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Old 12-31-2017, 03:29 PM   #1
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Diesels at idle; risky?

Odd title, right?
Anyway, I got to thinking about the way diesels are governed and how that might be an issue at idle, IF there is a driveline (or grounding) problem.

I have heard stories, that I cannot authenticate, of trawler diesels spinning off their mounts due to a line fouling the shaft.

So, perhaps my question is this. How much fuel flow is fed when a diesel is jammed up at idle (not allowed to reach idle rpm)? And, is this different with electronic vs mechanical injection?

One of the benefits of diesels are the wide range of fuel to air mixes that still allow combustion, and the constant rpm nature of the governers when in seas, towing, etc.
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Old 12-31-2017, 03:47 PM   #2
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My friends lobster boat sucked in a pool cover and a plastic trash can that stopped it immediately from 2000 rpm cruise. No damage to mounts, transmission, or struts either time.
300 hp Cat 3116 with mechanical FI.
At idle in gear it would just stop.
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Old 12-31-2017, 04:42 PM   #3
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If the mounts break, it’s because the mounts are broken.
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Old 12-31-2017, 06:17 PM   #4
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I have choked off a dozen or more engines for a variety of reasons and never broke or damaged a motor mount.

I will say high speed vessels where the props can be pulled back far enough can damage mounts.
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Old 12-31-2017, 06:26 PM   #5
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For most of our Diesel engines the torque capability peaks in the mid rpm range, and drops off at both high and low rpm. If the mounts can withstand full throttle torque they can resist idle speed torque too.
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Old 12-31-2017, 06:37 PM   #6
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OK, I see the urban legend of the spinning engine due to a loose dock line can be debunked.

One time I came across a D6 in the woods. I managed to start it and ran it around for awhile but couldn't for the life of me figure out how to stop it. Tried to stall it into a large tree, no dice. Long story short, I found the fuel cut-off.
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Old 12-31-2017, 06:43 PM   #7
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We wrapped a line around the shaft on our sailboat that killed the engine. Shaft, mounts, strut, transmison we’re all good. The cutless bearing was toast though. It broke the nitral loose from the navel brass. A new cutless bearing and we were good to go. We probably motored for 20 hours with no issues other than low rpms we had a little vibration till we could get to a yard to haul out.
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Old 12-31-2017, 07:16 PM   #8
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Quote:
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I have choked off a dozen or more engines for a variety of reasons and never broke or damaged a motor mount.

I will say high speed vessels where the props can be pulled back far enough can damage mounts.
Can imagine a quick/instant change from forward to reverse at higher-than-idle engine speed will be a problem.
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Old 12-31-2017, 07:25 PM   #9
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Can imagine a quick/instant change from forward to reverse at higher-than-idle engine speed will be a problem.
Actually I was referring to running aground at 25+ nmph.
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Old 12-31-2017, 08:39 PM   #10
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Actually I was referring to running aground at 25+ nmph.
Can't imagine a toy-but-true trawler traveling more than a small fraction of that speed.
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Old 12-31-2017, 08:47 PM   #11
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uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer
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Old 12-31-2017, 09:02 PM   #12
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my comment about running aground has nothing to do with tbe engine rotating more....

just yanking back on everything.....
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Old 12-31-2017, 10:35 PM   #13
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Quote:
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uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer
funny you mention that. Just 2 days I saw an IC engine designed to rotate the engine around the crank. It was a early aircraft engine. Two different radial designs did that. The crank was fastened to the frame, the prop attached to the crankcase.
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Old 12-31-2017, 11:47 PM   #14
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Quote:
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uhhhh....this is cartoon physics?

Thinking about it, what mechanics in an internal combustion engine, after having stopped rotation of the crank externally, would then cause the block to start rotating around the crank shaft?

Unless I'm missing something...or excess beer
Yup, the cartoon of the airplane spinning when the prop is stopped! With this logic, a powerful enough diesel would capsize the boat if the prop got wrapped....
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Old 01-01-2018, 12:54 AM   #15
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Commercial boats rarely have rubber motor mounts like yachts. Bigger steel fishing boats have the engines directly bolted to the engine beds. If something were to quickly stop a prop, because of the reduction gear, a coupling, the shaft or prop blades would fail. The clutch should release before an engine would break its mounts. Maybe before shaft or prop damage.
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Old 01-01-2018, 06:55 AM   #16
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"Just 2 days I saw an IC engine designed to rotate the engine around the crank."

Those early engines had great cylinder cooling!
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Old 01-01-2018, 08:15 AM   #17
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Bronze propellers will absorb a lot of shock from a propeller strike. Stainless propeller would be a different story but rarely see that on recreational boats.
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Old 01-01-2018, 08:26 AM   #18
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I have a trawler friend who backed over a line that had one end cleated.
He pulled the engine off it's mounts.
38 Marine trader with single Lehman 120.
It can happen.
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Old 01-01-2018, 03:35 PM   #19
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Quote:
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I have a trawler friend who backed over a line that had one end cleated.

He pulled the engine off it's mounts.

38 Marine trader with single Lehman 120.

It can happen.


Says a lot about the cleat
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Old 01-02-2018, 07:53 AM   #20
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"Says a lot about the cleat"

He pulled the engine off it's mounts.
38 Marine trader with single Lehman 120.

It sez even more about the boat build.
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