Advice saught - Modified ISO Propeller Shaft Taper

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manlyp

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I'm about to replace my propeller. The shaft taper on my existing (metric) prop is 29.2, 1:10, 73, 8, M20 x 1.5 (shaft diam, taper ratio, taper length, keyway width, thread) and the new one I'm thinking of buying is standard ISO taper of 30, 1:10, 80, 8, M20 x 1.5. I understand that ISO tapers reduce in diameter by 1mm for every 10mm of taper - hence the change in the first and third parameter above. I believe the non standard taper prop was fitted when the boat was new to push the shaft backwards to accomodate a rope cutter between the p-bracket and the propeller boss which I wish to retain.

My question is this: how does this work? By machine something off the boss how does this move the propeller backwards. Someone has suggested I buy a propeller with a standard ISO taper and fit a spacer between the gearbox and shaft but I'm reticent to do this as I think it will be a very difficult job to separate the clamp as it is badly corroded.
 
Machining material from the face of the propeller boss will not alter the final position of the prop on the shaft (or the shaft relative to the hull)- but it will give an increase in the distance between the machined face and the stern tube, hence giving clearance for the rope cutter.
Its not good engineering though, unless there are other factors to consider- ideally fit a suitable machined spacer between the gearbox half-coupling and the shaft half- coupling as suggested. An alternative may be to fit an R&D shaft damper.
Is there an issue with propeller blade clearances to the hull that may have left this as an easier fix?
 
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Thank-you but I don't understand some of your answer!
1. There is no question of machining material from the face of the propeller boss. My original propellor was machined with a non-standard ISO taper. Somehow this moves the propeller backwards. My question is, "how does this work"?
2. I've just looked at the R&D damper solution on-line. It appears to be a replacement for my existing coupling and, whilst it would push the propeller shaft backwards, it appears to be the answer to a problem I don't have - i.e. noise at low revs. It also looks to be a rather expensive compared with the simple spacer solution.
By the way there is no problem with propeller blade clearance.
 
If as you suggest in your first post that you are considering a new shaft, just have it made to the right length to give you the necessary gap aft of the P bracket to fit the cutter. A spacer (or an R&D coupling) is only necessary if you want to use the same shaft and increase the gap so that the cutter will fit properly. There is not a lot of difference in price between the spacer and the coupling, but the spacer comes in finer increments so you can get the gap just right. The taper is irrelevant provided it is the same on the shaft and prop.
 
You've not machined anything off the boss, you've just reduced the largest dimension of the taper and so the prop taper 'lands' sooner on the shaft taper. By doing so then the prop boss forward face stops further away from the p bracket then it would if the prop taper had a 30mm largest dimension.
 
ean_p said:
You've not machined anything off the boss, you've just reduced the largest dimension of the taper and so the prop taper 'lands' sooner on the shaft taper. By doing so then the prop boss forward face stops further away from the p bracket then it would if the prop taper had a 30mm largest dimension.
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I think I gedit. Thank you ean_p.

Have I got this right? To move the prop shaft backwards, when they machine the taper in the propellor boss, they don't remove as much material as they would with a standard ISO taper so that when they come to fit the propeller to the shaft, it "lands" as you say sooner and the prop sits further back. I guess you have to be careful not to start the taper too soon otherwise the thread on the back of the prop shaft won't be any use when its all put together! When they machine the boss, the taper angle and length are the same asstandard ISO, the taper just starts nearer the front face of the boss.

So I have a simple choice for my new propellor. Have it made with a modified taper in the boss or fit a spacer?

There is no question of changing my prop shaft!
 
For the life of me I cannot understand what people are talking about. The taper ratio is 10:1. It matters not if the shaft diameter is 30 or 300 mm, the taper ratio of 10:1 will be the same. If you took a boss with the dimensions quoted for the new prop, 30, 1:10, 80, and cut it in half across the diameter, its dimensions would change to something like 26, 1:10, 40. The aft half would still fit in exactly the same place as before and the nut would tighten perfectly.

Seems to me that when Northup said "Machining material from the face of the propeller boss will not alter the final position of the prop on the shaft (or the shaft relative to the hull)- but it will give an increase in the distance between the machined face and the stern tube, hence giving clearance for the rope cutter" he was exactly right. The prop fits on the shaft precisely where it was supposed to, but the boss is a little shorter at the front face than it was, accounting for the dimensions 29.2, 1:10, 73.
 
