safe rpm's

[Silver Eagle]

I have a 24 ft pontoon boat with a 90 hp two stroke engine (2007)This past wekend I was running 3900 rpms but I was only going 16.5 mph. How high can I go with out hurting the engine. The boat weighs in at 2245 lbs. There were just the two of us in the boat. I don't know the prop pitch.

 

[sschefer]

Re: safe rpm's

Two stroke motors do not like to be heavily loaded for long periods of time. The max WOT rpms should be attainable and if they're not, you need to re-prop the motor so that they are.

WOT rpm is not the "Red Line" of the engine as it might sound. These motors will easily go well beyond it. Merc recommends not exceeding the Max WOT rpms because it will shorten engine life since lean conditions may occur and wear could become accelerated.
** Side note: Max RPM can also mean max developed HP and in the case of stock motors usually does.

That said, the numbers are used to tell you that your prop is correctly sized for the motor or that your motor is correctly sized for the vessel. This is unrelated to USCG method for determining a hulls max hp rating.

The rule of thumb is 200 RPM per inch of prop but if it's apparent that you'll need to run a very small prop to achieve max rpm then it's time to look into a larger motor. The rule of thumb is just an estimate and several other factors are in play but you can usually use it fairly reliably.

 

[Dave1027]

Re: safe rpm's

I have a 24 ft pontoon boat with a 90 hp two stroke engine (2007)This past wekend I was running 3900 rpms but I was only going 16.5 mph. How high can I go with out hurting the engine. The boat weighs in at 2245 lbs. There were just the two of us in the boat. I don't know the prop pitch.

Has it always been that way? Look very closely at the hub part of the prop for numbers. They usually have the prop size stamped.

 

[starcraft1982]

Re: safe rpm's

To answer the question,and w/out reading your manual,I would say somewhere between 5000-5600rpms.Youll have to look for specs for that particular motor.Are you giving it full throttle?kinda unclear in your post.If you are,you better check the motor and/or prop.If you arent,give her H*LL!See what she'll do.Two strokes like to be ran WOT.Dont worry about getting to maybe 6000rpms.That may be a little much but shouldnt hurt any at all.But if it goes that high or beyond,youll want to start exploring different props to get back to recommended WOT number.

 

[Silver Eagle]

Re: safe rpm's

The motor says max rpm's 5500. So I was just checking to find out how much rev I could do for a long period of time. It's not laboring at 3900 at all. Maybe this weekend I'll open ot up just to see what it will do. My last boat have a 320 hp I/O so It would fly at 4,000 rpm's

 

[tfbrown1270]

Re: safe rpm's

Your motor is designed to run all day long at WOT and 5500 RPM (or there about), it is down right leisurely to run it at say 4500 to 5000 RPM.

Like already said it should be able to hit the WOT RPM after a minute or so of accelllerating with the pedal to the metal, ideally you should reach the forward stop of the throttle throw as you reach WOT RPM on calm water; at least that's what I have been lead to believe the past 50 years or so .

Your motor should be able to reach max RPM under normal conditions with full throttle, it shouldn't zoom past it or you are under propped or never get there cause you are over propped.

 

[sschefer]

Re: safe rpm's

Learning what I have now about how you are running the engine, you should do a full power run and see if it will hit max RPM at WOT. If it exceeds it then go bigger using the 200 RPM per inch of pitch rule. Same thing if it doesn't get there except drop pitch to increase RPM. Try to get the best possible readings you can. It should be smooth calm water when you're testing. Failing to do so can end up costing you a small fortune buying the wrong props.

Contrary to the rumour, your engine is designed to cruise all day long not run at WOT all day long. Running at WOT for short periods of 5 minutes or so is fine and is really a pretty long time when you think about it. You'll never see a commercial fishing boat running at WOT for extended periods and they get 1000 or more hours out of their motors. Take care of it and it will take care of you.

However, it's your motor so feel free to follow whichever advice you wish.

 

[Silver Eagle]

Re: safe rpm's

Thanks for the help everyone. This weekend I'm taking some honies for a ride to Lloyds Creek. The water is never smooth but maybe if the tide is high I'll try it out there. I'm going from Stemmers run Ramp out on the Chesapecke bay across the Sassafras tro Lloyds creek.