Loud "air Horn" noise from Shaft area

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Patha1

Member
Joined
May 3, 2021
Messages
10
Vessel Name
Chapter Tug
Vessel Make
Nordic Tug 37
On our 2000 Nordic Tug 37, we recently experienced a very loud air horn type of noise from the shaft / prop area. It only occurs at very high rpm ( basically max), after about 20 seconds and the boat planes.
Just finished haul out, replaced dripless, cutlass, the shaft, engine mounts (for other reasons including failed cutlass). Surprised when the noise re-appeared.
It sounds like air escaping a balloon, when you hold the opening, high pitched - but very loud, like canned air horn. As you back down 100 rpm the noise lowers in volume and pitch. Backing off 200 to 300 rpm makes it go away.

Appreciate anyone with same experience to share potential solutions.

I don't drive at max rpm, but in testing for my failed cutlass, we spotted this issue and if I ever get a survey this is going to show up.
 
Welcome to the forum. I don’t have an answer but anxiously waiting to hear others thoughts.
 
Patha1 said:
I don't drive at max rpm, but in testing for my failed cutlass, we spotted this issue and if I ever get a survey this is going to show up.
Click to expand...

I have had several surveys and one engine "inspection" mandated by my insurer. None ever ran the engines in gear, let alone near WOT. Why would you expect yours to do so? Maybe on a sea trial, but otherwise?
 
Interesting ,following to learn the cause.
 
Noise

The sound you describe is not quite the same as we had on our 37 but you may want to check the shaft bonding block located about half way along shaft alley. The block is mounted on a spring arm that bears on the shaft. Try tying it up off the shaft and see if the noise disappears. If it does, polish the shaft where the block bears with fine wet/dry sand paper and the contact face of the block. A touch of grease periodically helps as well.

If this is not the problem, make sure there is water flow to the fixed face of the shaft seal and try burping the seal to make sure there is no air pocket in the bellows.

I’m assuming everything was properly aligned when you replaced the engine mounts but check the shaft turns freely as well.

Let us know what you find.
 
koliver said:
I have had several surveys and one engine "inspection" mandated by my insurer. None ever ran the engines in gear, let alone near WOT. Why would you expect yours to do so? Maybe on a sea trial, but otherwise?
Click to expand...


My previous boats, every survey does a sea trial and with WOT as part of the inspection. Maybe I am using the wrong surveyor. :)
 
How old is the propeller or when was it last checked?
Occasionally propellers sing ie they make a similar sound at different rpms

If this is the case send diver down and file slight flat on trailing edges of the blades

We had this occure on a boat I installed brand new propellers
 
I always run the boat at WOT during a sea trail to ensure that it will make the rated RPMs.
 
Yes. Inspection should include WOT and a backdown test to observe the engine mounts and coupling. Oil samples are good too.

Sent from my SM-G715U1 using Trawler Forum mobile app
 
JpNomad said:
How old is the propeller or when was it last checked?
Occasionally propellers sing ie they make a similar sound at different rpms

If this is the case send diver down and file slight flat on trailing edges of the blades

We had this occure on a boat I installed brand new propellers
Click to expand...

"Singing" propellors was a phenomenon on a commercial boat I used to run from the day it entered service. The twin 48" four-bladed wheels would emit a high-pitched sighing noise, but only as power was being reduced. Under load or at cruise, the sound was negligible, but when decelerating from cruise to slow or idle, the soft propellor whine picked up and was audible from the pilot house, 100' away. It was never intrusive enough to warrant changing props - just a quirk.
 
Ive had many road trucks get a certin rythmic churn so to say to them going down the road.

Maybe a motor mount is starting to wear and adjusting the angle of your prop shaft to the cutlass? Something to rotation and binding would be my thoughts.
 
JpNomad said:
How old is the propeller or when was it last checked?
Occasionally propellers sing ie they make a similar sound at different rpms

If this is the case send diver down and file slight flat on trailing edges of the blades

We had this occure on a boat I installed brand new propellers
Click to expand...


Thanks for the suggestion - we just had it pulled, cleaned and re-worked. But will discuss this with the prop shop that did the work.
 
Blissboat said:
"Singing" propellors was a phenomenon on a commercial boat I used to run from the day it entered service. The twin 48" four-bladed wheels would emit a high-pitched sighing noise, but only as power was being reduced. Under load or at cruise, the sound was negligible, but when decelerating from cruise to slow or idle, the soft propellor whine picked up and was audible from the pilot house, 100' away. It was never intrusive enough to warrant changing props - just a quirk.
Click to expand...


interesting - Will look into the prop as potential source of my issue, thanks.
 
i second the advice to check for water flow to the dripless seal. it's high speed operation where you really need to be sure you're getting water to it or it can chatter (vibrate) and do damage to the seal. blocking water flow to the cutless can cause essentially the same thing too.
 
