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Metalastik Coupling Sheared (6.25")


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The Middle Level Commissioners have installed a shiny new landing stage at Ramsey 'basin'. I suspect that the old concrete mooring wall is now in the basin,as I hit something big and hard when trying to extricate my boat from the Ramsay 60ft 'winding hole' in reverse. I was unable to wind my 60ft narrow boat due to low water levels and also too much building debris in the water.

 

I sheared the metalastik coupling on the drive to the propeller shaft. My original metalastik coupling was at least 43 years old. Part number 21/642/02. 6.25" diameter. Probably the same part described by Aurthur Marshall on these forums in the past (and others) as 'unobtainable' 😞

 

If anybody has got a new or used 6.25" (approx) coupling like this, then please let me know !

 

Robush (the Metalastik distributor) say that they might be able to get some manufactured when their manufacturer returns from his summer break. However, this option is going to take several weeks I guess.

 

In the meantime Nick at Brinklow Boat Services is going to try and help me with a solution to get the boat moving again.

 

Note: These 6.25" metalastik couplings are not the same as the ones used on some old cars (smaller) nor the ones used on ex-working boats (these are about 7.5" diameter).

 

Ralph 

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FWIW I don't see why it strictly needs such a coupling, although it might reduce vibrations transmitted through the hull. You obviously have two Hooks (UJ) joints so they will cancel any cyclic vibration. I would try to find a clamp on shaft coupling that either does, or can be machined, to accept the UJ flange.

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When we were last there (2015?) the old moorings on the opposite to the new ones had fallen into the water. As they were the responsibility of Huntingdonshire District Council they wouldn't have done anything about it.

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1 hour ago, Tony Brooks said:

FWIW I don't see why it strictly needs such a coupling, although it might reduce vibrations transmitted through the hull. You obviously have two Hooks (UJ) joints so they will cancel any cyclic vibration. I would try to find a clamp on shaft coupling that either does, or can be machined, to accept the UJ flange.

 

Thanks Tony.

 

However, I guess that the metalastik coupling does provide the weakest link in the transmission if the propeller hits an 'immovable' object. Otherwise the Lister JP3(M) Blackstone gearbox or reduction box gets the hit ??

 

Ralph

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We had the metalastik coupling on Fulbourne fail some years ago. At that time the minimum order was 4 units, so we acquired 4, one of which has since been used. As they are no longer available we are hanging onto the remaining spares. But we did identify an alternative from another manufacturer - not identical and would require the coupling flanges to be redrilled, but at least that's another option.

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2 minutes ago, fladda said:

 

Thanks Tony.

 

However, I guess that the metalastik coupling does provide the weakest link in the transmission if the propeller hits an 'immovable' object. Otherwise the Lister JP3(M) Blackstone gearbox or reduction box gets the hit ??

 

Ralph

 

Or the clamp on coupling slips on the shaft. However, I take your point, but if it is an obsolescent or special order part it might be an idea to look for and alternative. It is also possible the clutch(es) in the gearbox may slip. I don't know the Blacklstone box, but I bet it is a multi-plate clutch for ahead and a brake band for astern. Almost certainly the whole of the guts revolves as an assembly in ahead and it is only in astern that small gears transmit drive.

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1 hour ago, David Mack said:

We had the metalastik coupling on Fulbourne fail some years ago. At that time the minimum order was 4 units, so we acquired 4, one of which has since been used. As they are no longer available we are hanging onto the remaining spares. But we did identify an alternative from another manufacturer - not identical and would require the coupling flanges to be redrilled, but at least that's another option.

 

We've used more than one of them 😞

 

When we broke our original one as a temporary 'fix' we bolted the coupling 'straight through'. This does of course mean that the gearbox is much more vulnerable

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Given it is obviously designed and fitted as a 'weak link' to protect the engine etc in the event of a solid prop foul, are you SURE there isn't a box of spare ones tucked away somewhere inside the boat?

 

 

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1 hour ago, MtB said:

Given it is obviously designed and fitted as a 'weak link' to protect the engine etc in the event of a solid prop foul, are you SURE there isn't a box of spare ones tucked away somewhere inside the boat?

 

 

There isn't in mine, and they were going to charge about 700 quid each for them with a minimum order. As mine was only damaged rather than destroyed we filled the damaged bits as best we could and bunged it back on.

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16 hours ago, MtB said:

Given it is obviously designed and fitted as a 'weak link' to protect the engine etc in the event of a solid prop foul, are you SURE there isn't a box of spare ones tucked away somewhere inside the boat?

 

 

Mike and Kate Fox, the original owners of Syrinx did leave quite a few useful spares in the locker under the rear side-bed. Alas no metalastik drive couplings. I suspect they were easily obtainable then, so why bother carrying a spare ??

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You could ask Vibracoustics in Leicester if this is in there range.

