TexasMark......prop-lash.

[walleyehed]

We had talked before on whether a prop loaded and unloaded between power strokes, and I've found some info that may explain my reason for saying the prop is always loaded when in fwd, or reverse for that matter..<br />Here is the example..Let's say we're spinning your merc 6000RPM, it's a 3-cylinder so thats 18,000 ignition charges PER MINUTE. This equates to 300 per second, now, if we take into account the slip ratio on the power stroke, but let's remember, the slip ratio is including the time between power strokes, so if we have 300 explosions per second, the prop slip never becomes negative between power strokes because of slip...it simply doesn't have time...therefore, it ALWAYS has a positive load on the prop. Even if your rig has 8% slip, it would have to drop to zero and past that to negative to have a reverse load....it is impossible to do that because our slip is average between power and exhaust, and another cylinder has already fired before it could ever reach negative slip.....

 

[Dhadley]

Re: TexasMark......prop-lash.

Been watching that thread and just to throw out some "food for thought' -- <br /><br />A flywheel on a motor does not turn smoothly once the motor is running. Every time a cylinder fires, it tries to push back or stop. You have compression and then an explosion trying to push the piston, crank and therefore the flywheel back. <br /><br />Obviously it's more pronounced at an idle. The slower it idles, the more pronounced it is. The lighter the flywheel, the more it's noticable at idle.<br /><br />Think about an I/O or inboard motor with a dampner plate that has the springs in it. I'm sure everybody has heard one that the springs are shot on. It sounds like the motor is coming apart at idle. Once it gets a bit of rpm the noise goes away. It doesn't have time to load and unload or kick back.

 

[Texasmark]

Re: TexasMark......prop-lash.

Hey guys, that's the point exactly. At "just in gear" idle, I get the noise...(650-675 rpm's per the book, in water, in F gear). Once I give it a couple hundred rpm's it's gone.<br /><br />So Kenny, your numbers are great, but 10x too high.<br /><br />Interesting that Merc has published a service bulletin on it. In all my engines this is the first time I observed it and the first time I ran this few cylinders (on this big of an engine), with this much prop, on this light a boat.<br /><br />Thanks for your concern. Open to any comments.<br />Mark

 

[Texasmark]

Re: TexasMark......prop-lash.

Hey guys, that's the point exactly. At "just in gear" idle, I get the noise...(650-675 rpm's per the book, in water, in F gear). Once I give it a couple hundred rpm's it's gone.<br /><br />So Kenny, your numbers are great, but 10x too high.<br /><br />Interesting that Merc has published a service bulletin on it. In all my engines this is the first time I observed it and the first time I ran this few cylinders (on this big of an engine), with this much prop, on this light a boat.<br /><br />Thanks for your concern. Open to any comments.<br />Mark

 

[Boilermaker]

Re: TexasMark......prop-lash.

Dhadley, Granted that the reciprocating mass of an engine does not turn smoothly at low speeds...but it should not "unload" or become a negative torque. The purpose of a flywheel is to "store" energy to help carry the crankshaft thru its revolutions at low speed...isn't that correct???<br /> The crankshaft & revolving mass should be very close to a constant speed spinning, but the pistons,wristpins & the small end of the rods have a linier motion...they have to accelerate...decelerate... stop.... reverse direction...accelerate...ect.... <br /> The revolving mass should be the crankshaft, big end of rods, flywheel, driveshaft, gearset, & propshaft.<br /> So how would the propshaft..with all this revolving mass attached to it NOT carry the prop thru a revolution even with a single cylinder engine??? How would a prop "UNLOAD" in gear if the boat is moving forward unless the boat is moving forward faster then the prop is spinning...a negitive slip ratio?????

 

[Texasmark]

Re: TexasMark......prop-lash.

Boiler...hear you and tend to agree, but there must be some relationship between the load to be driven and the size of the mass to keep it spinning.... And maybe it's not a negative torque, but just less (torque) than what was preceeding it and with the propeller pushing against the water (load) it backs off a little which allows the slip clutch to click/rattle; because there is deliberate slop in the F clutch assy.<br /><br /><br />Case in point, farm machinery. Look at the single and double cylinders that farmed the world for 10's of years and look at the size of the flywheels. Some of those old steamers weighed tons.<br /><br />Take my "single cylinder" square hay baler. It crams it's mouth full of hay and the slammer comes in and slams it into the chute in the same manner as these engines run, but the massive flywheel just keeps on spinning and the tractor hardly knows what happened.<br /><br />I agree in that the purpose of an engine/machine (of this type) is only to keep the flywheel moving as it does the work.<br /><br />But what about when the 3 cylinder outboard engine is idling and misses (misfires) when it's supposed to be firing....which a lot of engines do without injection at idle?<br /><br />Oh well, thanks for the brainstorming session.<br /><br />Mark