Prop nuts

[Graham_Wright]

The end of my prop shaft is tapered to 25mm diameter and is keyed.

It seems I have a choice of threads for the nut.

M24 X 3 (a standard)
M24 X 1.5 or
M25 X 1.5.

A 3 mm pitch sounds coarse.

Does anybody know what standard propshaft nuts are available as, even if I make one, it would be nice to know that the swindleries would have a replacement if needed?

Further;- what material? A stainless nut on a stainless shaft offers (in my experience) a risk of cold welding. But a phosphor bronze one would possibly be an invitation for corrosion. On the other hand, the prop is phosphor bronze so it seems as long as it is short from the point of view of corrosion.

Help!

 

[LORDNELSON]

Over the past 15 years on two yachts I have used bronze castellated nuts on stainless steell shafts with bronze propellors with standard M. G. Duff anode nearbye and have had no problem with corrosion. Probably an experienced mechanic could tell you by looking at the shaft what nut you need. If not borrow a thread key or measure as well as you can and buy a nut or two until you find which one fits.

 

[brianrunyard]

I assume this is a new propshaft, hence the choice of what thread to go for.
I aggee that 3mm pitch sounds a bit course.
As the shaft tapers to 25mm, I would go for M24 x 1.5.

 

[2Tizwoz]

My Volvo saildrive has a stainless nut retaining the bronze propeller . I assume this is the standard arrangement and there is also as standard an anode on the saildrive leg.

The stainless nut has thus far presented no problems. The saildrive is now 25 years old.

The only corrosion obvious requiring periodic attention (just replaced one after three years) is the replacement of the sacrificial anode. Molybdenum disulphide can be beneficial if galling is a problem.

 

[charles_reed]

Usually 316 stainless and BSP thread.

 

[Graham_Wright]

It just goes to show how carefuly you pen a post, confusion reigns!

There is no thread at the moment having just turned the shaft myself.

As an [ QUOTE ]
experienced mechanic

[/ QUOTE ] myself, I can therefore detect no thread!

Now I am no wiser.

BSP would have to be 5/8" (major diameter 0.902") which is an intermediate size and probably not available in the nationalistic reaches of the Mediterranean.

Where can I buy castrated,- sorry castellated nuts from (save me making one which is messy)?

Thanks for the tip on molyslip;- does this prevent cold welding?

 

[MedMan]

I have had a stainless castellated nut on my shaft for 15 years. I removed the prop a few weeks back to replace the cutlass bearing and there was not a hint of corrosion.

When I fitted a new prop 15 years ago I couldn't find any source of large stainless nuts so I had it made by an 'experienced mechanic'!

 

[supermalc]

I too had this problem. First of all, the original bronze nut on my bronze propshaft, which was worn and bent stripped it's threads. I was told it would be Whitworth or BSF so sourced a nut. Living in Lincoln, an engineering town I assumed this would be simple.....not so. I eventually got one from Boston Engineering, only to find the thread different. Eventually it turned out to be metric.....is 5/8inch 16mm.

When it came to making my new shaft last Autumn, I had difficulty getting a stainless nut, but found some Whitworth nuts in Lincoln, so turned the thread to suit. They are steel nuts, but I've put two on the shaft, drilling the outer one for a split pin.

I could only get 302 stainless (316 was much too expensive) and it has been in 'fresh' water for 8 months or so. I pulled some weed off the prop yesterday, and only a slight film of rust was visible on the nuts in clear water.

At this rate, I will be either 108, or dead long before they require changing ..... I've a couple of spare ones, so even if they do it's not the end of the world. I changed the nut before in the water. The prop can easily be reached if a low platform close to the water is available.

 

[Graham_Wright]

Yes! It should be simple shouldn't it?

I can go for M24 X 1.5 but, as it is non-standard, I don't have a die or a tap. The combined cost will be around £60 and they'll likely never be used again!

Maybe I can borrow them.

 

[2Tizwoz]

You may well have a nut and bolt merchant closer but Thomas Potter in Newcastle upon Tyne will no doubt supply a couple of 316 nuts at a reasonable price and post to you. Standard 'preferred size' metric nut will be cheapest.

Presumably with access to a lathe you can cut a thread with a single point tool ?

Thread galling / welding can be prevented using molybdenum disulphide.

 

[oldharry]

Talk to Lancing Marine, who made up a new shaft for me for this season. They can advise on correct combination of materials, and supply a suitable nut, at a very competitive price

 

[Strathglass]

When I had my propshaft made.

I supplied my folding prop to the manufacturer.

They tapered the shaft to suit the prop and supplied the matching nut.

This nut is in 316 the same as the prop shaft.

I don't know what thread they used but a 1" socket exactly fits the nut. This is on a 1 1/8" shaft.

Iain

 

[charles_reed]

Heavy weather

of a fairly simple conundrum.

Decide your thread, the most common fine one.

Take the shaft to an engineering shop, have them cut the thread.

Buy the nut from one of a number of specialist fixing stockists.

If having decided on the thread you'll use, you care to contact me with a PM, I'll be only too glad to obtain the nut for you from some of my contacts - it'll be 316 stainless and castellated and will cost >£2. I can even give you a number of contacts in the Midlands who would find cutting the thread utterly usual.

Meanwhile try Lake Engineering @ Poole, haven't got the laptop with the full telno and contact details with me here in Prague.
they usually only deal with trade and will give you far more authoritative advice than you can expect on this well-meaning but amateur board.

PS I've had shafts machined in a number of places and have not yet found a need for this level of soul-searching.

 

[TheoSr]

Considering that metric thread is self locking and that all propellor nuts should have a locking plate, I see no use for a fine thread. After all the prop is on a taper and if it fits, it does not need a lot of force to sit tight.
Never used castellated nuts with cotter pins, I like the end of the propshaft streamlined.
When I made a new propshaft for my Crabber, I used M16 on a 25.4 mm shaft. Thread cutting nut was not expensive.
For the streamlined nut, I bought a standard M16 brass nut, turned it till it fitted in a self made brass dome and brazed things together. Filing two flats to tighten it was easy.
For a 24 mm size I think I would screw cut it myself. If you make the complete nut out of two pieces, you don't have to cut in a blind hole.
Used "copper-ease" to lubricate the threads and never had problems.
Keep smiling
Theo.

 

[Graham_Wright]

Re: Heavy weather

So what came before "of a fairly simple conundrum"?

It really was a simple question!

My company is an engineering company, we can machine thread (anything up to 2 1/2" NPT (tapered) on machining centres) and anything you like on turning centres.

However, if, in the future the nut needs replacing, it would be nice to choose one which could be obtained easily. I could cut left hand M24.5 which would really fox the future but be a bit silly.

But after all the "soul searching" nobody has come up with a definitive answer "what is the standard?"

But thanks for all the feedback. It just shows what a bunch of individuals we are.