Prop Shaft Thread

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themons

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Hello:

I am trying to determine the prop shaft thread on a 25 mm shaft from a 1987 Westerly Corsair. M20-2.5 and 3/4 x 10 do not seem to fit properly.

Is it possible that the thread is either 5/8 or 3/4 Whitworth?

The nut is 33 mm across the flats.

Thanks

C. Chung
CS98
The Mons
 
The answer could be "yes" but you need to get an engineer to look at it, or if you are thinking of changing the prop, let the prop maker/agent look at it.
I'm told my boat has a 25mm shaft, a 3/4" Whitworth nut, a 1 in 10 taper and a 1/4" key. Probably. I could wish that the maker would stick with one system of measurement.
 
I'm told my boat has a 25mm shaft, a 3/4" Whitworth nut, a 1 in 10 taper and a 1/4" key. Probably. I could wish that the maker would stick with one system of measurement.

***************************************************

Why does everyone blame the poor manufacturer....Blame the folk in Brussels who dictate we buy grams of sweets and eat straight bananas...and also said we should forget tradition and use metric shafts and threads...

Sounds like you have fitted a new prop at sometime as no doubt your taper was originally 1:12 LOL

/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
If you want to determine the thread size youself, you can do this by measuring the outside diameter of the thread using a micrometer or a caliper with a dial gauge, and the pitch of the thread using a thread gauge which will have a separate guages for different TPI (threads per inch). With these two factors you should be able to identify the thread. I always use a book of Zeus tables. One factor to be aware of is that when threads are cut they can sometimes be cut at 85% which means that they will not have the full profile. This will result in the outside diameter of a 1 inch thread being 0.990", so be aware.
Best of luck.
 
The taper is definitely 1 in 20. The propellor hub is 70mm, the major diameter is 25mm and the minor diameter is 21.5 mm.

From my limited testing it appears that a Whitworth nut will fit quite nicely on an equivalent sized 3/4 x 10 coarse thread, but neither an M20 x 2.5, nor a 3/4 x 10 nut will fit properly on a Whitworth thread.

The differences would appear because the Whitworth has a 55 degree pitch whereas the metric and coarse thread have 60 degree pitches.

Being across the pond, nobody has heard of the whole Whitworth issue.
 
Quite likely to be ¾" Whitworth. A boat of your age could well have had a metric shaft with an Imperial thread. Interestingly I've just reengined with new shaft & prop and a 1" shaft was cheaper than 25mm so I went for the 1" shaft. I believe the thread is Whitworth.
 
All the above should help you a bit. However, to be pedantic, you really should check the TPI and measure the effective diameter - a procedure which involves fitting pins of a set diameter into the thread and measuring across them. Many years since I did that!

Another possible source would be to contact the supplier of the prop you are replacing. I cannot speak too highly of Darglow Engineering whom I contacted with the number stamped on my old prop. They checked the ledgers and within minutes gave me the shaft diameter, taper, thread, key dims., pitch and diameter!

Rob.
 
My threads are 12 per 22 millimetres, equating to 14 threads per inch. Thread diameter is 15.75 mm, equating to 5/8 inch. This makes it a 5/8 BSF. Whitworth is a coarse pitch thread, not really ideal for this application.

Not sure how you arrive at a 1:20 taper. My taper goes from 25.32 mm (1 inch) to 21.0 mm over a length of 56.5 mm (2.25 inch) which makes it a 1:12 taper. This could be described as the standard taper for UK boats.
 
Hello themons, When you say major diameter, do you mean the outside diameter of the thread, and it is 25mm. Then I would say that it is unlikely to be a metric thread, as ISO metric fine can be either 20mm,24mm,or 30mm O/D. ISO metric coarse can be 20mm,22mm,24mm,or 27mm O/D. Non have a O/D of 25mm.

If you convert 25mm to inches, you will get 0.984".
Which stating the obvious is only 16 thou under the nominal 1 inch

1" whitworth O/D=1.000" , Core Dia=0.840", TPI=8

1" BSF O/D=1.000", Core Dia=0.872", TPI=10

1" UNF O/D=1.000", Core Dia=0.898", TPI=12

1" UNC O/D=1.000", Core Dia=0.847",TPI=8

So as you can see the best way forward is now to measure the number of threads per inch and if they come to 8 then the other difference is that whitworth have a thread angle of 55 degrees, and UNC will have a thread angle of 60 degrees so you would have to check with a nut.

All the best
 
I think that I have it all figured out.

The nut corresponds exactly to the 3/4 inch Whitworth standard. This is 1.3 inches across the flats.

According to:

http://en.wikipedia.org/wiki/British_Standard_Whitworth

The Whitworth thread for the 3/4 size is 10 per inch. This is also coincidentally the same as the 3/4 x 10 coarse pitch. Because the M20-2.5 is so close, the metric nut would also thread on. The loose fit was on account of the difference between 55 and 60 degrees of the threads.

The 3.5 mm reduction in diameter over the 70 mm hub is a 1 in 20 taper. Westerly is apparently know for creating their own standards at times.

As a final confirmation, Caroline Hardy from Trafalgar (former association with Westerly) also kindly advised me that other Westerlys with 25 mm shafts had been fit with Whitworth 3/4 threads.

If I ever have to redo the shaft, I think that I will go completely metric.

If I am successful getting the AutoStream feathering prop to fit, I will keep the board posted on its performance.

In the mean time, I believe the mystery is solved. Thanks for all the input.
 
Here is an update to my Whitworth thread post.

This morning, I installed the new Autostream 3 bladed feathering prop. I had to have both the 3/4 inch Whitworth nut and 1 in 20 taper prop hub individually manufactured for my existing Westerly Corsair prop shaft.

I am pleased to say that both the hub and the nut fit perfectly on the shaft. I am very happy that I went to the extra trouble of insuring that the special nut was threaded with 3/4 Whitworth.

Seahawk, the Australian manufacturers originally wanted me to use a 3/4 UNC instead of the 3/4 Whitworth. Incidentally, the Kiwi manufacturers incorrectly say that these threads are the same. If you happen to have one of these types of threads, don't be tempted or talked into using 3/4 UNC.

With the exception of having to increase the pitch somewhat, so far, I am quite pleased with the performance of the Autostream.
 
it will be a volvo shaft and this is the shaft anode details which are the same as the propeller nut


17.17 833913 25 W 3/4"
Zinc anodes protect the propeller against corrosion. The anode is shaped like a propeller cone and should be fitted after the locking nut on the propeller shaft.

For shafts manufactured before V9338.
 
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