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Old 12-04-2020, 14:34   #1
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Flex Plate Failure

I have a rattling sound at idle in gear, to me it sounds like a clutch rattling, but of course we have no clutch, so I assume flex plate, which I assume has similar springs in it that a clutch does.
Couple of questions, how often do they fail to the point to where there is no drive?
Do they rattle awhile before they completely fail, and if so how long?
Iím at Normanís Cay, jumping to New Providence tomorrow, then Chub Cay after that, and then onto Lake Worth.
The last leg in particular there wonít be sailing winds, so I will need the motor but worry if Iíll have it.

I know flex plates can fail of course, but have no experience with one failing, so I donít know the frequency of such a thing or itís failure mode.
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Old 12-04-2020, 15:00   #2
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Re: Flex Plate Failure

Depends on the plate. Good quality one will last, a cheap automotive one not so much. I would replace asap while you can, pretty easy and cheap as far as things go. Just did ours by myself, pulled a cheaper one out with loose rivets that rattled a lot at lower speeds. Not sure how long it would have lasted but it had been doing it for a few years, lots of play in the old one. It may have taken one quick shift coming into dock or pushing into a wave who knows but Iím glad itís done so I donít have to find out. Worst case it fails catastrophically when you need it most and gets chewed up in the flywheel and you bust teeth off or worse snap the crank. They are considered regular maintenance but I canít remember the recommended interval. You could even try and replace with a cheap auto one where you are and get a good one when closer to home. The cheap ones work, they just donít last as long.
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Old 12-04-2020, 15:07   #3
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Re: Flex Plate Failure

What engine and trans do you have?

Besides the rattling, is there any extra whirring on start up? Is the noise from the rear of your eng./trans? (if yes, probably flex plate)

Does the rattle or high pitch noise go away w/slight increase of engine rpm or a consistently increase w/the rpm? If the rattle keeps time w/an increase/decrease then its most likely a broken spring in the flex plate about to come loose or maybe already in the bell housing.
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Old 12-04-2020, 15:09   #4
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Re: Flex Plate Failure

When I bought my boat, I brought in an engine surveyor. During the sea trial with the broker and owner, he told me that he "heard something." After we got back to the dock, he had me shift slowly from forward-neutral-reverse at idle. Iím half deaf, but he convinced the others that the damper plate was bad and that to fix it involved more work than just replacing the engine. In any event, when we got the old engine out and in the shop, we pulled off the transmission and v-drive to rebuild. As we slid it apart, we heard a clink as something hit the concrete floor. Upon looking, it was one of the damper plate springs. Several of the other springs were loose where the plates had rusted.

Since then, Iíve seen a damper plate on another engine where the rivets holding the drive shaft sleeve to the plate had broken, apparently by rusting.
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Old 12-04-2020, 15:23   #5
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Re: Flex Plate Failure

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What engine and trans do you have?

Besides the rattling, is there any extra whirring on start up? Is the noise from the rear of your trans? (if yes, probably flex plate)

Does the rattle or high pitch noise go away w/slight increase of engine rpm or a consistently increase w/the rpm? If the rattle keeps time w/an increase/decrease then its most likely a broken spring in the flex plate about to come loose or maybe already in the bell housing.

Only thing I have noticed is the rattling, and it’s only when in gear, and only low RPM, it goes away pretty quickly, say 1,000 or so. I left Long Island thinking I would be unable to buy fuel and it was low winds, so I was motorsailing with my code zero at 1,000 RPM, which gave me 5 kts to conserve fuel.
I think that made it worse. Got to Staniel Cay to find that their fuel dock was open 7 days a week, even during the shut down.
Engine is a 1987 4JHE with I believe a KBW20 transmission, I changed tranny fluid yesterday when I changed oil and it came out looking just like brand new fluid with no odor, I change it at a 100 hr interval with engine oil, I don’t believe I have a transmission problem, it shifts crisply and always has.

I’ve not listened to see where the noise is coming from, Since I’ve noticed it, I try to not let it happen, so when I put it into gear now I quickly add throttle to get to say 1200 or so.

I have a lot of experience with auto clutches, and believe a dampener plate is nearly identical, I knew about the springs of course and do know about the rivets that hold the splined center to the plate, but didn’t think about them failing, never seen that in a car.
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Old 12-04-2020, 15:30   #6
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Re: Flex Plate Failure

$100,000 question is, do you think it will get me home?
I assume at higher steady RPM of say 1500 to 1800 the springs are doing very little flexing, the engine impulses are quick enough to not be working them?
I believe that’s their job, to absorb or smooth out engine impulses and not transmit that shock to the drive train?
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Old 12-04-2020, 15:52   #7
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Re: Flex Plate Failure

Higher RPM should be less harsh on the plate. Realistically, any time the winds are decent enough to sail (even if a bit more slowly than you'd like to be moving), shut the engine down and sail. Less operating hours will reduce the risk of a failure.
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Old 12-04-2020, 16:10   #8
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Re: Flex Plate Failure

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$100,000 question is, do you think it will get me home?
I assume at higher steady RPM of say 1500 to 1800 the springs are doing very little flexing, the engine impulses are quick enough to not be working them?
I believe thatís their job, to absorb or smooth out engine impulses and not transmit that shock to the drive train?

