Running without prop.

[tolent]

Is it ok to run a 90HP outboard in the water without a prop? Sounds silly but the boat has been sitting for a long time and I want to blow the cobwebs out of it so to speak. With a 13 inch prop the prop wash is so strong that it blows all the water out of the trough I have it sitting in so by the time I can get it up to high throttle and keep it there for a while it runs dry and the water hose I'm using to fill it can't keep up. Taking it to a lake is not feasible right now. I'm just worried about it spinning without the balance of the prop if that is even a possibility. Just don't want to risk any serious damage to the motor.

 

[Chinewalker]

Um, no. It is NOT advisable to run it up without a load. Very bad things can happen. No different than revving it up in neutral. About the only beneficial thing "blowing the cobwebs out" can accomplish us you might clean up some lightly fouled plugs. If the plugs are already clean, run it for a bit on muffs and call it good until you can splash it.

 

[tolent]

Ok. Thanks.

 

[Scott Danforth]

Sitting for 13 years, the impeller should be replaced

 

[tolent]

If you mean the water pump impeller...Already done. Just knocking all the 'tune up' things down one at a time.

 

[82rude]

Running high rpm,s not under load can lead to catastrophic failure.Not advisable.

 

[tolent]

Ok. That idea is off the table.

 

[mlaajanen]

Running high rpm,s not under load can lead to catastrophic failure.Not advisable.

So I have heard aswell, and so I treat engines BUT why?
What is different with load and no load, and how much is enough load?

 

[82rude]

Maybe somebody else more versed in mechanics can explain it better but in basic terms it can cause a run away engine .Sorta like playing Russian roulette.You may get away with it for a few times but your number will come up so why take the chance.

 

[mlaajanen]

Maybe somebody else more versed in mechanics can explain it better but in basic terms it can cause a run away engine .Sorta like playing Russian roulette.You may get away with it for a few times but your number will come up so why take the chance.

What do you mean with run away?

 

[bruceb58]

You can run it at any RPMs without the prop as long as you don't exceed max RPM of the engine. Would love to hear people's theory that you HAVE to have a load!

Why you think you need to run it at high RPM is another issue.

 

[interalian]

You can run it at any RPMs without the prop as long as you don't exceed max RPM of the engine. Would love to hear people's theory that you HAVE to have a load!

Why you think you need to run it at high RPM is another issue.

For every "old wive's tale" there's an old wife.

Interested in actual experiences/reasons myself due to the number of times this has been repeated.

Edit: Leeroy has a theory: http://www.leeroysramblings.com/runaway_outboards.html

 

[F_R]

The key words there are as long as you don't exceed the maximum rpm it is rated for. Fact is that let's say it is rated for 6000 max rpm (under load). But without a prop (load) at full throttle, it may wind up to 10 or 15 thousand rpm. Way more that it is designed to withstand. At that speed, the pistons are accelerating from a stop to the other end of the cylinder, stopping again and accelerating toward the other end to stop again, over 350 strokes per second! Just think how many G's are exerted on those parts.

 

[racerone]

Has to do with clearance and needle bearings.---They do not like to bounce around.----More can be said but I do not have time today.

 

[bruceb58]

Just to add, I would never run a 2 stroke motor at high RPM while turning off the fuel. Not a good idea to run lean under that condition.

 

[interalian]

Has to do with clearance and needle bearings.---They do not like to bounce around.----More can be said but I do not have time today.

Enlightening. I think.

 

[bruceb58]

Seen a few automotive engineering pages that don't agree with that although they weren't discussing needle bearings since they were 4 stroke engines.

 

[interalian]

I read that with needle bearings and low-tension pressureback rings, 2-stroke outboards have extremely little rotational friction. No cams or other parasitic loads makes a very free-revving assembly.

Add no exhaust backpressure when running on muffs, and autoignition from hotspots (carbon deposits etc) in the combustion chamber which simulates advanced timing (equivalent to the ignition system advance which would give high RPMs), plus lean fuel drawing only through the rather large notch in the throttle plate but pulling fuel mostly through the idle circuit. Maybe?

Water supply on muffs will be inadequate for high RPMs too. Yikes.

 

[Faztbullet]

My question is why do you want to as it tells you nothing about engine condition/performance or power output.

 

[racerone]

They make / market ---Marine dynomometers---so that a motor can be load tested on dry land.-------Winding them up in nuetral proves nothing !!!