Propeller;- to key or not to key.

[lw395]

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

Well none, but it shows the principle, if you are a engineer.
It shows how the taper is the bit that does the work.
It's not easy to find info about props without keyways on the net, but I've seen quite a few, mostly older.
Fair enough a keyway is the norm now, but its not the only way.
And it does little to reduce props falling off.
If anything, isn't that more common than it used to be?

 

[Graham_Wright]

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.

Both then.

 

[Plevier]

How about Nord-Lock? Pairs of locking ramped washers.
http://www.nord-lock.com/

 

[Graham_Wright]

How about Nord-Lock? Pairs of locking ramped washers.
http://www.nord-lock.com/

Sounds promising - they do them in stainless I read. Not sure what grade.

I'll ask my fixings and fasteners supplier.

 

[Plevier]

254SMO EN 1.4547 duplex for use with A4 bolts in salt water environment.

 

[Leonidas]

Absolutely. All modern ships have keyless propellers and shafts. Initial good fitting to taper is essential. Classification societies provide guidance of the accepted standards of percentile contact of the cone of the shaft to the propeller hub. The propellers are secured with hydraulic nuts ( known as Pilgrim nuts) which are also used to provide an even push to the taper to preset limits at preset temperatures. There have been occasions that the propeller has "walked back" during astern operation, but this was only due to the incorrect tightening up of the propeller nut to its markings,

 

[Tranona]

Well none, but it shows the principle, if you are a engineer.
It shows how the taper is the bit that does the work.
It's not easy to find info about props without keyways on the net, but I've seen quite a few, mostly older.
Fair enough a keyway is the norm now, but its not the only way.
And it does little to reduce props falling off.
If anything, isn't that more common than it used to be?

Yes, your reference does show that it is technically a possible solution. However, keyways are used primarily because they are as simple and reliable way of locating the prop on the taper and are not reliant on close tolerances. In addition they still enable the prop to drive even if the nut does go slack.

Not sure where this idea that props fall off regularly comes from, nor how not having a key will stop it. Equally not sure why suddenly providing a "weak link" or a damper is deemed necessary. Tapered keyed shafts are robust and idiot proof. As I suggested earlier most stern gear damage comes from ropes and nets getting round the prop and there is a lot of damping in the drive train to prevent damage to the keyway and taper, from the engine mounts, flexible couplings if fitted. Cannot see how not having a key in any way would change this, as if it is tight enough to provide drive it will not slip. The negative side, as Snooks describes is if the nut slackens the prop can move on the shaft and drive is lost, whereas if you have a key, there will still be some drive, albeit rather noisy!

Cutters can reduce the possibility of damage, but inevitably there are situations where other parts of the stern gear can suffer damage, particularly P brackets, and as mentioned earlier if rope gets wound in around the shaft in the gap between the prop and stern bearing. You only have to see the effect of this type of wrap on an engine that is powerful enough to keep going and be pulled off its mountings, or worse still on some engines with ally mounting points leaving parts of the casings still attached to the mounts - but not the engine!

Saildrives are, of course different as the thrust is transmitted direct into the geartrain in the leg, and splined shafts in rubber bushed hubs are used - but that is another subject!

 

[Marsupial]

Yes, your reference does show that it is technically a possible solution. However, keyways are used primarily because they are as simple and reliable way of locating the prop on the taper and are not reliant on close tolerances. In addition they still enable the prop to drive even if the nut does go slack.

Not sure where this idea that props fall off regularly comes from, nor how not having a key will stop it. Equally not sure why suddenly providing a "weak link" or a damper is deemed necessary. Tapered keyed shafts are robust and idiot proof.

Absolutely agree, in over 50 years I have personally seen only 2 props lost both on power boats but have heard of others on racing power boats and sailing yachts - but its not a regular occurrence. l Based on what I have seen and heard lots of horsepower and shoddy workmanship is the best way to loose a prop. IF the securing nut does come loose because of nets, ropes, electrolysis or corrosion the key will/could/would enable the drive system to work. No key and there is nothing to stop the nut undoing if any of the above perils are encountered plus the shaft will/could/would spin and the prop wont turn at all.

fit a key

 

[john_morris_uk]

I feel slightly reluctant to post in the face of so much expertise...

