttle at 4000rpm down the highway.

 

[jerryjerry05]

Proper prop and rpm's is what it's about.
I don't think the 92 50hp had a rev limiter so you can over rev it.
​The 5500 is the max recommended operation.
Proper pitch is important too.
​My twin 1988/ 85's came with 17p.props.
​My motors ran about 52-5300
I wanted more speed/ performance.
Changed to 19p.
I dropped to 4800 rpm's and got more.
​I picked up 2-3 mph. and about 20% more fuel usage. MORE!!

 

[Chinewalker]

Prop it to reach the high end of your RPM range with a light load so that when you have the boat loaded with kids, wife, dog, gear, coolers, etc. you're still able to hit the range at WOT.