Propshaft

[Leslie CookParticipant @lesliecook71319]

can anyone out there tell me how to stop water comming in my boat through the prop shaft ive been told grease but how do you stop the grease pushing out of the motor end

[Leslie CookParticipant @lesliecook71319]

[David MeierParticipant @davidmeier28154]

Hi Leslie. You could try fitting an o-ring at the bottom of the shaft with a small brass washer next to the prop to take the thrust friction. I think that would provide less rotational resistance than a shaft going through a tube filled with grease.

David.

[ashley needhamParticipant @ashleyneedham69188]

Leslie. this is an old chestnut. Do not fill the shaft with grease, a light grease is all thyat should be required for lubrication.In theory a waterproof grease may help but i dont think i know anyone who uses it ! Almost the only thing you can do is to ensure you have the absolute minimum free play in the shaft end-end once you have nipped up the adjusting nuts.You could use nylon washers to get this down to virtually nothing, but these may need replacing now and then. if the inboard end of the shaft is below the waterline, then you will never stop a bit of water ingress, Best then to contain any seepage with dams or a bit of sponge. Ashley

[Paul TParticipant @pault84577]

Leslie

 

Is this a home made shaft?

Does the tube have capped and threaded ends?

 

If it is a home made shaft could you provide us with a basic drawing of its construction or failing that a set of close up photos so that we might provide you with an effective answer.

 

Ashley is right about using to much grease as this just slows the motor and both David and Ashley have hit the nail on the head with sealing washers / thrust washers.

 

Paul

[Peter FitnessParticipant @peterfitness34857]

Ashley, I’m curious as to why you are advising against filling a prop shaft tube with grease. I make my own shaft assemblies, and always install a capped grease tube in the main shaft tube, which is filled with waterproof outboard motor grease. Each end of the prop shaft is fitted with an o-ring, which helps prevent water intrusion, and keeps the bearings lubricated. I understand that there will be some power loss due to the slight friction caused by the grease, but unless you are running fast electrics, this power loss is almost negligible. The only disadvantage I have found to my method is that some grease can be extruded from the shaft, inside the hull, and centrifugal force can fling it around somewhat. To overcome this, I usually fit a small, curved, removable brass plate over the inside bearing.

 

I have used the above system in all my models, and have never had a water intrusion (via the prop shaft) problem. If I was at home, I would attach a photo illustrating my comments, but I am in Sydney at present, on our son’s PC, so I don’t have access to my photo library.

 

Peter.

[David MeierParticipant @davidmeier28154]

Hi Peter.

I agree with Ashley. If you are completely filling the tube with grease I think you would be surprised at how much power it is consuming. For the shaft to turn, the grease is in constant shear all along the shaft and this uses energy and is probably why the grease is being extruded out through the bearings. As a test you could try just greasing the bearings and run the motor on a lower voltage out of the water and observe the shaft speed. Then fill the tube with grease and repeat the test to see if it runs slower.

 

David.

 

[Peter FitnessParticipant @peterfitness34857]

 Hi David,

While I said some grease can be extruded in fact, very little is. I will do as you suggest, however, and test with and without grease – I may well be surprised.

 

Peter.

[Paul TParticipant @pault84577]

As Ashley has said this is a perennial problem that rears its head every so often and different modellers have many different solutions.

Just like Peter I build my own shafts, I use copper or brass tube with phos/bronze bearings at each end and threaded cap ends, for the shaft I use threaded rod with thrust and neoprene washers at each end.

I do use light lithium grease as the lubricant opting for 30% x volume fill of the completed shaft.

I have tested this setup for power loss by testing a free running motor against the same motor turning the shaft (both in and out of water) and found a maximum increase in power consumption of 7% and an equivalent loss in motor revolutions.

I appreciate that given the size of models I build a loss of 7% is negligible and that I have not tested the theory on any model of less than 4ft long but I see no reason why the principal should not apply.

Paul

[Bob Abell 2Participant @bobabell2]

I`m surprised that the commercially made prop shafts don`t come complete with STUFFING GLANDS and the FILLER STUB TUBES.

 

It would solve a lot of problems?

 

Just another brilliant idea from Bob!

[David MeierParticipant @davidmeier28154]

I agree Paul, with the sort of power that you have available the power loss due to grease is not an issue. I was thinking more of small power electric motors.

 David.

[David WooleyParticipant @davidwooley82563]

Hi leslie

One of the old tried and tested methods is to make use of an old discarded Humbrol tinlet. At the coupling end of the shaft a small hole is drilled out. A brass tube of appropriate diameter is soldered to the shaft.  This is then either inserted into the base of the tinlet and  soldered into place.  Or as in the photo a single reservoir  fitted with four lubrication points to {in this case} four shafts.   The reservoir can then be filled with light lubricating oil.

