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[Seaspray]

Hi all   

Not up to speed yet on the forum, whether it been asked or not so I'll just go ahead and do this thread.

I was looking at a model boats magazine and saw a picture of a shaft with 3 propellers on it.  That is in series ....one after the other.

Surely that won't work ?  I think  the second prop will cause a disturbance of water flowing from  the first prop and the third prop  disturbing the flow from both of them.

Unless the second prop is slightly bigger in diameter and the third one bigger than the second one. 

I think a better idea would be a propeller thats shaped like an auger with a reasonable length to it.   

Any commends on this theory ?

[Proteus]

turbinia had three and worked well

Proteus

[catengineman]

I have seen real "pods" with two propellors so I would say that the 'prop slip/ disturbance would have been looked at by the brains

But what would I know.

R,                http://www.nauticexpo.com/prod/schottel/double-propeller-thruster-22142-78601.html

[Colin Bishop]

A lot of the early attempts with multiple propellers on a shaft were not very efficient as they found when one fell off etc. Read up on early destroyer designs of the 1890s for the full story. There are some modern arrangements which work but I think these use contra rotating props. There is also a device called a "Grim Wheel" which was fitted to the QE2 when she was re engined with the idea of improving the efficiency of the main propeller. These fell off too! This is a very complex hydrodynamic subject and what works full size may not work on a model. Unless you are going for scale fidelity or simply want to experiment then stick to a single screw.

Colin

[John W E]

Hi there

It has been a long time since I visited the Discovery Museum, where the Turbinia is, and, I can remember reading an article regarding the Turbinia's setup of 3 props on one shaft.   It has something to do with cavitation - trying to eliminate it, due to the fact of the high revolutions the steam turbine was developing.   Alongside the article there were several wooden model hulls; for testing purposes - each model has a prop shaft with various props on of various shapes and sizes & a dirty great elastic band to drive the prop shaft which was wound up with a big ‘starting handle’ affair. 

I am not sure if these models are still on display; but, if they are – they are well worth a look and the articles are well worth reading.

Aye
John e
Bluebird

[Seaspray]

I see the Turbinia has 3 propellers on her shafts.

I thought this type of arrangement would suit a shaft(s) running parallel with the surface of the water.

Martin

[tobyker]

The auger type was tried in the early days (by Ericsson?) but fortunately it fell apart leaving only "blades" behind whereupon the vessel picked up speed.

[Martin (Admin)]

The Volvo Penta drives have 2 propellers, contra rotating on their stern drives.

[Seaspray]

That looks a nice setup and the propellers  look really meaty.



Martin

[tonyH]

Happened to be in Newcastle Discovery Museum on Saturday and the Turbinia, along with the early models, is still there.
3 props on each shaft and they are spaced at about 4-5ft apart along the shaft. All the props appear to be the same size and have very wide blades, the width being almost the same from root to tip. In fact they are almost square, just with radiused corners.
One thing that was very obvious was the fact that the hull drops down towards the stern, with a large fixed 'anti-cavitation' tongue sticking out acros the full width.

There's also a beautiful builders model of a TBD with 2 props on each of the 4 shafts.

Tony