Obviously not being there can't exactly tell what's going on, but it may be gear lash or something other than the plate if the sound goes away at an increased rpm.
Approx. 1700-1800 rpm seems reasonable (mid rpm for your motor) and should get you back.
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Old 12-04-2020, 16:24   #9
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Re: Flex Plate Failure

So your saying if it’s the plate, the rattle stays at higher RPM?
It’s actually always had this sound, but really at the threshold of hearing, I’ve heard that on most of the boats I’ve been on, it does sort of sound like diesel clatter.
Just now it’s much more pronounced.

What I guess I’m after is hearing from people that our boat did that, did it for a long time, finally had it fixed, or what I don’t want to hear of course is our boat did that, then a day or two after it started there was this grinding sound and we lost all drive.
It could I guess even be the Autoprop, but it’s such a familiar sound from my auto mechanic days. Drag racing of course it wasn’t uncommon to bust clutch springs.
Even then I don’t remember one completely failing to work, just sounded like Hell so we would replace the clutch.

I do realize of course that if I don’t use the motor it will last longer

Today foolishly I was sailing downwind and traveling with another boat, and 1200 RPM and I was doing 8 kts.
I should have just sailed at 5 to 6 and let him go.
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Old 12-04-2020, 16:36   #10
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Re: Flex Plate Failure

Take a long screwdriver and use it like a stethoscope to see if you can pin down the location. A rattle could be something simple like a loose air filter so worth eliminating a noise from somewhere else.

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Old 12-04-2020, 16:36   #11
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Re: Flex Plate Failure

All the torque transfers through the springs and rivets even at higher rpm, bashing into a good wave could push its limits. No one can tell if itís going to fail without opening it up. Ours went for a few years without issue but our rear main seal was leaking so it was getting a nice oil bath and the springs werenít rusted. Yours could be all cracked or completely fine. I suspect when you get around to pulling it you will look at it and say wow I canít believe it held up that long.
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Old 12-04-2020, 16:37   #12
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Re: Flex Plate Failure

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Originally Posted by a64pilot View Post
Today foolishly I was sailing downwind and traveling with another boat, and 1200 RPM and I was doing 8 kts.
I should have just sailed at 5 to 6 and let him go.
Címon, most of us buy boats to try to be young and kids again.
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Old 12-04-2020, 16:38   #13
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Re: Flex Plate Failure

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So your saying if itís the plate, the rattle stays at higher RPM?
The "rattle" (from the trans) should increase/decrease w/the rpm. If it goes away, it may not be the plate.
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Old 12-04-2020, 16:49   #14
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Re: Flex Plate Failure

Bill, If they are like clutch springs and I strongly suspect they are, they are the connection between two parts of the plate, all torture is transmitted through the springs.
The rattle occurs because when the engine is at low RPM, it’s actually jerking, not a smooth rotation, but actually many quick jerks.
If the engine RPM goes high enough it smooths out and the jerks are very much less apparent. It’s also why many engines will shake at idle, but visibly smooth out at high idle or higher.

That is why I’m thinking it’s the springs.

Pete, not being able to fix anything, I don’t want to make it make the noise why I try to pin it down, gut says get it through the RPM range where it makes the noise and do not stay in an RPM that it does.

rbk, I hope your right, I’ve taken car clutches apart that were like that, with broken springs in the bell housing and what ones were left were broken, but it still worked, just was noisy and on a car made the clutch jerky when you let it out.

There isn’t anything I can do unfortunately, pre COVID shut down days, I’d head over to Spanish Wells to R&B boatyard, but now I guess I’ll have to cross my fingers.

It doesn’t help at all that tomorrow is the 13th, and yes even though I believe myself to be intelligent, I am superstitious. At least it’s not Friday.
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Old 12-04-2020, 16:55   #15
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Re: Flex Plate Failure

See if you can find a part number and cross reference it to automotive and see if you can source one locally as a spare or have one mailed enroute to pick up. Itís not something youíd want to replace mod crossing but something easily done at anchor. Iím reality you could have it replaced in a day with minimal tools. As you know once the springs go all torque goes to the rivets and flywheel, it may hold but without dampening all the torque transfers to the crank and tranny.
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