The only time I gave lost a prop was when I tried to use one (temporarily) for racing that didn't have a keyway.

I understand the argument about keyways forcing the designer/engineer to oversize the shaft to allow enough 'meat' for the keyway to be machined. However I wonder whether the average yacht power train is so over engineered for that argument to be irrelevant. Perhaps an engineer will look up the desired shaft diameter in s/s for a 30 hp engine? I bet my 30mm shaft is far bigger than it needs to be...

Perhaps the telling argument is that tapers ( of whatever proportion ) would be fine PROVIDED the engineering is to close enough tolerances. Crawling under my boat I am never too sure about the tolerances on the propellors and shafts I have assembled, even when I have been measuring with a calibrated digital caliper... Hence my gut feeling to stick with a keyway. I'm open to persuasion though...

 

[GrahamHR]

Maybe the solution is out there in the lawnmower world. Hub keyed to the engine shaft and a proper solid contact between the blade and the hub , friction discs either side of the blade. Hit something and the blade slips relative to the hub.

 

[oldbilbo]

Now that OP Graham_Wright has determined his answer, might a 'bear of little brain' ask for some input....?

My MAB is currently sitting up on a cradle. The 12" prop was removed and polished, and is now ready for refitting. It has a keyway, and the tapered (?) shaft has a key, of course, taped in place so I don't lose it. I've purchased a shiny new Nyloc nut, thinking that would be appropriate. Due to this thread, I'm now none too sure....

I don't have the facility to drill the shaft-end to take a pin of any kind. Would the team recommend a Philidas nut instead? Are such available in corrosion-resistant stuff? Or would my Nyloc nut, aided by Loctite and the tight taper, suffice?

 

[GrahamHR]

Sorry, I can' help you re your specific questions. However, relating to someone else's earlier post about nyloc nuts only working when the nut had become loose due to vibration, what a load of bull stuff. They stay where they are. If they are loose enough to move, they will continue to move ( unscrew).

I know they are theoretically only to be used once. However, on the smaller ones a pin punch can be used to deform the locking ring. it works very well. On bigger ones the plastic ring can be removed and inverted so what was the outside is now the inside; result, one new locking ring !

 

[Tranona]

Now that OP Graham_Wright has determined his answer, might a 'bear of little brain' ask for some input....?

My MAB is currently sitting up on a cradle. The 12" prop was removed and polished, and is now ready for refitting. It has a keyway, and the tapered (?) shaft has a key, of course, taped in place so I don't lose it. I've purchased a shiny new Nyloc nut, thinking that would be appropriate. Due to this thread, I'm now none too sure....

I don't have the facility to drill the shaft-end to take a pin of any kind. Would the team recommend a Philidas nut instead? Are such available in corrosion-resistant stuff? Or would my Nyloc nut, aided by Loctite and the tight taper, suffice?

Not difficult to put a pin in it. Surprised the shaft does not have one already. Tighten your nut up until the prop is solid on the taper. Drill a hole through the thread of the shaft in line with the castellations using a cobalt tip drill - slowly. Put a stainless split pin through and your prop will not come off until you want it to. Standard practice! Other ways of doing it if you don't like pins. My shaft has a socket head grub screw in at an angle through the end of the nut and shaft to lock the nut in. Used that arrangement because the nut is buried in the hub of the prop so a pin method would not work. It is currently in a plastic bag hanging on a shelf in front of me in my study to remind me to put it in before I launch the boat!

 

[Leonidas]

Perhaps the telling argument is that tapers ( of whatever proportion ) would be fine PROVIDED the engineering is to close enough tolerances. Crawling under my boat I am never too sure about the tolerances on the propellors and shafts I have assembled, even when I have been measuring with a calibrated digital caliper... Hence my gut feeling to stick with a keyway. I'm open to persuasion though...

One can easily ascertain the fit by using Prusian Blue paste on the taper of the shaft, harden up the propeller nut without the key in place , then remove the propeller and check the blue impression left on the hub.

