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Old 02-23-2018, 12:38 AM   #1
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Odd noise on slowing from cruising speed

I know noises are always hard to describe; but here goes....

I have a recently-purchased 2008 Nordic Tug 32 with Volvo D6-280 diesel inboard drive and transmission. 550 hours and excellent service history and records. General condition of the whole boat is pristine.

Here's my issue: The drivetrains purrs like a kitten with a single exception - when slowing from cruising speed ( 8 knots; 1600 RPM) to idle (2 knots, 600 RPM) as the engine and transmission slow down there is a strange noise that comes from the drivetrain that somehow sounds unhealthy - maybe a faint "grinding" sound - only while slowing down. Noise isn't there at cruising speed, and also not there once completely slowed and idling; - also not there while accelerating. Only while decelerating... I didn't notice this nor did the surveyor when purchasing last summer. Maybe I'm just more attuned to everything.

Any ideas what could cause this? I'd love any suggestions what to look at?

thanks all.
Don
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Old 02-23-2018, 02:03 AM   #2
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Look at the stuffing box first.
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Old 02-23-2018, 05:36 AM   #3
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-- Is the engine the supercharger and turbo combination?
-- Running gear clean?
-- When was transmission oil last changed? Do you check it for level and color before each run?
-- Did you recently get new hearing aids?
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Old 02-23-2018, 07:00 AM   #4
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[QUOTE=Foster Nordic;638916]I know noises are always hard to describe; but here goes....

I have a recently-purchased 2008 Nordic Tug 32 with Volvo D6-280 diesel inboard drive and transmission. 550 hours and excellent service history and records. General condition of the whole boat is pristine.

Here's my issue: The drivetrains purrs like a kitten with a single exception - when slowing from cruising speed ( 8 knots; 1600 RPM) to idle (2 knots, 600 RPM) as the engine and transmission slow down there is a strange noise that comes from the drivetrain that somehow sounds unhealthy - maybe a faint "grinding" sound - only while slowing down.

Hi

I think I know the sound you are talking about and it is due to the shaft line where there is no flexible switch between the shaft-gearbox flange, The propeller brakes and transfers the vibration gearbox, my NT37 misses the nasty sound when I quickly drop a voyage 8 to 4kn and my tug not have flexible coupling.

I do not know whether this sound is a risk to the gearbox, I remember reading every NT doing a sea test where a crash stop test is carried out, with the reverse gear being switched directly from full forward speed and gearbox takes this.

I avoided the sound, put in a free connection, and when the boat decelerated the speed of 3-4kn gears in front, this way the transmission does not collapse and I think it is more durable. Another way to drive speed very slow down,

NBs


Exaple flexible coupling shaft/gearbox
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Old 02-23-2018, 07:22 AM   #5
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If it only happens when the throttle is chopped , not during a simple slowdown , the springs in the drive damper plate could be weak or broken.

If there are no strange sounds shifting slowly at idle from fwd to reverse , I would not worry , but I would slow the boat , not chop the throttle , unless it was an emergency.
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Old 02-23-2018, 07:47 AM   #6
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Damper plate on the engine would be one possibility. It's hard to describe, but does the noise sound like like it's dampened by fluids ( gear oil), or more open air? If you think it might be dampened by fluids, do an oil analysis on the transmission to see if you're getting any unusual wear metal.

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Old 02-23-2018, 09:00 AM   #7
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I was going to suggest doing a "back down test" to see if it might be a loose engine mount.
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Old 02-23-2018, 03:16 PM   #8
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Could it be the turbo dumping pressure?
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Old 02-23-2018, 03:34 PM   #9
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If the boat were mine I would go through a series of checks, 1st Engine alignment, must be within 2 thou on both planes ie. vertical and horizontal, 2nd Gearbox box fluid level & oil colour, 3rd stuffing box, any shaft play ? 4th check for any play (movement) in the carrier bearing if fitted.
Having checked all that and the sound still persists it sounds like wear in the cutlass bearings. Check rudder shaft bearings at the same time.
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Old 02-23-2018, 03:39 PM   #10
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Could it be the turbo dumping pressure?
It is noisy with mine. That is why knowing his induction setup as previously asked would be good.

But if a new thing ???
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Old 02-23-2018, 04:04 PM   #11
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Yes its a turbocharged induction engine. Why does that matter?
No new hearing aids.
Yes the transmission fluid changed recently.
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Old 02-23-2018, 04:05 PM   #12
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It happens even when throttling down slowly.
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Old 02-23-2018, 04:09 PM   #13
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Can you tell me more about the turbo dumping pressure - is there some sound from that - that is exactly the situation. This is my first trawler with turbo induction so maybe I'm just not used to that?...
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Old 02-23-2018, 09:25 PM   #14
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Maybe best to take a run up to Anacortes some sunny afternoon and have a prearranged visit with the right guy from NT. Internet guessing is not always the best way, hands and eyes on by a pro usually works for me on these funny type things.
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Old 02-23-2018, 11:22 PM   #15
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Quote:
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Maybe best to take a run up to Anacortes some sunny afternoon and have a prearranged visit with the right guy from NT. Internet guessing is not always the best way, hands and eyes on by a pro usually works for me on these funny type things.


I second this, or perhaps even phone NT. This electronically controlled engine may have a version of what us dinosaurs (I have not had hands on experience with modern diesels) would have called a variable speed (as opposed to limiting speed) governor. In a marine application the “droop” of the VS governor had to be set unique to each vessel or it would hunt or chatter on deceleration. Increasing the governor droop could be a quick setup change that would eliminate your chattering.
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Old 02-23-2018, 11:30 PM   #16
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When an engine is running under a good load the turbo will be wound up producing pressure.
If the throttle is dropped abruptly the turbo takes some time to slow, the pressure does not quit instantly. That pressurized air can back flow out the intake system causing some noise. On some engines it can be a bark or pop.
Just my understanding.

With you sitting atop the engine in a pilothouse you may be more aware of those noises than otherwise would be the case.

I suggest that you do as Sunchaser suggested and contact Nordic. It may not be unusual and if known and explained then it will be easy to deal with.

Otherwise you then know you need to do further checking.
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Old 02-24-2018, 12:54 AM   #17
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Hi,

At 8 kn speed at the pressures are still low, almost zero.

NBs
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Old 02-24-2018, 05:32 PM   #18
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Might be worth checking your prop shaft bearing for play. As you throttle down, you are unloading the prop, which is then free to rattle the shaft around a little until it loads back up at the lower speed.
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Old 02-24-2018, 06:05 PM   #19
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My money would be on the damper plate. If that is it, nothing will leave you more dead in the water than a damper. If you replace it, you should look at the R&R dampers as sold by PYI. They have a get-home feature if they fail.
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Old 02-24-2018, 08:13 PM   #20
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Sorry: By R&R I meant R&D dampers
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