Vibrating shaft at slightly high revs.

[OceanSeaSpray]

Originally Posted by Cheechako

Yes, here's a boat designed by a whole group of "proper" marine engineers

Actually, I think those are referred to as "racing jocks": the engineering standard is "will hopefully hold together long enough", but the boats still mostly do these days.

Incidentally, I didn't say there weren't good marine engineers amongst the "non-proper" ones, I only said to be a little inquisitive... not sensitive.

[Hoohaa]

Update.
Sorry for the delay. I still have to work for a living..
Yesterday I went out to the boat and on my old man’s suggestion, I used a bit of solid timber to lever against the shaft while it was in operation to get an idea of how much force would be needed to stop the wobble. Turns out it didn't take too much effort.
So I am thinking of mounting a pillow bearing about an inch in front of the stuffing box and realign the engine to that.
So here is a pic of a contender for the bearing.

Suggestions?
There is a couple of grub screws which clamp the bearing to the shaft. I suspect that they will corrode fairly rapidly. Is there a product the will stop the corrosion and prevent the screws from seizing?
And what do you think of this idea. I can install an additional bearing in the stern tube when I eventually slip the boat but for now, this seems to be A simple cost effective way to solve the problem. Or maybe I am just shifting the problem further down the shaft. But I think I don't have anything to loose
by giving it a go.

[DeepFrz]

That tiny little picture shows us nothing. I recommend you use a "built for purpose" unit.

[Hoohaa]

Originally Posted by DeepFrz

That tiny little picture shows us nothing. I recommend you use a "built for purpose" unit.

Just fixed the pic.
Built for purpose? Are you suggesting custom made or marine equivalent? I didn't think they could make the bearings in stainless..

[Cheechako]

I have lost track of the shaft length... how long between the engine and stuffing box?

[Hoohaa]

About 2ft or 600mm

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[Hoohaa]

Originally Posted by Hoohaa

Sorry. Try this one.

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Sorry. That should have been 380mm

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[Cheechako]

Hard to imagine 2 ft needing a pillow block. Was this a small shaft? Sorry, been too many days in between. Do you know the shaft isn't slightly bent? A timber might take the bend out while turning as you mention I suppose.

[Hoohaa]

Sorry. Should be 380mm

[Cheechako]

Originally Posted by Hoohaa

Sorry. Should be 380mm

Well, that's even less.... about 15". Definitely shouldn't need a support in there. Your timber may be solving the visual but I doubt it's indicating a problem solution....

[Hoohaa]

Originally Posted by DeepFrz

That tiny little picture shows us nothing. I recommend you use a "built for purpose" unit.

Did you mean the aquadrive you mentioned earlier?

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[delmarrey]

Originally Posted by Hoohaa

Sorry. Should be 380mm

6.7' wow! That's 1.7' longer then my shaft was. It's no wonder it's whipping. When I finally went to 1-1/8" shaft it cured the whipping problem.

A pillar bearing fwd of the packing would work but it'll present other problems. Mostly rust. It's just a band-aid. You would have to keep it well greased internally, and externally. And you'll have to get a bearing rated for continuous running at full rpm's. Also you would not want to use the set screws unless it were a tapered (cone/cup) type. The shaft moves fore and aft under operation. http://www.globalspec.com/learnmore/...block_bearings

[Hoohaa]

Good point about the shaft movement.
The old man sugested some sort of vynl or non metal bearing?? Does anyone know about them?

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[senormechanico]

Listen to delmarray. He really knows his stuff.

Steve

[jimbunyard]

Agree with Delmarrrey above, a pillowblock in that location is just a bandaid. If there are misalignment issues it might actually make them worse. If shaft whip or propeller configuration/balance issues are the problem it might help some.

One of the larger problems is that you're adding a direct mechanical link between hull and the drive line which will likely telegraph and/or magnify vibration, with an attendant increase in noise.

In the picture of an aquadrive below (which, by the way, is not intended to control shaft whip but to transfer propeller thrust to structural members in the hull, allowing the use of softer engine mounts) notice the rubber mounts isolating the mounting flange from the hull.



The most efficient approach to problem solving, in terms of time invested and money spent, remains "identify the problem, devise a plan to address and correct the problem, execute the plan"... It seems you're missing step one with the pillow block approach; if you're going to go through all that trouble to install the pillow block, why not try and figure out what the problem actually is first?

You can always use this to prevent rust, though it is a little messy...