RPM at WOT

[Jason Edwards]

I have a 1998 200 Johnson Oceanrunner. What should my rpm's be at WOT? Also, what is the max rpm that would be reasonably safe to operate at for a moderate period of time?

 

[angus63]

Re: RPM at WOT

Setup and prop for around 5800 rpm. Rev limiter will prevent overspeed. Two cycles like rpms, so running up to 6000 or so is not unsafe.

 

[imported_Curmudgeon]

Re: RPM at WOT

Two cycles like rpms, so running up to 6000 or so is not unsafe.

I keep reading that, and for the life of me I can't seem to find anything that supports such a premise. I also can't see why running any engine at max speed would be good for it. Safe, maybe, but certainly not conducive to longevity if routinely run that way. Yeah, I know, someone will chime in that they ran theirs at 9000 rpm for 1200 hours and everything was still within new specs ... we all have our fantasies. You should consult a dealer concerning what the WOT rpm range is for your engine. :/

 

[ob]

Re: RPM at WOT

They don't necessarily like to be run at high rpms constantly as much as they like being capable of easily attaining the upper limits of their rated rpm range ,as in not being lugged.Running an engine that maxes at say 4500 rpm but is rated for 5500 rpm is considerably harder than on the same engine running at 4500 rpm that is capable of 5500.Outboards unlike motorcycles or autos don't have the luxury of gear shifting to maximize torque through the acceleration range.And since most boats don't plane until after 3500-4000 rpm , where the engine begins making power ,overpropping can be a significant detriment in terms of engine load and subsequently,engine life.

 

[Jason Edwards]

Re: RPM at WOT

I sometimes fish large lakes such as Texhoma or Tablerock and getting to my "spot" in the morning can be as far as 15-20 miles away. I was just wondering if anyone had any suggestions on what the max rpm is for operating safely over a moderate period of time. Thanks for the reply's guys.

 

[rickdb1boat]

Re: RPM at WOT

These motors are designed to run full throttle with no ill effects. If propped correctly, it will run all day long at WOT with no harm. 5800 is not straining the engine. It will run much further than that.

 

[Jason Edwards]

Re: RPM at WOT

I am always amazed how quick of a response you get on this forum. Thank you very much.

 

[Dhadley]

Re: RPM at WOT

Remember, if you're lugging the motor at wide open, you're lugging it at every throttle position under WOT.

 

[imported_Curmudgeon]

Re: RPM at WOT

These motors are designed to run full throttle with no ill effects.

There it is again! Maximum design rpm of an outboard is just that, whether or not it's peachy to run at that speed for hours on end is another matter altogether. And I've still to see anything more more to support that premise than speculation and personal opinion. I'm quite sure manufacturers would be the first to advertise "run the **** out of our motors" if they were designed for it and actually performed better and longer. Not happnin, is it? :/

 

[Dhadley]

Re: RPM at WOT

Curm, we're not real sure of what you're getting at. Jason asked what his max rpm should be as far as set up. The answer was to look for 5800 at wide open. That will keep the motor from lugging at all rpm ranges. With todays fuel we pay a lot of attention to combustion temps. Lugging creats more combustion heat than the motor was designed to handle.

As far as rpm goes for that particular motor, the rev limiter is at 6200 so the factory has confidence that 6200 is OK. In fact some versions of that motor have the limiter at 6700.

His other question was what's the safest and most rpm to run for extended periods. The answer is that, all things being in good working order, it should run at any rpm you want to for as long as you want to.

If you really think about it, on a carb'd motor the timing will reach full advance before 4000 rpm. From there you're just adding fuel. So, at some point you'll be running on full advance and little fuel. In theory that should be bad.

We see many more of those type motors with scored pistons that are set up in the 5200-5500 range than we do on the same motor set up at 5800-6000.

 

[GlasV162]

Re: RPM at WOT

Curmudgeon and Jason, take this recommendation for RPM range seriously. For the last 5 seasons, I have had my motor (90 hp Evinrude) propped at what I thought was correct; it runs at about 5200 rpms at wot; I rarely run at that high an rpm because my boat planes very easily at 3000-3500 rpms; even for skiing, I rarely need to go much over 4300 rpms. However, with only being able to reach 5200 rpm, my motor has been lugging for 5 seasons. I now have 1/4 of a ring broken off, a scored cylinder, and am facing a costly rebuild this winter. You can bet next season I will buy the prop that will allow my motor to reach the correct wot rpm range. It's best to learn from mistakes.........as long as they're somone else's.

 

[Jason Edwards]

Re: RPM at WOT

Point well taken, the new prop will be in tomorrow. I went down in pitch from a 20P to a 17P to gain 500-600 rpm. We'll see what happens when its on the water.

 

[MASTER Brian]

Re: RPM at WOT

So what is the best way to determine the correct prop size to get the RPM's needed?

I believe my boat should run 5500-5800 RPM's. I need to look in manual. Do I just need to run the boat at WOT and see where it goes RPM wise?

As for running at WOT for long periods....I wish I could recall how my machinist explained this to me, but I can't. The jist is that 2 strokes are designed to run at WOT all day long. Now he did pipe in that if you did it day after day after day, the life of the engine might go down a bit...

 

[Jason Edwards]

Re: RPM at WOT

The guideline I use is 200 rpm for every inch of pitch. Others may not agree but it seems to work for me.

 

[rickdb1boat]

Re: RPM at WOT

Do I just need to run the boat at WOT and see where it goes RPM wise?

Yep. That's the way to do it...

 

[Dhadley]

Re: RPM at WOT

Remember -- dropping pitch to gain top rpm is always a last resort. Do that only after you've maxed out the set up with the existing prop.

5800 with a 17" prop is less efficient than 5800 with a 19" prop on the same boat & motor.

 

[rickdb1boat]

Re: RPM at WOT

5800 with a 17" prop is less efficient than 5800 with a 19" prop on the same boat & motor.

That's a great way to put it....

 

[GlasV162]

Re: RPM at WOT

Dhadley, what do you mean by maxing out the setup with the existing prop? Now I'm wondering if my motor should've had other adjustments. I have a '72 Glastron 16' V 162 Futura with a 90 hp Evinrude; currently it has a 17 in. pitch prop and reaches 5200-5300 @ wot. I have to admit a 15 in. pitch seems very low for this boat; it's an old boat but the hull is tight and does not take on any water. Also, what role does the diameter of the prop play in performance? Thanks.

 

[MASTER Brian]

Re: RPM at WOT

Remember -- dropping pitch to gain top rpm is always a last resort. Do that only after you've maxed out the set up with the existing prop.

5800 with a 17" prop is less efficient than 5800 with a 19" prop on the same boat & motor.

I also am curious what that entails? If a complete link & sync is done and the motor is fresh, are there other things to look at or not? Maybe I should start another thread on this. I just don't know if my boat is hitting max RPM's or not. I believe there is a table in the manual talking about props for this motor and RPM's.

 

[rickdb1boat]

Re: RPM at WOT

I believe Dhadley is referring to raising the engine on the transom (X dimension). Most times if you raise it up a bit, your RPM's will also increase. Trying this is preferred to just putting props on until it achieves the proper RPM range. Of course, there is a limit to how high it can go and whether performance is enhanced. It's a matter of trial and error to get the right height. They say that if you can run the entire trim range with the current set-up and not blow out(Ventilate) the prop, you can stand to raise it up some...One hole up at a time is the way most do it...Hopefully Dhadley will explain further...