Maybe a build up of air somewhere by the shaft. Then maybe the air pushing it’s way out could be the noise. Doesn’t make a lot of sense but it is a guess.
 
Bmarler said:
i second the advice to check for water flow to the dripless seal. it's high speed operation where you really need to be sure you're getting water to it or it can chatter (vibrate) and do damage to the seal. blocking water flow to the cutless can cause essentially the same thing too.
Click to expand...


When we splashed the boat after the work was complete, first thing we did was check water flow to the dripless, it was fine. We do have a theory that at high rpm water flow may be somehow slightly restricted (perhaps the heat exchanger) causing the noise. thanks for the comment.
 
Sounds crazy, but get a 3 foot section of small tubing, like aquarium air line hose or most anything, put one end in your ear and use it to trace the source of the noise. amazing but it can be quite precise and directional too. Anyway, that's what I suggest trying as a simple way to isolate where it's coming from and move on from there.


If it is in a high noise environment you can put the tubing end in your ear and put a pair of ear muffs on over that to block out a lot of external noise.
 
Last edited:
Piaro said:
Sounds crazy, but get a 3 foot section of small tubing, like aquarium air line hose or most anything, put one end in your ear and use it to trace the source of the noise. amazing but it can be quite precise and directional too. Anyway, that's what I suggest trying as a simple way to isolate where it's coming from and move on from there.


If it is in a high noise environment you can put the tubing end in your ear and put a pair of ear muffs on over that to block out a lot of external noise.
Click to expand...
Kinda like using a long screwdriver used as a stethoscope to determin bad pullies. Lay the tip on such area and put your ear to the handle. Just watch for whats spinning or moving.
 
Does the frequency of the noise get lower as you back down the rpm or is it just that the volume goes down? If the frequency changes then you're looking at a vibration issue, prop, shaft, bearing, etc. I've seen boats that put Prop-Speed on their prop(s) develop an rpm dependent noise, usually at a frequency equal to the shaft speed divided by the number of prop blades, i.e., one blade is vibrating differently than the others. In the few cases I've seen it was only at one specific rpm and by increasing or decreasing the rpm by maybe 100, the noise would go away... very strange.
 
Dose your motor have a turbo it could be the air line to it i had a small split in mine and it sounded just like an air horn at high rpm
 
RickyD said:
Could you be spinning a cutlass bearing??
Click to expand...


We were, and that caused a noise at 1800 rpm, but replaced everything now, , dripless, cutlass, mounts, shaft included. Its still there.
 
I vote for the grounding block.
I had the same problem, moved the block ... noise gone.
What type of grease are you recommending?
 
Last edited:
Patha1 said:
On our 2000 Nordic Tug 37, we recently experienced a very loud air horn type of noise from the shaft / prop area. It only occurs at very high rpm ( basically max), after about 20 seconds and the boat planes.
Just finished haul out, replaced dripless, cutlass, the shaft, engine mounts (for other reasons including failed cutlass). Surprised when the noise re-appeared.
It sounds like air escaping a balloon, when you hold the opening, high pitched - but very loud, like canned air horn. As you back down 100 rpm the noise lowers in volume and pitch. Backing off 200 to 300 rpm makes it go away.

Appreciate anyone with same experience to share potential solutions.

I don't drive at max rpm, but in testing for my failed cutlass, we spotted this issue and if I ever get a survey this is going to show up.
Click to expand...

The distance from the prop tip to the hull should be checked, insufficient clearance resulats in the noise you are describing, maybe have to trim the prop.
 
Bmarler said:
i second the advice to check for water flow to the dripless seal. it's high speed operation where you really need to be sure you're getting water to it or it can chatter (vibrate) and do damage to the seal. blocking water flow to the cutlass can cause essentially the same thing too.
Click to expand...

And again! The small diameter pipe thread-to-barb fitting aft of my dripless packing on one of my shafts became clogged with dried salt. At certain RPM settings, a loud howl was emitted. Changed to a larger diameter fitting and the problem never recurred.
 
Have same on my boat. It is the Prop singing. Usually only lasts until props are not really clean.
 
Check your grounding strap. Readjust the position and or a bit of sandpaper on the shaft.
 
Mine was shaft ground strap singing. Carefully pull it back and listen.
 
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