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Tranona said:
Why not? Shafts are relatively cheap, and you end up with a standard taper, standard prop and enough room to fit your cutter.
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Tranona. What is cheap to you could be expensive to me! A spacer costs about £75 and cutting the taper on the new prop hub is £90. I had a new shaft fitted in Coruna (N Spain) 2 years ago after bending it in the Bay of Biscay (and I now think the propeller) and it cost me 210€ + VAT + the engineer to fit it I'm still in Spain (Cartagena) and I'm up to £2300 just for the prop (feathering) so I'm trying to keep total costs down.
 
ean_p said:
You've not machined anything off the boss, you've just reduced the largest dimension of the taper and so the prop taper 'lands' sooner on the shaft taper. By doing so then the prop boss forward face stops further away from the p bracket then it would if the prop taper had a 30mm largest dimension.
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ean_p has it. Using your figures for the old prop, the max diameter is 29.2mm so is 0.8mm less than the diameter of the shaft. As the taper is 1:10 the prop will 'land' 8mm from the start of the taper ie giving you 8mm more room between the forward face of the prop and the end of the stern tube.
The answer to your question is surely to get the new prop bored out to the same dimensions as your old prop (if that is an option).
 
manlyp said:
Tranona. What is cheap to you could be expensive to me! A spacer costs about £75 and cutting the taper on the new prop hub is £90. I had a new shaft fitted in Coruna (N Spain) 2 years ago after bending it in the Bay of Biscay (and I now think the propeller) and it cost me 210€ + VAT + the engineer to fit it I'm still in Spain (Cartagena) and I'm up to £2300 just for the prop (feathering) so I'm trying to keep total costs down.
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Not my boat, but a new shaft is a fraction of what you are spending on a property. Ship for haporth of tar comes to mind. Best in the long run to do the job properly. Just my view from making a similar bad decision in the past.
 
I think that's pretty much what everyone is saying...Northup suggesting that material removal from the boss being bad practice and I explaining why his current prop has a lesser major diameter (29.2)then a supposedly 'standard' taper (30)


vyv_cox said:
For the life of me I cannot understand what people are talking about. The taper ratio is 10:1. It matters not if the shaft diameter is 30 or 300 mm, the taper ratio of 10:1 will be the same. If you took a boss with the dimensions quoted for the new prop, 30, 1:10, 80, and cut it in half across the diameter, its dimensions would change to something like 26, 1:10, 40. The aft half would still fit in exactly the same place as before and the nut would tighten perfectly.

Seems to me that when Northup said "Machining material from the face of the propeller boss will not alter the final position of the prop on the shaft (or the shaft relative to the hull)- but it will give an increase in the distance between the machined face and the stern tube, hence giving clearance for the rope cutter" he was exactly right. The prop fits on the shaft precisely where it was supposed to, but the boss is a little shorter at the front face than it was, accounting for the dimensions 29.2, 1:10, 73.
Click to expand...
 
ean_p said:
I think that's pretty much what everyone is saying...Northup suggesting that material removal from the boss being bad practice and I explaining why his current prop has a lesser major diameter (29.2)then a supposedly 'standard' taper (30)
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I guess it's the nomenclature that is confusing. I don't follow why the diameter has any relationship to the taper ratio.
 
it doesn't....but if you make the major dimension 29.2mm instead of the supposedly 30mm in any given boss then the front face of that boss will come to rest further 'down' the hill of a given shaft taper. At that point were the diameter of the shaft taper is 29.2mm.
Likewise if you turn the same boss taper so that the major dimension is 30mm then the front face of that given boss would be further 'up' the hill of the shaft taper, though that point were the boss taper was 29.2mm wide (now buried within the boss) would still come to rest on that point of the shaft taper that was 29.2mm wide. So by changing the major dimension you can alter the position of the boss front face...and of course this would also alter the position of the back face and it's relationship with the thread!
Maybe its they way I try miserably to explain it and is one of those things that a simple diagram would speak volumes.....though I know we're all saying the same thing!
 
alahol2 said:
ean_p has it. Using your figures for the old prop, the max diameter is 29.2mm so is 0.8mm less than the diameter of the shaft. As the taper is 1:10 the prop will 'land' 8mm from the start of the taper ie giving you 8mm more room between the forward face of the prop and the end of the stern tube.
The answer to your question is surely to get the new prop bored out to the same dimensions as your old prop (if that is an option).
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Thank you alahol2 and ean_p. Boring out the big end of taper of the new prop to 29.2 would give me the same space for the rope cutter as the old prop except that the boss of the new prop will be longer and there won't be enough room for the retaining nut! Shortening the boss/hub is not an option.

The solution now evolving is to have the propeller supplier deliver to Spain a new prop bored out to standard ISO 30 so that when it is assembled the front face of the boss is in the same position as the existing prop. To achieve this, the supplier will specify where the taper starts in the prop hub and the width of the spacer on the gearbox flange (about 13mm) which they will supply. This way they will be responsible for all the calculations and if the new prop doesn't meet requirements, they will be responsible for the costs of sorting it out!
 
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