Drive train whine

[Fish 0]

We've just finished a long and involved noise and vibration project on our old 38hp Izuzu. The saloon is now quieter than the cockpit, which indicates the sound proofing on the engine box has been effective.

 

We now get a whine from the drive train, I'm interested in what it is most likely to be.

At about 2,200rpm (cruising revs) there is a steady background thrum.

Lifting revs up to 2,800 there is a full on whine, load enough to warrant investigating.

Today was the first descent run since:

1) re-packing the packing gland. Not much heat coming off the stuffing box, and we inspected this at full noise, no obvious jumping or twitching from the shaft. Also, we have done the packing gland in the past with no problems, unlikely to have substantial stuffed that up.

2) replacing the previously stuffed cutlass bearing with a new proper Cutless bearing. Wouldn't have thought we could get a whine from a Cutless bearing.

3) put a spare bronze 3 blade prop on while we service the kiwi prop. This has no fouling proptection, has been on circa 6 months and may well have barnacles growing. We will be swapping back to the freshly serviced kiwi prop soon.

4) we also have a hydraulic gearbox. PRM160. It is a beast of a thing weighing in at 56 kg's. It makes modern mechanical gearboxes look like afterbirth.

 

We have never previously ran at the revs we ran at today to get the whine. That was due to the vibration issue we have now fixed. We used to cruise at 2,000 and not often go over 2,200. Today we took it all the way to 3,000, where th max revs is supposed to be 2,800.

 

Could it be that hydraulic gearboxes just whine, and we haven't heard it before over th engine noise, and because we couldn't run at higher revs?

 

If a prop is cavitating due to poor size for the boat, poor cupping, or just cause of crap on it, what would it sound like? The bronze prop is smaller diameter and pitch than our kiwi prop, it has loads of hull clearance.

 

The level of whine is directly related to rpm above about cruising revs. Is there anything here that would give the prudent mariner immediate cause for concern?

[Jon 360]

Sounds like your on the right track Fish but of its still there with the newly serviced prop then get a cup half fill it with water and move it along the drive chain sitting on the hull, where the water wants to jump out the most will tell you where the problem is.

Or maybe it's been there all along and now you can hear it.

You should get 10% less revs at full throttle underway than at full revs neutral if your props sized correctly i.e. if your getting 3000 underway you should get 3300 in neutral.

[wheels 543]

Could be the box. There is no such thing as a hydraulic box. The hydraulic part just engages a clutch one direction to engage Fwd and the other to engage Rvrs. The gears in the older boxes tend to be straight cut which makes them long lasting, strong and efficient. There is normally  Whine from a gear box.

The prop is another possible. I certainly would not worry about your issue till you replace he prop with the Kiwi prop. Props can whine, which is called "Prop Singing". Unfortunately it only knows one song being, Ï know a tune that gets on everyones nerves".
This is caused by the edges around the prop being too sharp. If you can easily access the prop at low tide for instance, then simply run a file around the edges and round them off. Both leading and trailing edges.

[180S 20]

I had a similiar experience with a whine in a Volvo D1-18 WITH MS10a gearbox.

Funny thing was it didn't whine in reverse. After a bit of investigation found out there is a idler shaft/gear dropped in to reverse rotation and the prop had the incorrect hand. Replace the prop with the correct hand and only have the whine in reverse now.

[John B 106]

I think you're onto it . Prop singing or its a sign from the gearbox.  But you can use nearly anything as a stethoscope.. Big screwdriver , lump of 4 x 2,  wooden oar or paddle or dowel. Track it with that..

[Fish 0]

Props can whine, which is called "Prop Singing". Unfortunately it only knows one song being, Ï know a tune that gets on everyones nerves".

 

Dear Lord, my engine knows that song, don't say the propeller has learnt it now....

[too_tall 15]

As wheels said, straight  cut gears make a fair whine. Strong, durable, industrial type strength but noisy.

[Fish 0]

Thanks for the feedback, good to know I'm on the right track.

 

We normally use a very large screw driver as a stethoscope, but I do like Jon's idea of a cup of water, easier to get into the cockpit lockers with.

 

I'm thinking it is the prop singing. Mainly cause the sound was coming more from under the boat than around the engine. Although it's very hard to pinpoint. That and the gearbox is now fairly well enclosed in sound proofing now.

 

The bronze prop is 16 x 12, where as our kiwi prop is 17 x 14.6, so a substantial difference. We didn't have any issues reaching max revs with the bronze prop, in fact we went straight past what was supposed to be the max revs, which makes me think the bronze prop is too small. It came with the boat.

We are actually going to try bigger foils on the kiwi prop, 18.5 x 15.2. See if we can't get a higher speed at cruise rpm. The engine is rates at 105 nm torque at 2,000 rpm, and being a heavy old engine we suspect it is conservatively rated and can give us a higher speed lower in the rev range.

 

Will report back once we've swapped the props around.