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Determining prop nut size


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Need some help finding the right size nut for my new prop. Found a table which says the thread diameter for a standard 1" shaft should be 18mm, but I'm having trouble working out how that translates to how the sizes are specified in the ones I can see online. Can anyone point me in the right direction?

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An 18mm thread should have a nut with an across flats dimension of 27 mm.  The thread pitch should be 2.5 mm. You usually need a stainless steel nut and A4 is the best nut grade, OK for salty watter.

So you could go into a fastener stockist and ask for an A4 stainless nut M18 by 2.5mm pitch and you will be talking sense to them.  M18 is not a preferred size though so a stainless nut may not be an off the shelf item.

It would be wise to get someone with a caliper and a thread pitch gauge to check the actual size of the thread on your shaft.  The nearest preferred sizes are 16 or 20 mm.

What happened to the old nut?

N

PS. If it is a 1inch shaft I would expect an imperial thread.

C

Edited by BEngo
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I thought there were (at least) 2 variations of common prop shaft size out there, from different manufacturers, and the angle of the taper is ever so slightly different. Fit the wrong prop to the wrong shaft (okay...technically right prop to wrong shaft, you know what I mean...) and they might appear to fit but are destined to work loose over time. Hence the surprise when you lose your nut, engage reverse and whizz!!!!!!!! The prop flings about 20 feet backwards out the boat at speed (okay, it won't do that, it will hit the rudder).

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Yes.  Not much to do with manufacurers, more about measurement Standards.  There is a metric taper of 1 in 10 on diameter and an imperial taper of 1 in 12 on diameter. The nut size wii depend more on the length of the prop boss than anything else.

N

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A really useful purchase is a digital vernier* caliper. Like this. Lots of uses if you are doing your own boat maintenance. For the amount of use it gets a cheapo one will be fine. You get them in places like Aldi every once in a while.

You can use it to measure the outer diameter of the thread and the pitch between threads. Measure the distance across a number of thread peaks, then divide by the number of threads. From this you can work out the nut required. The intertubes have tables of all the imperial and metric thread standards, so you should be able to find it. The caliper can also be used to determine the shaft taper. 1:10, or 1:12 so you get the correct prop.

Jen

* For the pedants. Yes I know that the digital ones don't have a vernier scale any more.

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5 minutes ago, Jen-in-Wellies said:

A really useful purchase is a digital vernier* caliper. Like this. Lots of uses if you are doing your own boat maintenance. For the amount of use it gets a cheapo one will be fine. You get them in places like Aldi every once in a while.

You can use it to measure the outer diameter of the thread and the pitch between threads. Measure the distance across a number of thread peaks, then divide by the number of threads. From this you can work out the nut required. The intertubes have tables of all the imperial and metric thread standards, so you should be able to find it. The caliper can also be used to determine the shaft taper. 1:10, or 1:12 so you get the correct prop.

Jen

* For the pedants. Yes I know that the digital ones don't have a vernier scale any more.

Not easy to do all that with the boat (prop-shaft) still in the water.

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2 minutes ago, Alan de Enfield said:

Not easy to do all that with the boat (prop-shaft) still in the water.

To be sure of getting the right part it really wants to be out the water and measured properly. If a prop has been lost once already especially.

Jen

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Beware of cheapo digital calipers.They measure OK but they have an unholy appetite for batteries.  The OFF button only shuts down the display so you need to remove the battery when it is not being used.

N

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On 31/07/2017 at 08:58, BEngo said:

Beware of cheapo digital calipers.They measure OK but they have an unholy appetite for batteries.  The OFF button only shuts down the display so you need to remove the battery when it is not being used.

N

 

I'll second this. I buy the button batteries in packs of ten now for all mine!

And as already hinted at, they won't like being used underwater. Not one bit! Get a proper vernier for measuring this thread with the boat in the water. Or a better idea which should work ok is get a lump of plasticine or something similarly pudgy, and press it into the thread underwater to take an impression, then measure the pitch on the plasticine impression on the bench with the digital vernier. Much easier!

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9 minutes ago, Mike the Boilerman said:

 

I'll second this. I buy the button batteries in packs of ten now for all mine!

And as already hinted at, they won't like being used underwater. Not one bit! Get a proper vernier for measuring this thread with the boat in the water. Or a better idea which should work ok is get a lump of plasticine or something similarly pudgy, and press it into the thread underwater to take an impression, then measure the pitch on the plasticine impression on the bench with the digital vernier. Much easier!

Best way to avoid a hernia and-or hypathermia.

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14 minutes ago, Mike the Boilerman said:

 

I'll second this. I buy the button batteries in packs of ten now for all mine!

