Over propping engine failure

[Karl_Childers]

I would like to confirm or disprove something I have heard. I have heard that over propping a boat will cause a catastrophic engine failure. Is this true? Why or why Not?

 

[justchange]

Re: Over propping engine failure

I'd think that the wrong prop could cause excessive engine rpm thereby causing the engine failure.

I wonder though. Shouldn't the ECM take care of this on a new[er] boat?

 

[CharlieB]

Re: Over propping engine failure

UNDER proppong a motor will allow excessive RPM.

A carb'd motor may not have an RPM limiter.

The question was about OVER propping a motor which puts excessive load on and will not allow the motor to pull up to rated RPM, greatly over stressing, and overheating, the motor, radically shortening it's service life and sometimes ends in glorious destruction.

Neither condition is ever recommended.

 

[Karl_Childers]

Re: Over propping engine failure

By over propping what I meant to say was: having too much prop (pitch) effectively slowing the motor down while at WOT. Example: only turning 5000 RPM when at WOT and reccomended WOT is 5500-6000 RPM. Also, I know this probably isn't an issue on an EFI, but what I am really curious about is a carbed 2-stroke. Thanks!!

Rob

 

[Karl_Childers]

Re: Over propping engine failure

UNDER proppong a motor will allow excessive RPM.

A carb'd motor may not have an RPM limiter.

The question was about OVER propping a motor which puts excessive load on and will not allow the motor to pull up to rated RPM, greatly over stressing, and overheating, the motor, radically shortening it's service life and sometimes ends in glorious destruction.

Neither condition is ever recommended.

OK, but will it lean it out?

 

[Fuzzytbay]

Re: Over propping engine failure

Lean conditions are not related to props. Leaning out, is caused by not enough fuel in the fuel/air mixture. This cause's too much heat, and melts pistons. Generally its caused by air leaks in the fuel system, or too advanced timing.
Over proping and under proping are both not good either, and all three can lead to an expensive repair.

 

[Karl_Childers]

Re: Over propping engine failure

I get the concept of a lean fuel/air mixture, thanks. My question was; if the boat is over propped, and you go WOT, it seems to me that the throttle butterflies would be open, but the engine would not be pulling enough vacuum to pull in enough fuel from the main fuel jet, because it is not spinning fast enough, but all the potential air could come down the venturi, thus more air than fuel.....is this correct?

 

[Fuzzytbay]

Re: Over propping engine failure

No that is not a correct assumption.

 

[JB]

Re: Over propping engine failure

Think of a stick shift car climbing steep grades at 30mph in 5th gear.

Nothing to do with mixture, just simply stressing the engine outside of its best operating range. Some can take it, some can't. The ones that can't break.