They also may be able to use your old metal plates and mould on new rubber.

They are run by ex metalastic people.

Alternative may be to re-engineer and use the ring donut rubber coupling as once used on hillman imp cars or the polyurethane drive disc manufactured by R and D

You can also get couplings that have an external rubber tyre, made by Centraflex I think 

 

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10 minutes ago, adrianh said:

Alternative may be to re-engineer and use the ring donut rubber coupling as once used on hillman imp cars

 

 

Also used on the Lotus Elan, and they broke regularly. DAMHIK.

 

 

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17 hours ago, adrianh said:

You could ask Vibracoustics in Leicester if this is in there range.

They also may be able to use your old metal plates and mould on new rubber.

They are run by ex metalastic people.

Alternative may be to re-engineer and use the ring donut rubber coupling as once used on hillman imp cars or the polyurethane drive disc manufactured by R and D

You can also get couplings that have an external rubber tyre, made by Centraflex I think 

 

Thanks for all the helpful suggestions. I phoned Vibracoustics this morning in Leicester, and they were going to email me back if they located anything suitable. We discussed making a new coupling but the Vibracoustics chap reckoned this would be serious ££££ (i.e. thousands !), as they'd probably have to make new jigs etc. Although their office is in Leicester (near Syston) their manufacturing facility is down south.

 

In the meantime I've ordered and received a Ford Granada 'doughnut' type vehicle coupling, as recommended on these forums several times by user 'bizzard' (thanks for all your previous information btw). Ordered a genuine QH vehicle part (part number QL9000).

 

 

This Ford Granada transmission coupling has 6 bolt holes with three forming a fixing 'triangle' on each side. Distance between the bolt centres along base of the fixing 'triangle' is about 85mm, which is close to the 83mm I measured on the drive boss on the boat. Not an ideal solution, as the drive to the propeller is going to be transmitted to the doughnut coupling through the fixing bolts and multiple washers, rather than via the original Metalastik coupling steel plates. But the transmission will certainly have a weak spot if the propeller cannot turn...

 

In the meantime I'm also getting a steel coupling machined to the dimensions of the failed Metalastik coupling, to use as an emergency if the QL9000 coupling ever fails. I'll probably also get a spare QL9000 as they're relatively cheap and readily available (by marine standards).

 

This all said, the best solution would be to find another Metalastik 21/642/2 drive coupling... Or get one made by Robush... 

 

Ralph

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On 29/07/2022 at 20:46, Dav and Pen said:

I seem to remember that when we brought Tadworth there was a shear Bolt which was supposed to break instead of the metalastic joint but it’s a long while ago now.

 

 

We had that arrangement on Pisces, the shear pin was made from a length of 3/8" mild steel rod. We had a box of them in the engine room, and it was my job to replace it when it sheared. It took less than five minutes to drive the old one out an drive a new one in,  a lot cheaper than fancy flexible couplings.

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35 minutes ago, David Schweizer said:

 

We had that arrangement on Pisces, the shear pin was made from a length of 3/8" mild steel rod. We had a box of them in the engine room, and it was my job to replace it when it sheared. It took less than five minutes to drive the old one out an drive a new one in,  a lot cheaper than fancy flexible couplings.

 

My thoughts exactly. Why anyone would think a rubber coupling apparently costing hundreds of pounds makes a sensible safety device is beyond me.

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On 28/07/2022 at 16:23, fladda said:

Thanks for all the helpful suggestions. I phoned Vibracoustics this morning in Leicester, and they were going to email me back if they located anything suitable. We discussed making a new coupling but the Vibracoustics chap reckoned this would be serious ££££ (i.e. thousands !), as they'd probably have to make new jigs etc. Although their office is in Leicester (near Syston) their manufacturing facility is down south.

 

In the meantime I've ordered and received a Ford Granada 'doughnut' type vehicle coupling, as recommended on these forums several times by user 'bizzard' (thanks for all your previous information btw). Ordered a genuine QH vehicle part (part number QL9000).

 

 

This Ford Granada transmission coupling has 6 bolt holes with three forming a fixing 'triangle' on each side. Distance between the bolt centres along base of the fixing 'triangle' is a thebout 85mm, which is close to the 83mm I measured on the drive boss on the boat. Not an ideal solution, as the drive to the propeller is going to be transmitted to the doughnut coupling through the fixing bolts and multiple washers, rather than via the original Metalastik coupling steel plates. But the transmission will certainly have a weak spot if the propeller cannot turn...

 

In the meantime I'm also getting a steel coupling machined to the dimensions of the failed Metalastik coupling, to use as an emergency if the QL9000 coupling ever fails. I'll probably also get a spare QL9000 as they're relatively cheap and readily available (by marine standards).

 

This all said, the best solution would be to find another Metalastik 21/642/2 drive coupling... Or get one made by Robush... 

