Propeller;- to key or not to key.

[PCUK]

You cannot do without a key. Is that clear enough? At some point the prop will free off' and spin on the shaft, could be hitting debris or just a violent move from forward to astern. You can use a nyloc nut and when it is fully tightened drill it and fit a split pin, or just use a standard nut and drill it after tightening. Castellations rarely line up exactly. Whatever you use you will definitely need a key.

 

[Graham_Wright]

You cannot do without a key. Is that clear enough? At some point the prop will free off' and spin on the shaft, could be hitting debris or just a violent move from forward to astern.

Right - so as Devil's advocate, if I fit a key and then after "hitting debris or just a violent move from forward to astern", what will happen then? The key shears? The prop disintegrates? The gearbox shatters? Perhaps just the engine will stall (hopefully).

To repeat - just as Devil's advocate.

 

[vyv_cox]

Right - so as Devil's advocate, if I fit a key and then after "hitting debris or just a violent move from forward to astern", what will happen then? The key shears? The prop disintegrates? The gearbox shatters? Perhaps just the engine will stall (hopefully).

To repeat - just as Devil's advocate.

People who have suffered this sort of unfortunate incident report that the P-bracket will bend, assuming you have one, and it is quite common for engine mountings to shear. Bending a blade of the prop also seems to be known.

As for the effect on an unkeyed prop to shaft, you only need to read the many posts in which people are unable to remove their props axially, when the key has no effect, to understand how powerful the tapered fit can be. Posters report hitting with large hammers, heating, pullers and various other ploys having no effect.

 

[alahol2]

Right - so as Devil's advocate, if I fit a key and then after "hitting debris or just a violent move from forward to astern", what will happen then? The key shears? The prop disintegrates? The gearbox shatters? Perhaps just the engine will stall (hopefully).

To repeat - just as Devil's advocate.

On each occasion that I've caught a rope round the prop the engine has stalled. It's then just a case of trying to clear the mess. I suspect a taper fit prop would probably have spun on the shaft and I'd then have had to at least dry out to re-fit/tighten. Mine's only a small engine though (13HP), a much bigger engine may well do more damage.
Perhaps it's psychological, I just wouldn't feel happy about no key and no pin through the nut.

 

[Graham_Wright]

People who have suffered this sort of unfortunate incident report that the P-bracket will bend, assuming you have one, and it is quite common for engine mountings to shear. Bending a blade of the prop also seems to be known.

I don't have a P-bracket but the stern gland is long.

As for the effect on an unkeyed prop to shaft, you only need to read the many posts in which people are unable to remove their props axially, when the key has no effect, to understand how powerful the tapered fit can be. Posters report hitting with large hammers, heating, pullers and various other ploys having no effect.

Surely the key would make removal even more difficult. The shaft is tapered with key or without key. Without the key, it might help to tap the pulley in rotation at the same time as using a puller axially.

All this makes me wonder if it is advisable to introduce a "weak link" as with outboards. But where? My CNC lathes have a trip-out arrangement such that if rotational resistance exceeds a limit, the trip operates. I guess the shaft could have a shear pin introduced somewhere perhaps in the pulley boss. One of my customers operates a chicken processing plant. They have some instances of "soft" keys for failure survival. Is that the way forward?

Or am I being over-protective and paranoid as usual?!

 

[ean_p]

The Morse is one of only many tapers in use in engineering etc...ref the ISO tapers used in machine tool holders and you'll find that the tapers are often much steeper. .

Morse tapers are used to transmit torque. The steeper tapers, such as International are for location and are self-releasing, dogs or keys are usual to transmit the torque.

I don't think that thats quite right.... as said the Morse is only one of many 'tapers' that are used for torque transmission and most of them require a method to break the grip of the taper....as an example many of the HSK type have no drive dog component relying simply on the taper for torque transmission and have a taper of 1 in 10 which is I believe a common taper for props

 

[Graham_Wright]

I don't think that thats quite right.... as said the Morse is only one of many 'tapers' that are used for torque transmission and most of them require a method to break the grip of the taper....as an example many of the HSK type have no drive dog component relying simply on the taper for torque transmission and have a taper of 1 in 10 which is I believe a common taper for props

All mine, predominantly CAT40, have a drawbar and drive dogs. BT40 is the same. They do release (if they don't there is a bang and you get the noose out!).

 

[earlybird]

I don't think that thats quite right.... as said the Morse is only one of many 'tapers' that are used for torque transmission and most of them require a method to break the grip of the taper....as an example many of the HSK type have no drive dog component relying simply on the taper for torque transmission and have a taper of 1 in 10 which is I believe a common taper for props

True, & slight fred drift but HSK is a very specialised and precise machine tool drive taper, not really comparable with the average propeller/ shaft combo.

 

[lw395]

You cannot do without a key. Is that clear enough? At some point the prop will free off' and spin on the shaft, could be hitting debris or just a violent move from forward to astern. You can use a nyloc nut and when it is fully tightened drill it and fit a split pin, or just use a standard nut and drill it after tightening. Castellations rarely line up exactly. Whatever you use you will definitely need a key.

