What happens if you travel under max rpm?

hedp

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Jul 28, 2013
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I recently bought a used 2000 Mercury 115 horse and the owner said he had two props for it. One was the original and the other was a specialty prop that he bought for towing people for skiing but he really didn't like using it because it raised the rpms too high and he didn't like going over 4,000 rpms (I think he said 4,000rpm).


Well, I chipped the 1st prop and am sending it in for repair and because of that I put the other prop on. I haven't gotten it to full throttle yet but at 3/4 it runs about 3,500-4,000 rpm. I thought this would be fine. I checked the max rpm on the side of the motor and it says 5,250rpm.



So, I'd think I could open it up to full and probably be under 5,250rpm, probably closer to 5,000. Is that bad though? Should I be way under the 5,250rpm mark? He seemed really concerned about not using it at full throttle.
 
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Frank Acampora

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Re: What happens if you travel under max rpm?

Outboard engines have two speeds: Off and on! LOL. The maximum rating is there so you can correctly choose the prop for the engine. While exceeding the maximum rating will not hurt if it is only moderate, fuel mileage will suffer. HOWEVER, going significantly below the maximum RPM range at full throttle will "lug" the engine resulting in increased carbon and varnish deposits, stuck rings, possible overheating, etc. SO: It is best to install a prop that lets the engine reach its full rated RPM (usually expressed as a range--5000 to 5500 with rated horsepower developed at 5250) THEN, if you prefer to cruise at less than full throttle, go ahead, be happy.
 

hedp

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Jul 28, 2013
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Re: What happens if you travel under max rpm?

Outboard engines have two speeds: Off and on! LOL. The maximum rating is there so you can correctly choose the prop for the engine. While exceeding the maximum rating will not hurt if it is only moderate, fuel mileage will suffer. HOWEVER, going significantly below the maximum RPM range at full throttle will "lug" the engine resulting in increased carbon and varnish deposits, stuck rings, possible overheating, etc. SO: It is best to install a prop that lets the engine reach its full rated RPM (usually expressed as a range--5000 to 5500 with rated horsepower developed at 5250) THEN, if you prefer to cruise at less than full throttle, go ahead, be happy.


So you're saying even though it says max rpm is 5,250 you can still go up to 5,000 all the time without hurting it?



A guy on another forum said that that model is known for having a crankshaft harmonic problem.


Have you heard of that? Is there anyway to fix or avoid that?
 

Noltz

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Sep 13, 2012
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Re: What happens if you travel under max rpm?

There's no simple way to fix a harmonic issue on a crankshaft. I'm honestly surprised they'd even know that. Only "fix" for RPM induced oscillation is to change the RPM or the mass/design of the rotating object. Obviously moving the throttle stop is easier :)

I can't speak to "hurting" an engine when it's spinning at near max RPM. Demands on boat engines can be similar to aircraft engines though; They're rated at a given horsepower and a given RPM and are designed to do so constantly.
 

foodfisher

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Re: What happens if you travel under max rpm?

The thing to hurt running near max RPM's is your pocketbook.
 

Fed

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Re: What happens if you travel under max rpm?

Take it for a run & see what it will rev to, if it goes over 6500 rpm back it off a little.
Does anyone know if these are fitted with rev limiters?
 

Sea Rider

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Sep 20, 2008
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Re: What happens if you travel under max rpm?

Hedp,

Go for a wot spin with you alone and lightly loaded boat and check max achieved rpm, if it's close or under 5250 it's fine, if more you're over reving and will need to back throttle down. If' your engine is achieving safe max wot revs, you can cruise at any speed between idle to wot with no engine issue.

Happy Boating
 

Home Cookin'

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Re: What happens if you travel under max rpm?

many coastal boaters seldom run WOT constantly with no ill effects. your max rpm rating is tested at WOT and you want to hit close (w/in 500) to it to know you have the right prop, because the wrong prop at WOT can harm the motor (over rev or lug).

I tested nearly 10 props on my boat and found a wide range of performance in the middle speeds.

I was told that if you run 1000 RPM over the max, every minute of running is the same as an hour on the engine life. My hunt club wore out a new Yamaha that way in a couple years; mechanic said it looked like a commercial use (crabber, etc) motor.
 

Silvertip

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Re: What happens if you travel under max rpm?

The other thing to be aware of is that the PROP is what controls maximum speed AND WOT rpm. A prop with too much pitch is one that will not allow the engine to reach its recommended WOT rpm with what you consider an average load. That prop is therefore wrong for the boat and is lugging the engine at ANY rpm. A prop with too little pitch will allow the engine to exceed the recommended WOT rpm and it will likely reduce speed dramatically since you need to throttle back to keep rpms at or below the maximum recommended. It will not "lug" the engine but speed will be affected. Hole shot may be excellent however but you absolutely must watch the tack to avoid over reving. This is why the prior owner had two props. One for towing and water sports (towing) and one for cruising.
 

hedp

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Jul 28, 2013
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Re: What happens if you travel under max rpm?

The other thing to be aware of is that the PROP is what controls maximum speed AND WOT rpm. A prop with too much pitch is one that will not allow the engine to reach its recommended WOT rpm with what you consider an average load. That prop is therefore wrong for the boat and is lugging the engine at ANY rpm. A prop with too little pitch will allow the engine to exceed the recommended WOT rpm and it will likely reduce speed dramatically since you need to throttle back to keep rpms at or below the maximum recommended. It will not "lug" the engine but speed will be affected. Hole shot may be excellent however but you absolutely must watch the tack to avoid over reving. This is why the prior owner had two props. One for towing and water sports (towing) and one for cruising.




I'd say you hit it right on the head. The second prop allows it to rev over the maximum. I tested it today. But the original owner said something like that and that he used it for towing.
 
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