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[tonyH]

Hello all!,

I'm after a couple of 45-50mm two blade coarse pitch (1.5/1.6) carbon props to modify for testing a home made jet drive but I've noticed that some of them are poorly centred and wobble on the hubs. Has anyone any experience of different makes and their quality (or lack of)?

Also, I'm trying to match a prop shaft which is M4 that comes with an 8mm tube and plain bearings. I'ts likely to be a commmon make but I've no idea which.

Any help would be much appreciated.

Tony

[Martin (Admin)]

Only company I know that make carbon props is Graupner.... Robbe might do some.

[John W E]

hi ya Tony

I have not encountered many problems with propellers having the threaded boss 'off-centre' but I have encountered plenty of problems with propeller shafts which have had the threads put on out of alignment.  What I have had to do many times is cut the 'bad piece of thread' off and then re-thread them.

this being on 5mm shafts and 4mm shafts.

aye
john e
bluebird

[tonyH]

Thanks Martin,

That's what I thought. They don't have to be carbon but they must be rigid and the revs would be too high for brass. Steel racing ones would be best but I need to sort out the best flow first before finalising the system.

Tony

[tonyH]

Hi John,

I started planning the tests using a few Graupner plastic ones and about 50% had the brass inserts either off centre or skewed. I'll go up a notch to get more rigidity, hence the carbon possibility.
As you know, the 'centrality' in jet drives has to be pretty tight, certainly <0.5mm

Tony

[John W E]

hi ya there TonyH

I am busy looking through an 'old' Ripmax Modeller's Handbook from the Dark Ages i.e. 2003 don't I keep some rubbish     and in the pages concerning Model Boat propellers there is a Company who are called Radioactive - there is nothing else but - they have  2-blade glass-filled nylon standard propellers; what the glass-filled means - I am unsure?   However, reading through the text it says 'the glass-filled nylon props have a much higher strength and rigidity than their nylon counterparts - making them ideal for the higher powered Marine Engines.'    The 2 bladers go up to 62mm.

So try and google 'Radioactive' company and you may get an alternative prop supply.    Worth a try.

Also, there is 'Kyosho' who also do props - they may be worth a 'google' but, I am unable to see any carbon fibres ones advertised.

Lastly,  there are 4-bladed carbon hydro props.   Now that goes underneath Graupner - not sure if Graupner manufacture them.

Let us know how you get on. 

aye
john

[warspite]

The use of glass is the same as in glass fibre, the mesh of fibres increase the strength of the substrate to handle greater forces, unless anyone a knows better reason 

[tonyH]

Hi John & Martin

I've done as suggested and Radio Active props are still on the Ripmax lists. Most are the standard red plastic but the glass filled ones tend to have either too narrow a blade or have too low a pitch.
I've ordered a couple to play with anyway and will also order some Graupner ones to test via Astec.

Any ideas re. the prop tube? Lord only knows where I bought it but the reasons for the 8mm type are that (a) it is inherently more rigid than the 6mm and (b) 'cos I've already fitted one to the MkI version.

Tony

[d-jnana]

Hi Tony
If its any help, I have found that the red props sold in my local model shop are plain moulded nylon and do tend to be poorly centered on thier hubs. On the other hand the black ones are glass "loaded" and seem to be of a better quality overall. They last longer, seem to flex less, and are better centered. The shafts I have bought locally however are awful. The tubes and bearings are OK, but the shafts treads eem to be cut by hand and visibly "wander."
If you have access to a lathe then I would suggest you have a go at making your own shafts. Its solved a lot of the problems I have encountered in the past with vibrations caused by out of true props. The other advantage is cost. I buy my materials from B&Q, but do select carefully, some customers are not that caring about how they handle the stock, bent stock makes for....well you know.
Good luck with it all.
GARY

[d-jnana]

Hi Tony,
Yes your right, from an engineering point of view, a tube of greater external diameter, with the same wall thickness will be more rigid than a thinner O.D.
GARY.

[tonyH]

Thanks Gary,

No lathe, so making best use of 'off the shelf' items, such as 40mm plumbing fittings etc. If the plan works, the materials cost per drive unit should be less than a tenner.

Tony

[bogstandard]

Tony,

If you can actually see the props Graupner use in their jet drives, they are actually machined down from larger glass filled racing props.

This does two things, It gets rid of any 'wobble' on the blade ends, and they can have very close fitting props to tube. Your best way might be to search out a friendly who has turning facilities. Or experiment with a clamped down electric drill and a file. But they must be turned on the shafts that are to be used with each prop.

If you are trying to make 'efficient' jet drives, you have to take into account that a converging venturi is required to speed up the flow and increase thrust, otherwise it is just a prop stuck in a tube, and you might start to suffer with ventilation/cavitation problems, as you can't get enough water to the prop for it to compress it using a constant diameter tube.

John

[tonyH]

Hi John,

The design is based on the Kamewa FF375 which seems to be the simpler version, having no 'bulge'.
The reduction at the nozzle end seems (assuming that the Rolls Royce sketch is fairly accurate) to be about 2:1 cross section area, so the venturi is approximately that. I'm electro forming the venturi and the steering nozzle in copper and the MkI reversing buckets are made up from sheet brass.

I've only seen pics of the Graupner rotors, but they seemed to be something like you describe and it is now a case of trial and error for the pitch/revs/etc. in a test-bed hull.
I've also got to sort out the internal vanes to clean up the flow but I think that this may be just another case of tin-bending.

One thing that has come to light is that a lot of the Kamewa drives have a downthrust angle of 3 degrees. I've assumed that this is because of the absence of trim-tabs.

It's good fun trying this out and hopefully the boat (the easier bit) will be worth it. (See attached)

PS Any ideas on the prop tube, or even who could supply just the bronze bushes?

[John W E]

TonyH - hi there

With regard to your propshafts, I have 2 propshafts here - one from RipMax with a 4mm shaft in - and one spurious may (which I just bought from the Hobby Shop).    The RipMax one, the outside prop tube diameter is larger than the Hobby Shop prop shaft -  :-\ - hum so it looks as though they are not all standard prop tube outside diameter size.

What I was going to suggest is; seeing if you could manage to get some brass tubing which would fit over the top of your prop tube with a snug fit - any - maybe securing either end of the tube to your original prop tube with maybe some soft solder.   This would reduce some flexing in the smaller prop tube - obviously the more tubes you could get over the top the better.

One thing I have also noticed is; on the cheaper prop tube, the phosphorous bronze bushes have just been forced into the ends, and, therefore slightly swelling the ends of the prop tubes.  So obviously, you need to reduce this back to the original diameter with the use of an electric drill, a file and some sandpaper, before you could fit the tube over the top.    That is one idea, I do hope it is of some help.

If it is an original Rolls Royce drawing or sketch, it will be pretty accurate, but, one possibility is that they may have missed out on some information. 

aye
john e
bluebird

[d-jnana]

Hi Tony,
just a thought, but, on the prop side,  have you tried Propshop? Could be pricey, but I believe they are very good.
GARY

[tonyH]

Hi John & Gary,

John, I may have to follow the 'sleeve' route. I've only used less than half of the original tube/shaft sofar with, of course, both the bushes. I've got a few 4mm shafts with the smaller diameter tubes so it may well be a case of further butchery.

As far as the drawing is concerned, it's the standard pdf brochure put out by Rolls.

Gary, I'd thought about brass and spoke with George (Sitek) who I've had more help from that Propshop in the past. He limits his to 5000 rpm as far as I remember and I may posibly exceed this. Also, in view of the relative costs, the carbon/glass varieties would be better at the experimental stage.

Thanks again

Tony