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[RAAArtyGunner]

Not being an Engineer or boat savvy, I am intrigued by exposed shafts.

Some boats have them some don't.

The prop tube is a means of sealing against flooding or water ingress where the shaft exits into the hull. I suppose it would also be possible to pump them, (the tubes), dry if necessary
.
 I see the advantage of expose shafts within the vessel where such protection is not required.

However, externally to the hull, a shaft in a tube would be better protected than an exposed shaft but an exposed shaft would also be cooled by the surrounding water.

So the question for the Engineering types is why do some vessels have them and others don't.

[NFMike]

I thought tubes were just a model thing. I have been known to be wrong though   

Stuffing a tube with grease to keep water out is a bit of a bodge - using grease to protect a bearing from water/corrosion is OK but for sealing the hull .. ??
Proper boats/ships have a sealing gland of some sort and I'd think putting it at the end of a tube just makes it really hard to maintain. But maybe modern practice and materials have changed the game.

Why would the shaft need cooling?

[tony52]

When an exposed shaft is used there is an 'A' (or other type of) bracket at the prop end of the shaft. Make sure the thrust washer and a collar are fitted at the top end of the shaft, where the shaft exits the hull and not against the A'' bracket.

[Perkasaman2]

Generally speaking, exposed shafts were standard on performance boats with inboard engines in order to reduce drag. The shaft support bearings were lubricated by the water. (Unfortunately ,I cannot remember the name of the bearing material used.) The shaft supports were also profiled or aerofoiled to minimise drag and improve efficiency. Boat modellers tend to use fully enclosed shafts and only sometimes exposed shafts, however, the supports don't usually have any bearing s other than some close fit brass sleeve which is also lubricated by the water.

[Colin Bishop]

Unfortunately ,I cannot remember the name of the bearing material used

Are you thinking of the wood Lignum Vitae?

Colin

[BarryM]

Am I correct in assuming that your question is about full-size practice? If so, the essentials are as follows.

Once vessels became propelled by paddles and then screw propellers which required hull penetration(s), there had to be a means of keeping the water in its proper place outside the hull and supporting the weight of the drive shaft(s) and screws/paddles.

In the case of paddles, this was accomplished by bearing ‘brasses’ either side of the paddles with a packed gland where the shafts penetrated the hull. Where screws are involved, the methods are more various.

Putting it in simple terms and without delving into the realms of shaft liners and rubber seals, single screw vessels use a sterntube through which the tailshaft passes. In earlier designs, the weight of the shaft and screw was taken by staves of lignum vitae (hardwood), dovetailed around the tube and forming a bearing. The for’d end of the sterntube was sealed by a packed gland.  The stern tube allowed water ingress at the aft end to lubricate and cool the bearing and usually a seawater feed was arranged to cool and provide lubrication of the gland.   A trickle of seawater from the gland into the bilge was the norm and, when this increased, it was a sign the lignum vitae was wearing.  (I recall one vessel where the weardown was so excessive that the lateral movement of the tailshaft loosened and then fractured the twelve 1.5” dia. holding-down bolts of the aftermost shaft bearing.)

Later developments involved white metal bearings and oil-filled tubes which obviously had to be sealed from leakage by outer and inner seals. A positive pressure within the tube was provided by a header tank. Still later a combination of old and new resulted in the introduction of synthetic bearing staves.

When multiple shafts are employed, then obviously the shafts are led to port and starboard of the hull to provide clearance for the screws to operate.  Up to a point, the support and bearing arrangements mirror those of single-screw vessels and the tailshafts/sterntubes are contained within fairings secured to spectacle frames. For reasons of hydrodynamics and weight, these are not continued all the way to the screws although the fairings usually provide less drag than the ‘P’ or ‘A’ brackets which are employed aft of the stern tube.  The shaft support brackets, whether ‘A’ or ‘P’, also locate the shaft, act as bearings and take the weight of the screw. The exposed prop shaft surface exposed to seawater may be protected against corrosion by a paint coating, other surface treatment, sleeves and/or cathodic protection.

Barry M

[Jerry C]

Perkassaman, Cutlass bearings. Fluted rubber or Neoprene inside brass tube. Water lubricated. ?
Jerry.

[Perkasaman2]

Hi Colin, BarryM and Jerry. Jerry's suggestion is, I think, correct. I do recall being surprised at the material in that it seemed at first  unlikely and unusual in this application. I'm sure that it was discussed in one of John -Bluebirds's excellent build threads some time ago but I cannot recall which one. I'll troll through them some time. Lignam vitae may have been used in some way on the shafts where they exit from the hull. A fluted cutlass bearing was used in the brackets lubricated by the ingress of water.

[Colin Bishop]

As stated above, on a model the crucial thing is to take the thrust where the shaft exits the hull and not on the 'A' frames close to the propeller as this only provide support for the shaft and does not absorb propulsion loads. This means that you need a 'collar' on the shaft against the tube where it exits the hull.

Colin

[NFMike]

As stated above, on a model the crucial thing is to take the thrust where the shaft exits the hull and not on the 'A' frames close to the propeller as this only provide support for the shaft and does not absorb propulsion loads. This means that you need a 'collar' on the shaft against the tube where it exits the hull.

Colin

You need to allow for reverse as well of course ...

[Colin Bishop]

You need to allow for reverse as well of course ...

But that is at the inboard end and no different whether there is an exposed shaft or not. In a model the thrust needs to be taken on each end of the prop tube which transmits it to the hull. But a lot of people rely on the motor bearing to take the astern loading!

