Advice needed for amateur in trouble please!

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girlatsea
Bilge Dweller
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Joined: Fri Jul 16, 2010 3:47 am

Advice needed for amateur in trouble please!

Postby girlatsea » Fri Jul 16, 2010 3:57 am

Dear Engineers,

Sorry to bother you all with such a basic question, (you look like the best authority to help me) but in the building of new sterngear (for a basic boat, say a 20m retired trawler, just for instance.. say.. ) is a "bush inner" and a "bush outer" the same as "inner tube end" and "outer tube end" respectively?

Any advice very gratefully received! Also, if they aren't the same things, any idea how much I ought to pay for replacements "inner tube end" and an "outer tube end"? Roughly? Nearest thousand? Thank you so much.

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JK
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Currently located: East Coast, Canada

Re: Advice needed for amateur in trouble please!

Postby JK » Fri Jul 16, 2010 7:41 am

Do you have any blueprints with this boat? You want to pull out the sterntube drawing and shaft general arrangement. This gives you a idea of sizes and where the bits and pieces are.
As for prices, that is a difficult one to answer because it is depedent on materials, shaft size and length of the bearings.

Very simply you have the sterntube fitted in which the shaft runs. In the tube the bush is located, this can be one piece or 2 pieces, fitted forward and aft in the tube. Or, in your case the "inner" which is forward and the "outer" which is the aft (nearest the propeller).
The shaft needs to run on a bearing which is water lubricated. The bearing can be a bored out solid tube or staves fitted into slots. It can be Thordon or rubber.

I am not sure if the yard is using two different terms interchangeably. You have to get them to clarify .
If you have a boat at a yard doing shaft work and are unsure what is going on, your best bet may be to to call a local Naval Architect company and hire one to be your representative for that work. You will be less likely to be ripped off and have the work done that you need. Seriously, it will be money well spent.

I have attached the drawing from Reids General Engineering that shows a general idea of what I am saying. In this case the bearing is Lignum Vitae which you will not see used anymore.
Attachments
sterntube.pdf
Good Old Reids
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girlatsea
Bilge Dweller
Posts: 3
Joined: Fri Jul 16, 2010 3:47 am

Re: Advice needed for amateur in trouble please!

Postby girlatsea » Fri Jul 16, 2010 8:21 am

Dear JK,

Many thanks indeed for your very swift and helpful reply - I have the blue-prints from the 70s but naturally the parts are not labelled in a way that a non-expert can understand.

The reason I am asking the question is that I think I am being charged twice - once for the bush inner and outer and then again for the tube ends inner and outer...

I think I will follow your advice and contact a naval architect, but I do have a picture of how the propellor and things looked before it went in the water and the outer thingy looks pretty old, it's bronze, I think, but is dull and tarnished.

Many thanks again,

Kind regards,

JFC
Mechanic
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Joined: Sun Dec 13, 2009 12:00 pm

Re: Advice needed for amateur in trouble please!

Postby JFC » Fri Jul 16, 2010 8:43 am

Hi,
The inner and outer probably refers to two bearings -"inner" being the one closest to the engine, and "outer" being the one closer to the propeller.
For the application you are speaking about I suggest you log on to some of the usual suspects for that size of boat and materials. A good place to educate yourself will be on the Morse Bearing website http://morserubber.com/ that will at least provide you with the proper descriptions of what they are referring to even if they use their own boatyard slang, or another good site for you to read about is Thordon Bearing at service@thordonbearings.com

I don't think you are paying twice, but they may be talking composite bearing (outer bushing and inner bushing bonded together), or others where there is a brass bush fitted to your boat and a composite material is pressed into that which "holds" it. The composite material is water lubricated and your shaft actually runs in that.

Usually the argument is around the wear-down or clearance between the shaft and the bearing material with boatyards. They say it needs replacing because they say it is too much etc....their are certain rules of thumb that are used and generally they favor the shipyard. You need to provide the size of shaft, the RPM, the measured wear-down in the bearings (hence the clearance).

Either way you will be smarter than the average boater when you are done with all this.

I hope this helps.

girlatsea
Bilge Dweller
Posts: 3
Joined: Fri Jul 16, 2010 3:47 am

Re: Advice needed for amateur in trouble please!

Postby girlatsea » Fri Jul 16, 2010 9:02 am

Thank you again for your very detailed and helpful reply - following your advice I'm going to get reading; I'd be happy to be smarter than the average bear, never mind the average boater by the time this is done!

Many thanks again


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