Thank you Crowthers

[Chewbacka]

I remember a few years ago being held up at a lock whilst a guy in his narrowboat was replacing the shear pin, said he caught the prop on the cill when going down.

[MoominPapa]

On 13/12/2017 at 09:59, Mike the Boilerman said:

 

A flexible coupling is a lot better than none but not quite what I meant.

If a prop jam occurs stopping the engine dead, the energy that bends the prop is the kinetic energy present in the rotating engine flywheel, crankshaft etc, rather than the basic engine power which just pushes the boat along. 

Consequently, a truly massive instantaneous force gets transmitted along the prop shaft at the instant of the jam and a coupling which can slip under this extreme stress is a Very Good Thing and might well save the prop from getting bent. Think of it as is the mechanical equivalent of a fuse in an electrical circuit. 

A 'slip joint' in a propshaft in its simplest form would be a plain shaft clamp coupling with the woodruffe key taken out and the clamp bolt tightened sufficiently to grip during normal operation but not tightly enough to continue to rigidly grip in the event of a prop jam. 

Edit to add:

A 'friction joint' might be a better term for what I'm suggesting.

Isn't this a function of the gearbox? After all on a marine gearbox the gears are selected using friction clutches, which will slip at some torque value.

Once, just as I was swinging under the bridge as Great Haywood, there was a big bang from under the counter, followed by a seriously loud whirring sound from the engine room. I instinctively pulled astern, and a huge lump of timber shot out behind. It couldn't have done anything other than jam the prop if caught. On selecting ahead again, normal service was resumed. I always assumed that the forward clutch in the Lister Blackstone gearbox had slipped to avoid dumping all the energy in the engine and flywheel into the jam. I'd guess that all marine boxes are built to do that, but a modern high-reving engine might not be able generate enough torque to make the clutches slip before it stalls.

 

MP.
 

Edited December 21, 2017 by MoominPapa
Posted too many times.

[Cheshire cat]

Well, we had a 2.2 litre Nanni attached to a PRM260 and no form of flexi coupling on a shared boat and got through three drive plates in two years. Sometimes the whotsit hits the immovable object. A supermarket trolley as I recall on one occasion.

[Tony Brooks]

11 hours ago, MoominPapa said:

Isn't this a function of the gearbox? After all on a marine gearbox the gears are selected using friction clutches, which will slip at some torque value.

snip

 

MP.
 

Not all do, some are more like a car's synchromesh gearbox where the initial engagement is by cone clutch but then a dog clutch ring slides over teeth to mechanically lock the clutch.

[MtB]

12 hours ago, Chewbacka said:

I remember a few years ago being held up at a lock whilst a guy in his narrowboat was replacing the shear pin, said he caught the prop on the cill when going down.

 

That sounds odd. The skeg and rudder would normally protect a propeller from hitting the sill. And why did he have it in gear in a lock anyway! Because he was trying to get away from over the cill I guess...

 

12 hours ago, MoominPapa said:

Isn't this a function of the gearbox? After all on a marine gearbox the gears are selected using friction clutches, which will slip at some torque value.

 

That's a good point. I guess my Kelvin J2 gearbox would slip the cones, and and so would the Gleniffer. And the old Parsons mechansical box under the floor.

I'm not so sure about my Velevet Drive though, or my PRM D260, both these being hydraulic boxes. I imagine Tony would know for certain.  

[Tony Brooks]

Unless you mean Wotever, no he would not. I am about 98% sure all the hydraulic boxes, including the Velvetdrives would slip but as for mechanical boxes I would want to check in the manual or look inside the box. I know the BMC DCC box locked mechanically as the gear was selected and I have seen what looks suspiciously like a machinery gearbox fitted on an old Lister that did the same.

My post was a  caution not assume all  gearboxes will slip, some may not. I wonder if Richard can add anything.

[BEngo]

I can say that the likelihood of a J2 box slipping will depend on how the blades get jammed and on the set up.

If the box is taking the thrust., as it is designed to be installed, then any jam which causes the bladrs to bite into something more solid than water ( say a bit of wood) causes a big increase in thrustand the engine stops to the accompaniment of loud chuffs and black smoke. Stuff like rags  and side fenders cause a massive snarl up and the clutch slips. If it does this in reverse it is also difficult or impossible to wind the box out of gear!

DAMHIKT.

If the box does not take the thrust then it will slip at some point.

N