Prop and shaft tapers. AmI missing something?

PuffTheMagicDragon

PuffTheMagicDragon

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In my quest for a replacement propeller I came across a site that offered reconditioned props. I duly sent an email enquiry, part of which is reproduced here:

"...Original prop was a left-handed 14 x 15. Boss length is 70mm. Shaft is 25mm tapering down to 18. Keyway on the shaft is 6mm (but may be milled wider if necessary). Not sure about DAR required.

My question is this. In your reconditioned props section you are showing one which is described thus: "15 x 11 LH 2 blade 25mm shaft tapering to 18mm , taper 1:12 , boss length 70mm , key 8mm DAR 0.36" Is the taper correct because I would have thought that the bores and the boss length (identical to mine) would indicate a 1 in 10 taper."

Today I received a reply from Steel Developments Propellers that said:

"attached is information regarding tapers. From the information you have given I would suggest you have a 1:12 taper but you need to double check your measurements if you believe it is 1:10 "

My measurements are accurate. Surely it is a simple calculation. 25 - 18 = 7. 7 over a length of 70 is the same as 1 in 10. What am I missing? :confused:
 
Fuddled my brain up with this one.

I think you should not be using the boss length to work out your sums but the length on the taper, which will be longer.
 
doug748 said:
I think you should not be using the boss length to work out your sums but the length on the taper, which will be longer.
Click to expand...

...or at least not mix the two. So shaft large and small diameters, and length of tapered part, or prop boss large and small holes and boss length.

Pete
 
My brain fuddled as well.

If you look at the shaft, it is tapered on both the top and the bottom.

Shouldn't you look at the difference in the radius not the diameter.

However, if I am right, it makes it a 1 in 20.

Better brains required
 
I tried (with an accurate vernier guage) to work out if my taper was 1:10 or 1:12 unfortunately it can be quite difficult to tell the difference because of even small inaccuracies combining in a compound manner. In the end I had a 1:12 taper plug machined up with which to check the shaft. It was very easy to tell that it was 1:12 (when I tried it on a known 1:10 shaft if was obviously wrong).

The norm is that a 1 inch shaft will have a 1:12 taper and a 25mm shaft a 1:10 taper. Thus the shaft diameter is a good clue from which to start. I still have the plug guage if you want to borrow it.

Yoda
 
doug748;3562493... I think you should not be using the boss length to work out your sums but the length on the taper said:
This information came from my ASAP catalogue and is contradicted by jamesbells site, so there we have it. So to speak.

You could double check it with a bevel gauge.
Click to expand...
 
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yoda said:
The norm is that a 1 inch shaft will have a 1:12 taper and a 25mm shaft a 1:10 taper. Thus the shaft diameter is a good clue from which to start.
Click to expand...

The shaft is definitely a 25mm and the boat was built in France, which would indicate that the Metric standard would be more likely. Tomorrow I shall try to measure the angle with my Mitutoyo vernier-goniometer and see what result it gives.

Thanks for the offer of loaning the plug. Unfortunately it would not be practical because I am some two thousand miles to the South. I had stopped short of making a plug because I thought that determining a taper would be a piece of cake. The shaft is still good and will be retained, otherwise I would just buy the prop and machine a shaft to suit.
 
Update

This morning I measured the angle that is formed between the flank of the shaft taper and the shaft itself:

shfttapercropped.jpg


As far as I can make out the angle is 2 degrees and just over 45 minutes which means that the included angle of the taper is just over 5* 30'. Now a taper of 1 in 10 is equivalent to 0.1 which, when inverse tan is applied, gives an angle of 5* 42,6' whereas a slope of only 1 in 12 would give an angle of 4* 45,8' a difference of almost one degree.

I am therefore concluding that the taper on my shaft is most likely to be 1 in 10 and not 1 in 12. Do you agree?
 
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PuffTheMagicDragon said:
This morning I measured the angle that is formed between the flank of the shaft taper and the shaft itself:

As far as I can make out the angle is 2 degrees and just over 45 minutes which means that the included angle of the taper is just over 5* 30'. Now a taper of 1 in 10 is equivalent to 0.1 which, when inverse tan is applied, gives an angle of 5* 42,6' whereas a slope of only 1 in 12 would give an angle of 4* 45,8' a difference of almost one degree.

I am therefore concluding that the taper on my shaft is most likely to be 1 in 10 and not 1 in 12. Do you agree?
Click to expand...

YES
 
The problem as I see it is that you were never in doubt about your measurements, it is the firm's measurements or calculation that is in doubt!
 
Sometimes even the professionals get it wrong.
I replaced my engine complete with new prop & shaft bought as a package from a well known east coast firm.
The prop and shaft didn't marry, so I took them back.
After a bit of a wait a harassed engineer came and asked, rather abruptly, what the problem was. I handed over the shaft and prop.
He then used a technical term I hadn't heard before.
" This fits like a pr**k in a sleeve, where did you get this prop?"
"From you"
"Oh, where did you get the shaft?"
"From you"
"Oh, we had better re-machine it then":D
 
Thank you all for your support and comments. I have settled on 1 in 10 and shall look for another prop supplier. A pity because the cost of a reconditioned prop is miles more attractive than that of a new one.
 
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