 

[Graham_Wright]

Not difficult to put a pin in it.

Not difficult to drill through the shaft (as said - slowly and with pressure) BUT! The chances of the hole coming out precisely through the diameter are effectively nil!

Your best chance is to drill through from both sides (diametrically opposite). However, when one bore meets the other there is a possibility (probability?!) that the drill will snatch and/or break. Take it very easy as you feel the break-through approaching.

(These pearls based on many bad experiences. In production, we drill in an indexer - i.e. half way at 0° and the other half at 180° and that following using a centre drill. This may not be an appropriate method in situ on a prop shaft!).

 

[john_morris_uk]

One can easily ascertain the fit by using Prusian Blue paste on the taper of the shaft, harden up the propeller nut without the key in place , then remove the propeller and check the blue impression left on the hub.

Quite! Except you perhaps miss my pint about the 'crawling under the boat bit'. You might find the problem with the fit, but what are you going to do about it when you are under the boat in the boat yard with the prop in one hand and the shaft in front of you? The tolerances for the fits that provide drive are very small.

Keyways might be agricultural and unnecessary when the tapers match perfectly, but its proven technology. The only downside that's been mentioned is that the shafts have to be that much bigger to allow for the keyway to be machined. I haven't noticed a desire for thinner prop-shafts on boats and I posed the question a little while ago about how much over engineered boat prop-shafts are?

I've no inside knowledge either way - and its clear that the engineering to produce friction fit tapered props and shafts is easy enough in many circumstances. I just wondered whether the reason we stick with keyways is that its not easy to produce the correct tolerance for a drive without keyway without both parts in the machine shop on the lathe? Matching things up from even the best attempts at measurement from under the boat in the boatyard might be a challenge too much?

 

[lw395]

Quite! Except you perhaps miss my pint about the 'crawling under the boat bit'. You might find the problem with the fit, but what are you going to do about it when you are under the boat in the boat yard with the prop in one hand and the shaft in front of you? The tolerances for the fits that provide drive are very small.

Keyways might be agricultural and unnecessary when the tapers match perfectly, but its proven technology. The only downside that's been mentioned is that the shafts have to be that much bigger to allow for the keyway to be machined. I haven't noticed a desire for thinner prop-shafts on boats and I posed the question a little while ago about how much over engineered boat prop-shafts are?

I've no inside knowledge either way - and its clear that the engineering to produce friction fit tapered props and shafts is easy enough in many circumstances. I just wondered whether the reason we stick with keyways is that its not easy to produce the correct tolerance for a drive without keyway without both parts in the machine shop on the lathe? Matching things up from even the best attempts at measurement from under the boat in the boatyard might be a challenge too much?

Not really, the tapers are pretty standard, they don't need to be fitted individually.
It will either be the right taper, need a bit of damage removing, or be completely wrong.
Even if there is a key, having the taper a good fit is still essential, other wise the prop may fret loose, or crevice corrosion is likely.

I think you are right, there is plenty of metal in most propshafts to cope with a keyway, they are made big because they have long unsupported sections, get bending stresses from the engine etc

 

[2Tizwoz]

http://www.youtube.com/watch?v=2ozh4JjoWm4

Fitting with blue.

 

[oldbilbo]

http://www.youtube.com/watch?v=2ozh4JjoWm4 Fitting with blue.

Hmmm. Velly intellesting, but...

Howzabout some Loctite ) http://www.loctite.co.uk/your-solution-4371.htm ) or some of the Sika ( http://tinyurl.com/lfq6aq8 )....?

KISS....

 

[john_morris_uk]

http://www.youtube.com/watch?v=2ozh4JjoWm4

Fitting with blue.

I found that a really interesting and informative video clip. So much so that I have lost time out of my life I will never get back watching some of his other clips...

He makes the point that the keyway and key is there for when you hit something, but the taper properly lapped together should transmit the drive. Makes perfect sense. It also makes me wish I'd lapped my prop hub to my shaft when I fitted it. However, six years on and it's still going strong so it can't have been that bad.