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Teleman

Question for all the Smart ones.

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Teleman

What running condition causes the most power-train stress and highest torques to pass through the dampening plate and propeller shaft.  Running WOT with a lightly loaded boat or Running a fully loaded boat surfing at 2500 RPM and at 2/3 throttle? Explain why

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oldjeep

What load causes the most stress on yoir truck drivetrain.  Driving empty at 60 or trying  to pull a tree out of the ground? 

The boat drivetrain is working less hard on plane than pushing water like a bulldozer. 

Edited by oldjeep

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Pnwrider

Also, no modern boat is turning 2500 rpms when “fully loaded” surfing.

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Teleman
1 hour ago, oldjeep said:

What load causes the most stress on yoir truck drivetrain.  Driving empty at 60 or trying  to pull a tree out of the ground? 

The boat drivetrain is working less hard on plane than pushing water like a bulldozer. 

This is true for a Truck, but is it true for a boat? My truck driving at 60 will not have WOT, while my boat is. My truck has a transmission to change the output torque, my boat doesn't. I guess I don't see the correlation.

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oldjeep
13 minutes ago, Teleman said:

This is true for a Truck, but is it true for a boat? My truck driving at 60 will not have WOT, while my boat is. My truck has a transmission to change the output torque, my boat doesn't. I guess I don't see the correlation.

The correlation is that wot with no load puts as little torque stress on the driveline as cruising down the highway. 

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justgary

Hmmmm.  The forces balance, which means the boat only goes as fast as it can with a given power driving it, so the drag balances the thrust.

I'm inclined to say that the stress is maximum at the peak of the torque curve.  That is by definition the most instantaneous torque the engine can provide.  The RPM that max torque occurs will vary slightly due to conditions, but should be around 4200 RPM or so for most inboard engines.  It doesn't matter in the boat is light and doing 42 MPH or surfing heavy at 11 MPH since the torque is the same. 

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oldjeep
11 minutes ago, justgary said:

Hmmmm.  The forces balance, which means the boat only goes as fast as it can with a given power driving it, so the drag balances the thrust.

I'm inclined to say that the stress is maximum at the peak of the torque curve.  That is by definition the most instantaneous torque the engine can provide.  The RPM that max torque occurs will vary slightly due to conditions, but should be around 4200 RPM or so for most inboard engines.  It doesn't matter in the boat is light and doing 42 MPH or surfing heavy at 11 MPH since the torque is the same. 

I don't think so. That would be like saying that the stress on your drive shaft is the same with inflated tires vs flat tires, or driving up the driveway with or without pushing 1000lbs of snow on your plow. The stress is highest at the max point of resistance.  A point of resistance that you will never get to without the plow. 

Edited by oldjeep

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ahopkinsVTX

So isn’t the question what stresses the engine more? Seems to mean running the engine WOT at max rpms is extremely stressful because the hull is actually being driven back into the water due to the prop shaft angle. Is that more stressful than surfing at 2/3, 3/4 throttle turning 3400-3900 rpms and high weight load? No clue, but I doubt the answer is cut and dry either way. 

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jjackkrash

Boats don't coast like a car on wheels.  Look at the RPMs.  That is how hard the engine is working.  The faster it is turning the more fuel its burning and the faster it is wearing.  

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Teleman
1 hour ago, oldjeep said:

I don't think so. That would be like saying that the stress on your drive shaft is the same with inflated tires vs flat tires, or driving up the driveway with or without pushing 1000lbs of snow on your plow. The stress is highest at the max point of resistance.  A point of resistance that you will never get to without the plow. 

This can't be similar. I can tie my boat to a big dock and floor it at WOT and go no where, or go for a joyride at WOT. Is the stress the same?.... if the engine is at the same RPM in both situations,  is the stress not the same?

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justgary
1 hour ago, oldjeep said:

I don't think so. That would be like saying that the stress on your drive shaft is the same with inflated tires vs flat tires, or driving up the driveway with or without pushing 1000lbs of snow on your plow. The stress is highest at the max point of resistance.  A point of resistance that you will never get to without the plow. 

You add a lot of variables with low tires and snow plows.  But on second thought, I need to change my answer to power, not torque. 

The resistance on a non-planing boat changes roughly with the square of velocity.  The boat will find the speed that balances the power applied, not the torque.  If the boat is light, it goes faster.  But the stress is identical for an identical applied power. 

Even with your low tire example, keep the applied engine power the same, and the car with inflated tires will go faster, balancing the load and stress.  The stress will be the same. 

Note that power is RPM*torque, so we're not talking about just holding RPM constant. 

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Teleman
3 hours ago, oldjeep said:

The correlation is that wot with no load puts as little torque stress on the driveline as cruising down the highway. 

Next time you go to the highway, try driving at WOT for 15 minutes on a level road.... I'm pretty sure you will back off on the peddle after 20 seconds....

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justgary
12 minutes ago, Teleman said:

This can't be similar. I can tie my boat to a big dock and floor it at WOT and go no where, or go for a joyride at WOT. Is the stress the same?.... if the engine is at the same RPM in both situations,  is the stress not the same?

The engine won't be at the same RPM in those two situations, and the drag of the dock is not proportional to the square of velocity. 

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oldjeep
14 minutes ago, Teleman said:

This can't be similar. I can tie my boat to a big dock and floor it at WOT and go no where, or go for a joyride at WOT. Is the stress the same?.... if the engine is at the same RPM in both situations,  is the stress not the same?

No, it is not the same because you are now forcing the prop to cavitate. 

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oldjeep

Lets try this another way.  How many nautique prop shafts have broke just tooling around vs sacked to the max? 😁

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justgary
3 minutes ago, Teleman said:

Next time you go to the highway, try driving at WOT for 15 minutes on a level road.... I'm pretty sure you will back off on the peddle after 20 seconds....