And as already hinted at, they won't like being used underwater. Not one bit! Get a proper vernier for measuring this thread with the boat in the water. Or a better idea which should work ok is get a lump of plasticine or something similarly pudgy, and press it into the thread underwater to take an impression, then measure the pitch on the plasticine impression on the bench with the digital vernier. Much easier!

The plasticine trick was what I was going to suggest, MtB beat me to it. 

On some boats you can remove the prop shaft back into the engine hole, having disconnected it from the coupling and gearbox. Just be ready with a bung of some sort to block up the ensuing flood!

6 minutes ago, bizzard said:

Best way to avoid a hernia and-or hypathermia.

And it is best to avoid a hernia, I am still recovering from a recent operation at the moment. NO heavy or even medium lifting.

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On 7/30/2017 at 15:49, BEngo said:

An 18mm thread should have a nut with an across flats dimension of 27 mm.  The thread pitch should be 2.5 mm. You usually need a stainless steel nut and A4 is the best nut grade, OK for salty watter.

So you could go into a fastener stockist and ask for an A4 stainless nut M18 by 2.5mm pitch and you will be talking sense to them.  M18 is not a preferred size though so a stainless nut may not be an off the shelf item.

It would be wise to get someone with a caliper and a thread pitch gauge to check the actual size of the thread on your shaft.  The nearest preferred sizes are 16 or 20 mm.

What happened to the old nut?

N

PS. If it is a 1inch shaft I would expect an imperial thread.

C

Thanks, that's helpful.

11 hours ago, Jen-in-Wellies said:

To be sure of getting the right part it really wants to be out the water and measured properly. If a prop has been lost once already especially.

Jen

Obviously that would be the ideal, but can't really do that right now for various reasons. Boat coming out in about 6 weeks anyway to be transported down south so I can probably get it checked at the same time but my priority right now is just to get it moving. The taper is standard for the shaft size, so the nut is very unlikely not to be.

The prop is fitted (it was challenging but I have quadruple checked and measured everything along the way so I'm 99% sure it's fitted right) so it's just the nut and split pin that need to go on.

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You say the taper is right but is that a 1in 10 right taper, which would point to a metric thread for the nut, or a 1 in 12 right taper, pointing to an imperial thread. 

18 mm is close to 3/4 in, where there are a selection of threads available and , possibly apart from BSF, the nuts will be readily available, unlike an M18 nut.

The plasticine or blutak trick will get you close enough.  Youmay need to buy a couple of different nuts to try but they are not expensive.

How are you going to secure the nut to avoid a repeat of the blades departing when the nut comes undone?  Normally there is a hole drilled through the nut and the shaft, which is then fitted with a split pin. It will be nigh-on impossible to drill this hole in the water.

N

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I have just purchased a brass nut from Scottish Propeller Service Ltd ( falpropellers.co.uk). They are very helpful and have stock or manufacture quickly. They also have castellated nuts

I would expect the thread to be BSW unless you have a fairly new shaft - possibly 3/4 or 7/8 inch.

If you are measuring a thread the diameter will be less than the actual thread size, for 3/4" expect something like 0.730 inch diameter.

You will probably need a good heavy washer machined to go between the Prop and the nut if the nut hexagon is small compared to the propeller - I can help you if you are stuck for this.

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If you are buying a forged mass produced nut beware that the threads are not necessarily true/square to the face. Not good if you want to avoid stressing/bending the thread.

I always buy a proper fully machined nut to ensure accuracy in this application, standard Metric are OK although BSW are hard to get and BSF or non standard Metric impossible.

 

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10 hours ago, BEngo said:

How are you going to secure the nut to avoid a repeat of the blades departing when the nut comes undone?  Normally there is a hole drilled through the nut and the shaft, which is then fitted with a split pin. It will be nigh-on impossible to drill this hole in the water.

The new nut castellations may line up with a hole already there, but also quite possibly not if the same nut isn't available or identifiable. In this case, I'd be looking for a quality Nyloc nut, at least til the boat comes out of the water. Some would throw their hands up at that suggestion, but nevertheless they are quite widely used in this application and it's the taper that does the work. 

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37 minutes ago, Sea Dog said:

The new nut castellations may line up with a hole already there, but also quite possibly not if the same nut isn't available or identifiable. In this case, I'd be looking for a quality Nyloc nut, at least til the boat comes out of the water. Some would throw their hands up at that suggestion, but nevertheless they are quite widely used in this application and it's the taper that does the work. 

Beat me to it - I was going to suggest the same.

In addition / alternative - if the threads are long enough, add a second 'lock nut'. It won't come off then.

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11 hours ago, bizzard said:

A touch can be rubbed off the washer or the back of the nut on a piece of emery on a flat surface to align the split pin.

Or get bigger Stilsons ;)

Or even an adjustable spanner  

 

Edited by WotEver
Added yootoob
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