Propeller shaft sealing

[Terence WestonParticipant @terenceweston89666]

After crashing several model aircraft, I’m switching to model boats. My first model will be Billings’ Smit Nederland 528, which I’ll be starting in a couple of weeks. It looks to me that there’s no great effort by the designers to prevent water ingress along the prop tube. It looks fairly simple to do, and I’m proposing using V seals, (face seals)which are cheap and easy to fit. Does anyone have experience of these?

Ihave a well-equiped workshop, so if anyone has a design, I’d be grateful if you could share it with me.

Also, can anyone advise the maximum propeller speed for this model; Billings are pretty useless at answering even simple technical questions … I was thinking of using brushless outrunners or inrunners, partly because they develop high torque at low revs, and their overall efficiency is high. Does anyone have experience of these? Again, I’ve no idea of the torque/speed requirements, and Billings were unable to help.

Thanks

Terry

[Terence WestonParticipant @terenceweston89666]

[Bob Abell 2Participant @bobabell2]

Hello Terry

I`ve often thought about sealing the prop tube………Other than the usual grease and close fitting thrust washer

What about a Stuffing gland?………As used on steam engine conn rods?

I`ve got a sketch in one of my galleries somewhere, I`ll try and locate it

Bob

[Bob Abell 2Participant @bobabell2]

Found it………..Any interest?

[Terence WestonParticipant @terenceweston89666]

Hello Bob

I did know about stuffing boxes, but I figured that the V seals are relatively low friction, and seal better when there’s pressure across them (i.e., water pressure presses the seal lip against a face). They don’t use a garter spring, and provided that the sealing face is smooth, they’re very cost-effective. I’ve used these seals in industrial applications at 3000rpm in aggressive liquids (the rubber was Viton), so thought that they might be a better/more modern alternative to a stuffing box. Equally, there are quite a number of rotary shaft seals in different designs, and I’m a bit surprised that the model boat fraternity don’t appear to use them. I’m very new to this, so I might find out that in the real world the tried and tested might be best …

Thanks for the drawing – it looks like you’ve doen your time in a drawing office!

Regards

Terry

[Peter FitnessParticipant @peterfitness34857]

Terry, it may well be that the reason the average boat modeller uses a fairly basic form of prop tube sealing such as Bob mentions is because, in most cases, a more sophisticated approach is not necessary. I have no experience with high speed racing type craft, but I have found filling the prop tube with water proof grease, and using O-rings and thrust washers at each end, has been very effective in all my models.

Peter.

Edited By Peter Fitness on 19/01/2011 21:11:24

[Terence WestonParticipant @terenceweston89666]

Hello Peter

Thanks for the info. on shaft sealing. It does seem that the method you advocate is likely to be the best tried and tested, so I will re-design the drive on my Billings kit accordingly. You mentioned thrust washers: do you mean bellevue type? Or do you put a compression spring bearing against a plain washer? Or do you just rely on the natural resilience of the O-ring? There are loads of ways to use O-rings for shaft sealing, but if you’ve sorted it, it would save me building in unnecessary complication. Re. stuffing boxes: when I was an apprentice in the food industry, I had to maintain quite a number of brine and syrup gear pumps, all of which had tallow impregnated rope in the stuffing box. They needed frequent tightening to reduce leaks, and wore out the shafts pretty quickly. The shafts were made from a hardenable stainless steel too, so I’m been a bit wary of using this method. Obviously it works for many people, but

I like to sort out how best to do things before I charge into building a model, since it’s much easier that way rather than butchering a half-built model. And since this is my first boat, I’m grateful for all the advice I can get from seasoned builders.

Kind regards

Terry

[ashley needhamParticipant @ashleyneedham69188]

No need to fiddle with the prop shaft, various threads on this topic, water ingress..no consensus of opinion I am afraid.

A bit of grease (not too much) and O rings or possibly nylon washers is all it needs. the thrust washers Peter uses are, I imagine simply ordinary washers, Brass could be good. I only use the as supplied sort. Adjust the nuts up as tight as free running will allow and thats that.

My vote is for nylon bearings and stainless shafts. Not a universally held opinion (see above)

Bound to be a tiny bit of water coming in. Shouldnt be a problem if the boat is waterproofed inside.

Dont forget any fancy seals are only going to create drag.

Ashley

[Terence WestonParticipant @terenceweston89666]

Ashley

Thanks for info. I agree about the stainless steel shaft- Billings supply a brass shaft in their kit, which I was going to replace with SS anyhow. Might try nylon washers- its good for bearings. I seem to remember that a clearance of 0.1%xdiameter is sufficient running clearance to prevent water leaks. (This is the clearance used on old-fashioned hypodermic syringes where a glass plunger operated in a glass cylinder. A pressure of 3 bars could be generated easily, with no leaks.) However, on a 4mm shaft, the clearance would be only 4 microns! So a non starter. I’ll stick to O-rings.

Thanks again

Regards

Terry Weston

[Peter FitnessParticipant @peterfitness34857]

Terry, I simply rely on the natural resilience of the O-ring and, as the grease I use is outboard motor type, there is no tendency for it to emulsify should there be any slight water ingress.

Some would argue that grease creates drag, which it probably does, but as my boats are not high performance craft, any such drag is not an issue.

Below is a photo of one of my home made prop tube assemblies, which include a slide on cap which I fill with grease.

Peter.

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