The car will find its max speed where the forces balance at WOT.  I'd do it in my truck.  It's not near as sleek as a car. 

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Teleman
6 minutes ago, justgary said:

The engine won't be at the same RPM in those two situations, and the drag of the dock is not proportional to the square of velocity. 

So I ask, and maybe you answered,  what situation is there the most stress on the prop shaft and drive train. WOT or surfing.....?

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justgary
3 minutes ago, oldjeep said:

Lets try this another way.  How many nautique prop shafts have broke just tooling around vs sacked to the max? 😁

Good question.  Were they propped correctly?  Now we *are* talking about torque. 

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justgary
1 minute ago, Teleman said:

So I ask, and maybe you answered,  what situation is there the most stress on the prop shaft and drive train. WOT or surfing.....?

WOT produces more power. 

I think that @oldjeep is onto something.  I'll run this past the aero folks at work.  A propeller is a rotating wing, where lift and drag turn into forward thrust and torque.  The problem with using the wrong prop is that whatever power doesn't get turned to thrust gets turned to torque drag.  Torque is what breaks shafts. 

Like I said, hmmmm.  All of this depends on the prop used in each case. 

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Teleman
25 minutes ago, oldjeep said:

No, it is not the same because you are now forcing the prop to cavitate. 

Good point on the cavitation situation while pulling the dock ( or not in this case).  While all ski and surf boats have the cavitation problem going on and eroding the prop. The RPM's will increase due to the lack of "Bite" due to the cavitation, if we haven't  destroyed the engine from over revs I assume it will come to an equilibrium along the HP vs. RPM curve and have stabilization. Thus the stress will be somewhat lower than WOT with no cavitation.

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oldjeep
5 minutes ago, Teleman said:

Good point on the cavitation situation while pulling the dock ( or not in this case).  While all ski and surf boats have the cavitation problem going on and eroding the prop. The RPM's will increase due to the lack of "Bite" due to the cavitation, if we haven't  destroyed the engine from over revs I assume it will come to an equilibrium along the HP vs. RPM curve and have stabilization. Thus the stress will be somewhat lower than WOT with no cavitation.

I doubt you are going to get to wot with the boat tied to the dock unless the prop folds over from the stress. 

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Teleman
14 minutes ago, justgary said:

WOT produces more power. 

I think that @oldjeep is onto something.  I'll run this past the aero folks at work.  A propeller is a rotating wing, where lift and drag turn into forward thrust and torque.  The problem with using the wrong prop is that whatever power doesn't get turned to thrust gets turned to torque drag.  Torque is what breaks shafts. 

Like I said, hmmmm.  All of this depends on the prop used in each case. 

I think a prop is missing many key functions that a rotating wing exhibit. A prop does not use Bernoulli's effects for the thrust. It is solely using the rotation of the blade to move the water backwards while the counter force moves the boat forward.

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justgary
1 minute ago, oldjeep said:

I doubt you are going to get to wot with the boat tied to the dock unless the prop folds over from the stress. 

Except that I can put a prop on the boat that will allow 5000 RPM at WOT.  Not much thrust produced in that case. 

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D_Turner

#1 The torque measured at the crankshaft in your two instances are not going to be the same.  WOT throttle at max RPM vs 2/3 throttle at 2500 rpm.

The torque measured at the crank and applied to the input shaft of your transmission on an engine at WOT at a given rpm is always going to be the same no matter what the engine is connected to.  Now behind that you can have lots of variables that can induce more or less stress into the driveline.  The resistance of the propeller to turn in water is one. 

Take a drag car launching at the same RPM, but with different tires.  On street tires, the torque will probably over power the tires and the tires will spin.  On slicks, the tires are going to resist spinning and transfer more of this power to the ground.  A LOT more 'stress' is then placed on your driveline and you can break rear ends, transmissions, etc.  The engine is supplying the same torque to the input shaft of the transmission in both situations, but the resistance to this torque is very different and where the varying stresses in the driveline would occur.

Mechanical Resistance is related to friction, meaning it is a measure of how difficult it is for an object to move in a fluid (or surface) under a force.  The force arising from friction or viscosity will oppose mechanical torque.

Quote

I can tie my boat to a big dock and floor it at WOT and go no where, or go for a joyride at WOT. Is the stress the same?.... if the engine is at the same RPM in both situations,  is the stress not the same?

With the above said.....possibly.  I have never messed with the hydrodynamics of props and what exactly happens at different prop rpms.  But with a single gear ratio, the prop will always be the same rpm no matter the speed of the boat.  So I would think the only factor affecting it then would be the speed of the stationary water hitting the face of the prop under a forward moving condition.    I would think the changing of props is kinda like changing of tires in the drag car example.  One prop will create more resistance to turning than the other.  I would look into the mechanical resistance of a prop and different speeds/rpms.  Even water temperature would affect water viscosity which would change the stress on the shaft at a given torque....although minuscule.      There is a lot to prop pitches and such also.  A lot to think about, but with the same prop, at same rpm, only difference I see in the two cases is the forward moving prop hitting stationary water.  How does that change the mechanical resistance?      Hmmmmm

As for Nautiques breaking shafts when weighted heavier....I would surmise that the boats are requiring more throttle(more torque produced at the crank) and/or people changing props that are creating more mechanical resistance at said torque.....which creates a weak link.

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justgary
Just now, Teleman said:

I think a prop is missing many key functions that a rotating wing exhibit. A prop does not use Bernoulli's effects for the thrust. It is solely using the rotation of the blade to move the water backwards while the counter force moves the boat forward.

It absolutely does use Bernoulli's principle. 

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