Colin

[Martin (Admin)]

As stated above, on a model the crucial thing is to take the thrust where the shaft exits the hull and not on the 'A' frames close to the propeller as this only provide support for the shaft and does not absorb propulsion loads. This means that you need a 'collar' on the shaft against the tube where it exits the hull.

Colin

This is an interesting read:  http://www.nauticalweb.com/superyacht/513/tecnica/propeller.htm

[boatmadman]

Interesting discussion.


I make my own shafts, with an exposed tail and 'P' bracket. I make the bushes out of colphos, which is a high lead content bronze and compatible to water lubrication.


I butt the thrust washer against the 'P' bracket bush, with a reversing thrust collar on the inside against the end of the shaft. I have used this arrangement on a number of boats, fast launches and big tugs, with no issues at all.


I do, however, make sure I have a good penetration right through the keel with the 'P' bracket flange, which is secured with grp.


Ian

[RAAArtyGunner]

Am I correct in assuming that your question is about full-size practice? If so, the essentials are as follows.

Once vessels became propelled by paddles and then screw propellers which required hull penetration(s), there had to be a means of keeping the water in its proper place outside the hull and supporting the weight of the drive shaft(s) and screws/paddles.

In the case of paddles, this was accomplished by bearing ‘brasses’ either side of the paddles with a packed gland where the shafts penetrated the hull. Where screws are involved, the methods are more various.

Putting it in simple terms and without delving into the realms of shaft liners and rubber seals, single screw vessels use a sterntube through which the tailshaft passes. In earlier designs, the weight of the shaft and screw was taken by staves of lignum vitae (hardwood), dovetailed around the tube and forming a bearing. The for’d end of the sterntube was sealed by a packed gland.  The stern tube allowed water ingress at the aft end to lubricate and cool the bearing and usually a seawater feed was arranged to cool and provide lubrication of the gland.   A trickle of seawater from the gland into the bilge was the norm and, when this increased, it was a sign the lignum vitae was wearing.  (I recall one vessel where the weardown was so excessive that the lateral movement of the tailshaft loosened and then fractured the twelve 1.5” dia. holding-down bolts of the aftermost shaft bearing.)

Later developments involved white metal bearings and oil-filled tubes which obviously had to be sealed from leakage by outer and inner seals. A positive pressure within the tube was provided by a header tank. Still later a combination of old and new resulted in the introduction of synthetic bearing staves.

When multiple shafts are employed, then obviously the shafts are led to port and starboard of the hull to provide clearance for the screws to operate.  Up to a point, the support and bearing arrangements mirror those of single-screw vessels and the tailshafts/sterntubes are contained within fairings secured to spectacle frames. For reasons of hydrodynamics and weight, these are not continued all the way to the screws although the fairings usually provide less drag than the ‘P’ or ‘A’ brackets which are employed aft of the stern tube.  The shaft support brackets, whether ‘A’ or ‘P’, also locate the shaft, act as bearings and take the weight of the screw. The exposed prop shaft surface exposed to seawater may be protected against corrosion by a paint coating, other surface treatment, sleeves and/or cathodic protection.

Barry M

  Thank you.
Martins link is very helpful and appears to indicate, if I am understanding it correctly, that the hull shape determines if the vessel will have exposed shafts and how long the exposure is example Merchant vessels as opposed to fast craft.

[BarryM]

Multi-screw vessels will normally always have exposed shafts whatever the hull form. Only the length will vary according to the hull form.
Barry M

[Colin Bishop]

Multi-screw vessels will normally always have exposed shafts whatever the hull form. Only the length will vary according to the hull form.

I don't think that is correct, just look at the stern of the old Queen Mary for example. The four props were all in bossings and most twin screw merchant ships were too. It was warships that tended to have exposed shafts and 'A' frames.

Colin

[NFMike]

As stated above, on a model the crucial thing is to take the thrust where the shaft exits the hull and not on the 'A' frames close to the propeller as this only provide support for the shaft and does not absorb propulsion loads.

I make my own shafts, with an exposed tail and 'P' bracket.

I think on a scale model that, provided the P/A frame is well fixed, the thrust loads won't be a problem. These sort of things don't scale linearly. (For a very powerful 'fast' boat a more substantial thrust block might be advised of course.)

I don't think that is correct, just look at the stern of the old Queen Mary for example. The four props were all in bossings and most twin screw merchant ships were too. It was warships that tended to have exposed shafts and 'A' frames.

This is the point I was making initially in #1. Prototype vessels either have exposed shafts or the hull is shaped around the shaft until close to the prop. In the latter case on most vessels I think there will probably be at least crawl space to access the bearings and seals. I don't think you will find anything like the tubes we use in our models on full-size vessels.

[RAAArtyGunner]

I think on a scale model that, provided the P/A frame is well fixed, the thrust loads won't be a problem. These sort of things don't scale linearly. (For a very powerful 'fast' boat a more substantial thrust block might be advised of course.)
This is the point I was making initially in #1. Prototype vessels either have exposed shafts or the hull is shaped around the shaft until close to the prop. In the latter case on most vessels I think there will probably be at least crawl space to access the bearings and seals. I don't think you will find anything like the tubes we use in our models on full-size vessels.

Yes that example was mentioned in the informative link, see reply 11 the shaft/s are faired into the hull shape.

[ballastanksian]

So the conical 'housings' on my HMS Rupert are actually called 'Bossings'?