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Author Topic: What grease?  (Read 9036 times)

Colin Bishop

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Re: What grease?
« Reply #25 on: May 06, 2010, 08:12:32 pm »

Maybe we ought to get back to basics a bit here.

There are essentially two reasons for oiling/greasing the prop tube.

The first is to reduce friction between the bushes at the ends of the tube and the revolving shaft. In most models this really only means the inboard end as the water itself will act as a pretty effective lubricant with most bushes at the outboard end.

The second is to help prevent water finding its way up the tube and entering the boat. Now, with most of the sorts of boats we run it isn't the grease or oil which stops the water although it does help. On the end of your shaft you will have a propeller(!). Inboard of that you should have a lock nut to stop it unscrewing and falling off. Inboard of that you should have a washer of at least the external diameter of the prop tube. This could be brass or nylon etc. When the boat is going ahead the propeller will push the washer up against the prop tube and and effectively seal it which will very effectively stop water getting in. Unless you have some sophisticated engineering setup with internal thrust blocks etc. it is the pressure of the propeller on the washer and thence to the prop tube which actually takes the load and drives the model forward. As it is submerged the water acts as a lubricant.

But when you go astern (which will normally only be a small proportion of the time spent going ahead) the propeller is pulling on the shaft so the load is taken on the nut and washer that you should have installed at the inboard end. if you haven't then the load will be taken on the motor bearings which is maybe not a good idea except at very low power. Now, if there is a lot of 'play' on the shaft; i.e. you can move it up and down appreciably, then going astern will move the outboard washer on the shaft away from the outboard end of the prop tube which may allow water to enter which is when your grease/oil comes in handy. So the best practice is to tighten up both ends of the shaft until it is locked in place and then ease off just sufficiently to allow the shaft to move freely. This will minimise the likelihood of water entering when going astern.

Some water will get in, it is inevitable, even on full size craft so the oil/grease in the tube will help prevent corrosion while the model is laid up at home.

Colin
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hazmat

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Re: What grease?
« Reply #26 on: May 06, 2010, 09:50:26 pm »

Lovely! All the answers in all of the colors and technical  descriptions to boot.
All the comments greatly appreciated. (even the 'witty' ones).

Thanks for all your help

Paul

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Peter Fitness

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Re: What grease?
« Reply #27 on: May 06, 2010, 10:46:13 pm »

Just to be different  %) I use outboard motor grease. It's waterproof and quite cheap, as a half kilo tub will last for years because, after all, not much is used in a prop tube.

Peter.
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jabba

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Re: What grease?
« Reply #28 on: May 07, 2010, 01:27:32 am »

i use castrol grease from car spares shop.
 no water gets up my shaft.
 jabba.
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tt1

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Re: What grease?
« Reply #29 on: May 07, 2010, 01:58:27 am »

Excellent info Colin! am on my first model and now at the stage where, as a newby, I've found your thread really helpful - so thanks for that :-))

        Anyone ever tried silicone grease as used by plumbers for lubricating 'o' rings and pipe seals?  cheap, tremendous temperature range, waterproof and safe even for potable water.

                               Regards, Tony.
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andyn

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Re: What grease?
« Reply #30 on: May 07, 2010, 01:52:06 pm »

I've got a tube of Dow Corning 7 Silicon grease and it works blilliantly. Silicon grease tends to be a bit more expensive than lithmoly grease though.
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sheerline

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Re: What grease?
« Reply #31 on: May 07, 2010, 01:55:37 pm »

Like nd3500, I tend to go for the cv joint grease (Lithium) and I use it on the subs. It is low on viscosity and  despite being totally submerged in unsealed shaft assemblies it remains as a  coating on the rotating and bearing surfaces. It is not water soluble either and as with all propshafts, it is not a good idea to fill em up to the brim with any grease as it will create drag and sap amps, especially on high speed direct drive units. Thats whyI use it on the subs, as Amps count for everything.
As my old Pappy used to say.. "enough is as good as a feast"
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dodgy geezer

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Re: What grease?
« Reply #32 on: May 07, 2010, 02:34:50 pm »

Traditional lubricants advice is to use a Lithium-based grease in the presence of water. Thus grease for bike-chains etc is lithium based. Such a grease is often called 'water-pump grease'.

However, these greases are usually high-viscosity, which is a bit two-edged if you're looking to pack a prop-tube with it. If I have to put a prop-tube below the waterline, I usually use one of those submarine seals....
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sheerline

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Re: What grease?
« Reply #33 on: May 07, 2010, 03:37:38 pm »

Hi Dodgy,the only product I have laballed 'water pump grease' is a Shell product and is a thick yellow waxy type grease which we used in the 'Lady May', our old 18ft motor launch. This was used on the full size propshafts and of course, in the water pump. The lithium grease I use on the subs is is a much lower viscosity and even a little of it leaves a fine film on the bearing surfaces which doesn't go away even though continued submergence and constant running. It also gets used in the dive unit packing glands and does not affect the rubber seals either so all in all it seems a very suitable product. I might add, it does not leach out into the water as the sub shafts are delibeartely designed as unsealed sealed units as they are not designed to keep water out.
There are so many lubricants to choose from, it's a bit of a minefield but I guess if you find one which works in your application you tend to stick with it... (no pun intended here).
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BJ

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Re: What grease?
« Reply #34 on: May 07, 2010, 04:07:54 pm »

product I have laballed 'water pump grease' is a Shell product  This was used on the full size propshafts and of course, in the water pump.

Search Shell Marine Grease for stern tubes and you get Shell Strombus MP
Uses : Speciality marine product.
Product Code : 901LU760.
It is a 17 page Adobe.pdf file for "light" reading
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dodgy geezer

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Re: What grease?
« Reply #35 on: May 07, 2010, 08:00:23 pm »

Hi Dodgy,the only product I have laballed 'water pump grease' is a Shell product and is a thick yellow waxy type grease which we used in the 'Lady May', our old 18ft motor launch. This was used on the full size propshafts and of course, in the water pump...

I remember it well...My father was a lubricants troubleshooter for Shell, so one of the perks of my childhood was access to all kinds of slippery stuff...that, and trips to foreign climes...
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sheerline

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Re: What grease?
« Reply #36 on: May 07, 2010, 08:39:40 pm »

Just read through my last post to you blokes, can't believe the spelling mistakes and typo's I found. :police:
Yoo muss awl fink Iym a nidi yot! :embarrassed:
I'd like to blame the computer as it's very old now but you know what they say "rubbish in... rubbish out"!

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poddy

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Re: What grease?
« Reply #37 on: May 07, 2010, 09:05:00 pm »

My turn.

I'm playing the 3-in-one card.

http://www.wd40.co.uk/index.cfm?articleid=1933

White lithium grease sounds pretty exotic.


Dunno if it's any good though.
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dougal99

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Re: What grease?
« Reply #38 on: May 08, 2010, 12:29:16 pm »

My turn.

I'm playing the 3-in-one card.

http://www.wd40.co.uk/index.cfm?articleid=1933

White lithium grease sounds pretty exotic.


Dunno if it's any good though.


Used it successfully for years. Preferred the old tube though. aerosol a bit violent  <:(


Doug
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triumphjon

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Re: What grease?
« Reply #39 on: May 08, 2010, 06:39:59 pm »

ive always used oil , normally 3in 1 type although im now useing semi synthetic motor oil ! the problem with grease is when cold its very thick which in turn makes anelectric motor labour ( meaning it will draw more amps )  !  greaseis ok when you have an ic motor .
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