Dave Wooley

[Mike DavidsonParticipant @mikedavidson22772]

 

 

 I know that my last post was ambiguous, butfor my propshafts, I use MS4 grease which is a trade name for Midland Silicone I met this grease through work, and items of electronic equipment were proofed with this grease which proved to be highly effective, what I do is to take the prop shaft out of the tube and squirt a little grease into thre tube from the prop end. Then, I put my finger over the motor end of the tube and push the shaft back in from th buisness end You might find that you generate as force too strong to keep your finger in place, and the grease will escapebut silicone grease is so waterproof that what remains in the tube will be enough. I have made a greaser, which is a short piece of threaded tube with a bolt of the same thread, which can be used like a grease gun as the bolt is screwed in, it pushes the grease out with great force. This can be put in place before final assembly, just like the little oiling cups used on steam engines to deliver the lubricant just where it is needed…………………………Mike …..D

[ashley needhamParticipant @ashleyneedham69188]

I am not a fan of oil in the tube. If water can get in, the oil is bound to get out into the water, and to boot oil emulsifies with water, especially being stirred up by the shaft. I think there shouldnt be a problem with water ingress if your shaft is lightly greased at the bearings and is adjusted for minimum play. Judging by the number of posts on this subject I imagine the main problem is sloppy bearings, either from too large a tolerance at manufacture (these commercial shafts) or wear.Ashley

[David WooleyParticipant @davidwooley82563]

Hi Ashley Yes oil can and does escape  if there are ill fitting bearings and glands.  I have used both types of lubricant  and experienced no problems with grease and few problems with  oil. With grease it can and does compact  over a period of time and may have some effect on the motors efficiency . Cleaning the the prop tube from time to time and replacing the crease should farestall that problem . In the past I’ve also inserted PTFE bushes with in the tube this also helps.

Dave Wooley

[Bob Abell 2Participant @bobabell2]

I`m a bit surprised, nobody has commented on my Stuffing Gland idea…………….Guaranteed no leaks!

 

Bob

[Dr John BoothParticipant @drjohnbooth43899]

Hello Bob

 

Do a quick sketch of your idea with a few notes and I will run it through a design programme to see how much power, if any, is lost by the system.

 

John

[Bob Abell 2Participant @bobabell2]

Hello John

 

Are you admitting that you don`t know what a stuffing glands is?

 

It`s used on all steam engine valve and piston rod glands…………….So it`s not really my idea………………Might have been James Watt`s!

 

Bob the Confessor

[Dr John BoothParticipant @drjohnbooth43899]

Hello Bob

Believe it or not I have used stuffing box technology in my professional life, it was quite odd really bringing together science from the Victorian age together with science from the space age.

Do you think that IKB would have approved?

Looking forward to seeing your drawing.

John

[Bob Abell 2Participant @bobabell2]

Hello John

 

Here`s the sketch of the stuffing gland showing a screwed gland nut………………but for existing boats, the gland nut could be a slide fit over the prop tube, with external screws applying the axial thrust…ie….screw into the hull

What does the team think?………………..and don`t say….get st*****ed!

 

Bob the Engineer

[Colin BishopModerator @colinbishop34627]

I’ve seen that method used in practice in full size yachts. Usually you had to tighten it just a little bit every day or so to prevent/reduce water ingress but not too much otherwise it could bind on the shaft and reduce power.

 

How often would you need to adjust it at model size? I’d worry that it would be easy to tighten it just that bit too much.

 

Colin

[Paul TParticipant @pault84577]

This design works very well for me, the two neoprene washers keep the water out and have little effect on the rate of spin nor the power draw of the motor.

A minimal slug of grease keeps everything lubricated.

The trick with the washers is to have them slightly smaller than the dia of the shaft so you get a snug fit.

 

Paul

[ashley needhamParticipant @ashleyneedham69188]

Chaps, I find this very over the top..my small lcm3 has a cut down prop tube of 1 1/2  inches, and is entirely below the waterline, guess what, …it lets in water! but only a teaspoon or so in 20 mins sailing. The r/c gear and motor is stuffed into the rear end in an area perhaps 4″ by 5″, and to be honest,this much water ingress is just not a problem. After an hours sailing there might be a tablespoon of water, but the wood is varnished and the gear is off the bottom,no problem. The question is…how much water intake was the original poster suffering, or indeed, how much water do ANY of you ship, and why is it such a problem? all this stuffer box business and all !   Ashley 

[David MeierParticipant @davidmeier28154]

It is not so much over the top Ashley, more up the tube. You have a point though, I put a couple of tissues under the motor/ shaft couplings, to soak up and confine any water that comes in on my plastic fizz boat.

 

Paul. Is the thrust washer between the prop and the neoprene washer? In the sketch it looks the other way around. The sketch might have got corrupted in the posting.

 

I think this thread is a great response to Leslie’s original question.

 

David.

[Bob Abell 2Participant @bobabell2]

Paul…………………To really understand your creation, we need a decent cross section!

 

J`know…………….summut like mine!

 

Bob the Engineer  (ret)

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