Model Boat Mayhem

Mess Deck: General Section => Full Scale Ships => Topic started by: RipSlider on November 29, 2007, 06:56:41 am

Title: How do full sized boats stuff tubes work?
Post by: RipSlider on November 29, 2007, 06:56:41 am

Hello.

Just a quick question.

With model boats, we usually use a fairly steel shaft inside a stuff tube with not very good quality bushes at each end to connect the motive power to the prop.

I'm wondering now they work in full sized ships? I'm guessing that there must be other mechanisms of water proofing apart from packing a tube with grease, and I'm also guessing that there are bearings etc in the mix.

I'm thinking more along the lines of big ships, such as tankers, cruise ships, warships etc.

Many thanks

Steve

Title: Re: How do full sized boats stuff tubes work?
Post by: John W E on November 29, 2007, 08:35:40 am

Hi all

Just a couple of 'scans' from a book to give you a basic idea of how a stern tube works on a full sized ship.   Not all ships have this set up though of a Lignum Vitae bush, and, also you will notice that there are oil packed stern tubes and these tend to be in Naval vessels.

Hope this is of some help.

aye
john e
bluebird

Title: Re: How do full sized boats stuff tubes work?
Post by: Bunkerbarge on November 29, 2007, 09:43:59 am

The Lignum Vitae stern tube arrangement has now long since been taken over by an oil filled stern tube. 

The arrangement is a stern seal, which usually consists of a pack of lip seals of two facing inwards to keep the oil in and three facing outwards to keep the water out on the outside of the vessel and then another lip seal on the inboard side of a couple more seals.  Between this is an oil filled stern tube with a lubrication system and a white metal bearing arrangement.  If there is an external "P" bracket or a "A" bracket this could well have it's own pair of seals. oils system and more white metal bearings.

Alternative seal arrangements could be a face type seal and even some of the more sophisticated ones have an inflatable tyre incorporated into them to enable components of the seal to be changed with the vessel in service.

As you can imagine this is prone to leakages and many a ship has had fitted lower oil system header tanks in an attempt to slow down the loss of oil from a leaking seal.  There is even a special oil produced by Vickers called Hydrox which is designed to allow water to leak in and emulsify the oil which with maintain it's lubricating properties up to about an 80% water content.

The traditional oil filled stern tube is still very common however, as with so many things the circle is coming round again and we are returning to a "stave" type of bearing.  The differrence is nowadays though that the staves are made of a plastic material and incorporated into a backing so they can be slid in and out as two pieces.  The significant differrence though is the fact that these are water cooled and lubricated so sea water is simply pumped through the bearing from dedicated pumps in board and there is therefore only a requirement for a single seal arrangement on the  inboard side.  "A" bracket and "P" Brackets are fitted with similar bearings and also water supplied down through channels inside the castings which do not require a seal and simply allow the sea water to escape to the environment.

This type of stern tube arrangement is becoming very popular as the environmental concerns are removed completely.  Also the plastic materials are proving to be very resilient and lasting extreemly well with very little wear down over many years.

This is a popular manufacturer with cruise ships and a typical arrangement:

http://www.thordonbearings.com/markets/marine/merchant/compac.html

Title: Re: How do full sized boats stuff tubes work?
Post by: mackem1946 on June 20, 2009, 02:24:07 pm

I spent part of my appreticeship boring out lignum vitae lined phosphor bronze, stern tubes. Oh happy days in North East England. When we used to build REAL ships.
 wages 2 pounds 3 shillings a week.

Title: Re: How do full sized boats stuff tubes work?
Post by: tobyker on June 22, 2009, 12:37:02 am

Thank you, BBs both. I can see how the water is kept out of the hull and the bearings the shaft revolves in, but where exactly is the thrust from the props taken in to the hull?

Title: Re: How do full sized boats stuff tubes work?
Post by: DARLEK1 on June 22, 2009, 01:03:59 am

Hi there, the thrust is taken up by something called a thrust block, basically a bearing system that moves internally sort of ish. It doesn't matter so much on a model though. :-)
 Paul...

Title: Re: How do full sized boats stuff tubes work?
Post by: gondolier88 on June 23, 2009, 05:35:28 pm

Hi,

Quote from: DARLEK1 on June 22, 2009, 01:03:59 am

Hi there, the thrust is taken up by something called a thrust block, basically a bearing system that moves internally sort of ish. It doesn't matter so much on a model though. :-)
 Paul...

This is true with electric RC models as most motors include an integral thrust bearing- however some IC engined models and also large steam powered models need thrust bearings.

These bearings are not to take the thrust from the motor/ engine etc - they take the thrust up from the propellor and stop linear forces on the propshaft unduly wearing, or mis-aligning any finely toleranced articles in the power plant.

A typical thrust bearing is arranged thus;

PROP - Shaft through stuffing gland - Shaft in ship - Thrust block- Small length of shaft- Powerplant Coupling - Powerplant

Also it can be arranged thus;

PROP - Shaft through stuffing gland - Shaft in ship - Powerplant Coupling - Powerplant - Forward mounted thrust block (ie. takes thrust through engine- however this arrangement is rare)

Hope this has answered any questions. :-)

Greg

Title: Re: How do full sized boats stuff tubes work?
Post by: catengineman on June 23, 2009, 05:41:27 pm

Also on the real ships they some times incorporate dampers between gear box and power unit or between gearbox and thrust block or both (rare)

If you use the rubber tube type couplings then that will act as a damper.

R,