 

Ralph

I doubt the QL9000 will ever fail as long as you don't let it get any oil or grease on it. I've fitted a few including my own years ago and all still going strong.

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I'm with Tony on this, I don't think you need a flexy coupling there, the UJ's om the shaft should be enough so some sort of ordinary coupling should work. If that is not the case then you might have to think again, if that plummer block allows the shaft to move backwards and forwards then some couplings are not good with that movement, Fennerflex tyre couplings don't like it for example.

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38 minutes ago, Bee said:

if that plummer block allows the shaft to move backwards and forwards then some couplings are not good with that movement, Fennerflex tyre couplings don't like it for example.

 

I would fear the cardon shaft might dogleg under thrust, although it seems to work OK on many boats. If that plumber block does not take thrust I would change it for one that does.

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On 31/07/2022 at 09:55, bizzard said:

I doubt the QL9000 will ever fail as long as you don't let it get any oil or grease on it. I've fitted a few including my own years ago and all still going strong.

I went over to the boat last night to try the QL9000 for size. Unfortunately it does not fit into the existing mounting holes of the failed Metalastik 21/642/2 drive coupling - it is several millimetres too small 😞 So the QL9000 part is not suitable as a replacement for a Metalastik 21/642/2 drive coupling. However, there are loads of other similar 6-hole rubber drive couplings available for various vehicles, with a good range of bolt hole circle diameters. 

 

The QL9000 (old Mk1/2 Ford Granada) coupling probably fits the Lister LH150 gearbox set-up OK, which the R+D flexible shaft couplings brochure (910-052) show as being 98.5mm 'bolt hole circle'. I think that the QL9000 coupling is equivalent to the Metalastik Rotoflex (i.e. car) drive coupling 21/1004/1 which has a 'bolt hole circle' of 100mm (and also large fixing holes - to provide some useful flexibility).

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On 31/07/2022 at 20:23, Bee said:

I'm with Tony on this, I don't think you need a flexy coupling there, the UJ's om the shaft should be enough so some sort of ordinary coupling should work. If that is not the case then you might have to think again, if that plummer block allows the shaft to move backwards and forwards then some couplings are not good with that movement, Fennerflex tyre couplings don't like it for example.

 

Nick at Brinklow Boat Services has 'machined/engineered' a solid steel coupling to replace the failed Metalastik coupling. I am going over to collect the steel coupling tomorrow, and I'll be installing the steel coupling during the next couple of days. Hopefully, I'll then be able to escape from Ramsey and get off the Middle levels and back onto the GU via the Nene. 

 

I'll also discuss the thrust bearing situation with Nick.

 

My engine is a JP3(M) and it is bolted very firmly using very thick engine support beams and fixed to the hull.

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On 31/07/2022 at 09:48, Tony Brooks said:

 

My thoughts exactly. Why anyone would think a rubber coupling apparently costing hundreds of pounds makes a sensible safety device is beyond me.

 

Thanks for your comments.

 

I would agree for a coupling costing hundreds of pounds. However, if you can get a good quality (i.e. German) auto rubber coupling replacement for (say) thirty pounds then I'll probably fit one before I reverse any distance on the BCN 😉 Syrinx's previous Metalastik coupling lasted without any problem (apart from becoming obsolete) for over 40 years, before it failed. Syrinx is a Malcolm Braine boat, and I presume that Malcolm was involved in the engineering of the drive etc.

 

My brother has a late 1990s Tooleys-yard build boat (sorry I've forgotten the builder's name). My brother's boat has a very similar drive arrangement to Syrinx - but his boat has a steel adapter plate where Syrinx has/d the Metalastik coupling. Horses for courses I guess...

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The catch with a rigid coupling is that alignments in all planes must be exact,or you will have bending forces ...that inevitably lead to component fracture from fatigue.....You will have to check all around the flanges with feeler strips ,as you do on a proper boat...............the rubber couplings are not a safety device,but rather an alternative for correct alignment.

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8 hours ago, john.k said:

The catch with a rigid coupling is that alignments in all planes must be exact,or you will have bending forces ...that inevitably lead to component fracture from fatigue.....You will have to check all around the flanges with feeler strips ,as you do on a proper boat...............the rubber couplings are not a safety device,but rather an alternative for correct alignment.

 

I disagree with that. There is a cardon shaft with two hooks joints between the rubber coupling and the engine. That will accept angular and radial misalignment.  I don't have a data sheet for that particular coupling but most only allow for a degree of angular misalignment, with no radial. Despite what certain manufacturers claim the only sure way to deal with a degree of angular and radial misalignment is to use a coupling with two flexible elements such as that cardon shaft, the long Centaflex, or the Aquadrive type.

 

What such a rubber coupling may do is help absorb any cyclic oscillations in the drive line, but the worse that would do on this particular setup is to possibly cause more vibration when underway.

 

 

Edited by Tony Brooks
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