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07-17-2015, 07:24 PM
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#1
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Engine vibrations at idle
Quote:
Originally Posted by FlyWright
Don't expect much difference at idle, Mark. They are twin 4-cyl Perkins. It's what they are supposed to do at idle.
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FlyWright and I have "brotherly" teasing concerning our boats' relative levels of boat vibration between our two boats. My 4-cyinder JD transmits much less vibration. Is it due to steel versus fiberglass hull? Different engine mounts? Or different engine makes?
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07-17-2015, 07:46 PM
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#2
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Guru
City: Cape Cod, MA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 1,306
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JD has balance shafts right?
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07-17-2015, 07:48 PM
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#3
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Enigma
City: Slicker?
Join Date: Oct 2007
Posts: 16,565
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Greetings,
MR. mp. " Is it due to steel versus fiberglass hull? Different engine mounts? Or different engine makes?" Could be all or none of the above. It could also be sympathetic vibrations specific to one's individual vessel. I would be very surprised if two identical boats (factory built side by side at the same time) "vibrated" exactly the same. Slightly thicker resin in one bulkhead tab, three instead of four fasteners holding a locker together etc. A plethora of causes.
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07-17-2015, 08:01 PM
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#4
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Master and Commander
City: Vallejo CA
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Join Date: Sep 2010
Posts: 12,559
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Quote:
Originally Posted by Brooksie
JD has balance shafts right?
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Don't all engine makers strive to balance the shaft?
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Kar-KEEN-ez Koot
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07-17-2015, 08:13 PM
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#5
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Guru
City: Cape Cod
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Quote:
Originally Posted by markpierce
Don't all engine makers strive to balance the shaft?
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Different animal. Engine design often has a counterbalance installed on the main crank, on the flywheel or possibly on the cam shaft. (Most often on the main crank).
These are NOT all the same counterbalance. Often when rebuilding the 'guy' grabs the wrong one (stb engine, versus port) or other model. I have had 3 Detroit Generators shake themselves to pieces in short order.
Although I have not found the specific RPM on my Perkins 4.236, There does seem to be a certain RPM that shakes the stainless galley stove and the toaster oven. Luckily, when I am up on the flybridge the vibration goes away!
On the other engines I run (EMDs), they definitely are bad vibrating between 700 and 750 rpm. It is a well known phenomenon that the operator must avoid that RPM window if at all possible.
Are you referring to vibration at idle in neutral or clutched in gear? Same in reverse as forward?
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07-17-2015, 08:17 PM
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#6
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Guru
City: Fort Myers, FL... Summers in the Great Lakes
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The balance shafts are seperate rotating shafts that offset some of the vibration in our 4 cylinder JD engines.
Ted
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07-17-2015, 09:08 PM
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#7
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Guru
City: Cape Cod, MA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
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Quote:
Originally Posted by O C Diver
The balance shafts are seperate rotating shafts that offset some of the vibration in our 4 cylinder JD engines.
Ted
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That's what I am talking about, even my old '62 Greymarine has them, so does JD, but not Perkins.
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07-17-2015, 09:23 PM
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#8
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Balance shafts ... news to me ... thanks for the education!
So ... which popular marine engines do or do not have balancing shafts other than JD (does) and Perkins (doesn't)?
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07-17-2015, 10:12 PM
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#9
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Guru
City: Cape Cod, MA
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Quote:
Originally Posted by markpierce
Balance shafts ... news to me ... thanks for the education!
So ... which popular marine engines do or do not have balancing shafts other than JD (does) and Perkins (doesn't)?
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Can't say really, like listing them. Except to say only 4 cyl and under would normally have them as 6's are naturally balanced. My 5 cyl MB diesel seems well balanced too. But in a 4, you have 2 pistons rising and two falling at all times so unless it has opposed cylinders, you have vibration of the first order.
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07-17-2015, 11:40 PM
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#10
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Technical Guru
City: Wilmington, NC
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4cyl 4stroke has a second order vibe. Balance shafts run 2x crank speed to cancel. This second order vibe is only evident up in the higher revs. Not the same as idle shake, balance shafts will do nothing there.
Idle shake is from the gap in compression strokes/firing pulses- slower engine turns, bigger each "bump" feels.
Usually only the larger displacement 4's have balance shafts, and only then if they run up there in rpm.
Idle shake can be managed with well selected resilient mounts. And even then, some boats are better than others. A gazzilion variables.
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07-18-2015, 04:16 AM
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#11
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Guru
City: Adelaide
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Quote:
Originally Posted by Ski in NC
Idle shake can be managed with well selected resilient mounts. And even then, some boats are better than others. A gazzilion variables.
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Exactly.
The amount of vibration is affected not only the quality and condition of the engine mounts, but how well they are adjusted.
Often, prop shaft alignments are done using just the engine mount adjustments, sometimes setting them at the top or bottom end of their range. This greatly reduces their effectiveness.
The weight distribution between mounts must also be kept even, especially side to side, as all mounts are designed to carry a certain range of weight.
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07-18-2015, 05:36 AM
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#12
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Guru
Join Date: Oct 2007
Posts: 22,553
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With nor without a balance shaft a 4 cyl will never be smooth ay all RPM.
The mfg usually try to create the harmonics at a speed not usually operated at.
So a slower than book idle might be the cause of shaking, because that's where the engine mfg put it.
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07-18-2015, 05:47 AM
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#13
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Guru
City: Sydney
Join Date: Jul 2015
Posts: 1,646
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Talking about " harmonics" Ive seen different harmonic pulleys fitted to the same make of engine and in some cases the harmonics pulley had been machined to except a v belt to run a water pump this can also upset the idle balance
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07-18-2015, 05:49 AM
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#14
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Guru
City: South Florida
Join Date: Sep 2014
Posts: 1,088
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All very good and valid replies. And there may be other sources of vibration like pulse rate from the exhaust system. I'm assuming we are talking about idle in neutral.
Engine manufacturers make engines and buy mounts from various sources. Often the manufacturer doesn't know where the engine will end up. To narrow that down if it's a marine engine is it going in a planing hull or displacement? What's the gearing? Will the mounts take thrust or not? Point being if the engine is installed with factory supplied mounts they are most probably very conservative towards keeping the engine secure and usually at the expense of isolation.
Engine mount selection for optimum isolation involves taking many factors into account. Weights, CG, mount relation to CG, torque, thrust (or not) RPM range, plus a safety factor. Engine manufacturers don't usually do this. Then as mentioned the mounts have to be installed properly, loaded evenly and loaded square. Age also plays a part as creep will degrade isolation performance over time, and for natural rubber mounts 10 years is about the working lifespan for optimum isolation.
It's probably the mounts, but might not be.
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07-18-2015, 07:33 AM
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#15
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Enigma
City: Slicker?
Join Date: Oct 2007
Posts: 16,565
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Greetings,
Correction to post #3. Rather than sympathetic vibrations, I meant harmonic vibrations....brain fart.
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