It's clear enough that you've got no idea.
There are a great many props out there giving trouble free service on un-keyed shafts.
A key may be a good thing, probably is, but it's no substitute for everything being in good order and properly assembled.

 

[MASH]

Let's look at this from another direction. Does a keyway do any harm? It was mentioned earlier that it can introduce stress points and thus potentially be a source of a fatigue failure, but are yacht shafts really prone to that? How common - or otherwise - are shaft failures associated with a keyway on yachts?
If it is very unusual then what's the problem?

 

[snooks]

We were motoring up the river Trieux and hit a hefty branch with the prop. It forced the key into the keyway, there was just enough slack for the prop to come loose on the shaft. So while I had plenty of drive ahead, astern I had nothing as the prop would unscrew itself off the shaft and do nothing. Made berthing quite fun!

I'm sure it wasn't the best system to begin with, but I wouldn't want to risk it happening again.

 

[PCUK]

It's clear enough that you've got no idea.
There are a great many props out there giving trouble free service on un-keyed shafts.
A key may be a good thing, probably is, but it's no substitute for everything being in good order and properly assembled.

Utter nonsense! Without a key it's not properly assembled. Taper is for alignment, key is to stop turning. Two essential items of prop' shaft assembly.

 

[lw395]

It's mostly the friction in the taper that stops it turning. The key is little bigger than a shearpin.
Thousands of boats out there with no keyway. Millions of un-keyed tapers working fine in other bits of engineering.
But the taper needs to be made accurately enough.
Keyways are obviously good when alignment matters, like a camshaft.
Or when you need to hold the outer part still to torque the nut up.

 

[ean_p]

True, & slight fred drift but HSK is a very specialised and precise machine tool drive taper, not really comparable with the average propeller/ shaft combo.

Why?...... it is one of many tapers that transmits torque without the use of dogs and was chosen as it has a similar taper profile to a shaft taper. It may well and does have a much greater degree of precision but that is because its role in a machine tool demands that degree.

 

[Tranona]

Maybe I have led a sheltered life, but I don't think I have ever seen a shaft/prop without a keyway - unless it is on a spline. Perhaps you could provide a source to support your assertion about "thousands"?

There is no need for any "weak" link in a conventional shaft/prop like on a splined drive. If the prop hits something on a small hp engine it will usually stop the engine or damage the blade. The biggest danger is rope etc getting between the prop and the bearing housing and either solidifying or winding round the shaft and pulling the engine back on its mountings.

Fitting an effective rope cutter to prevent this and keeping the prop clear of debris is pretty bullet proof, except perhaps against damage from hitting a solid object with the blades.

 

[lw395]

Maybe I have led a sheltered life, but I don't think I have ever seen a shaft/prop without a keyway - unless it is on a spline. Perhaps you could provide a source to support your assertion about "thousands"?

.....

http://highseasyachtservice.com/2010/08/schaffran-propellers-hydraulic-fit/

Just one example a bit upscale from most of our boats.

 

[Tranona]

http://highseasyachtservice.com/2010/08/schaffran-propellers-hydraulic-fit/

Just one example a bit upscale from most of our boats.

And how many Countess 33s (OPs boat) might be fitted with one of those? - or any other yacht auxiliary?

 

[Rigger Mortice]

? Accepted that the pin never lines up with the best castellation, the answer surely is to introduce a resilient spacer, a substantial spring washer, between nut and prop boss.

The last thing I would want to do is to introduce anything which would allow movement along the shaft with a tapered fitting. I think that would be an open invitation to the prop coming free.

Having seen the affect on a keyway on a shaft where a flywheel came loose it's pretty obvious that a key simply helps get alignment right. A sliver of metal won't resist much torque. Admittedly the forces involved with a flywheel will be greater, but I think that a key is pretty much redundant in helping the mechanical strength of a tapered fitting.

Personally, I would have no hesitation in fitting a prop without a key. Properly done it's not going to loosen up.

 

[Graham_Wright]

Time to knock this one on the head!

(Wouldn't it be nice though, if dogma could be avoided in these requests for help?)

I am not going to fit a key.

I am going to fit a pinned nut with a compressible washer between it and the prop.

I am also going to introduce a weak link at the connection between the propshaft and the Aquadrive which, at the moment, relies purely on the clamped joint friction (no keyway). If there is a catastrophic collision with an "immovable object", the failure will be inboard. That will still probably not deny me the opportunity for another dive (but hopefully in clearer water than last time!).

 

[myquest]

Right Graham, start sucking on that egg! The issue that you would like to clarify in this instance is Nylock or pin. The taper is unburstable WITHOUT a key IF it the two components are kept in compression at he correct level. MY two penneth is that the Nylock stops the nut vibrating off if it comes loose, whereas a pinned nut prevents the nut coming loose in the first place.