Lugging the Engine

[Norman Buus]

I just read an article about selecting the proper prop. The author said you could seriously damage the engine by choosing too fast a prop which would cause the engine to lug. Is this for real? Is there any test data to substantiate? How quickly will it damage the engine?<br /><br />I've got a 4.3 OMC that my repair manual says should be kept under 4500 rpm. What if I overprop so I can only get 4000 rpm - will that kill the engine quickly?

 

[Reel Poor]

Re: Lugging the Engine

Lugging an engine can definately cause serious engine damage ie, excessive rod and main bearing wear, thrust bearing wear, and piston skirt wear. It can also cause the engine to run at higher temps. Not to mention the extra gas you will burn trying to turn the over pitched prop. Running oversized props also puts undue stress and srain on the drive system. This can cause bearing and splined shaft damage. It,(the drive) will also run hotter than it was desinged to. <br /><br />Running to small of prop will not cause damage if you keep the engine RPM's under the factory recomendations (no full throttle operation). However boat speed will be decreased at any given RPM (still staying within specs). Low pitched props are good for hole shot, ie towing, pulling skiers, or carring heavy loads (like MIL).

 

[Bondo]

Re: Lugging the Engine

RP's Quite Right,............<br /><br />And,...<br />I'll add,...... The Major Issue with Over-Propping is,.... <br />Detonation,.........<br /><br />You can Easily Blow Holes Thru the tops of your Pistons,.....In a matter of Minutes................

 

[TilliamWe]

Re: Lugging the Engine

I'll tell you what lugging does first on a Vortec engine, at least on a 5.7 It causes the valves to get hot, they then bend, and pull too far up into the head. That causes them to lose their seal with the valve seats, and you get cylinders with NO or very low pressure. Oh, and eventually it cracks the heads. Ask me how I know!

 

[Norman Buus]

Re: Lugging the Engine

Bondo - Good point. Pinging would surely cause problems in a hurry.<br /><br />TilliamWe - How much were you overpropped? Was there any pinging?

 

[TilliamWe]

Re: Lugging the Engine

My situation had to do with growth on the bottom of the hull. But my engine normally ran 4800rpm WOT, and I could only make 4000. And yes, there was "pinging". Actually detonation, which sounds awful! I got away with it a couple of times. But eventually, I lost all compression in one cylinder, and was way down on another. When the heads were removed, multiple exhaust valves were bad, and a crack was starting in the starboard head. Reconned one head, bought a reconned starboard head. $160 to recon the one, $185 for the replacement head.