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Weak Link


eid

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I'd like to make my bow fender safe by inserting some weak links. I haven't decided yet between cutting through one side of the chains, or using cable ties.

 

I have loosened the side chains enough that the fender can easily rise right out of the way. Can anyone see any possibility of this failing?

 

I still need to allow the fender to release downwards anyway. I was thinking of putting weak links in all but one side chain, so that the fender will drop off to one side in a "situation".

 

Any thoughts would be welcome.

 

fender2.jpg.0ff7bf3d2136569a526151f01f1a096a.jpg

 

 

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2 hours ago, captain birdseye said:

I use carabinas, easy to snap on and off and essentially an open link so release when under too much strain

Are these proper mountaineering carabiners, or the sort of thing that is used in a dog lead? Mountaineering krabs have a typical breaking load of around 2000kg and essentially just don't release when put under to much strain. Climbers would die if they did!

 

Jen

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35 minutes ago, Jen-in-Wellies said:

Are these proper mountaineering carabiners, or the sort of thing that is used in a dog lead? Mountaineering krabs have a typical breaking load of around 2000kg and essentially just don't release when put under to much strain. Climbers would die if they did!

 

Jen

i take it you're a climber, if you're using that short form?  And British?  [as opposed to 'biners' on my side of the ocean]

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21 minutes ago, Wittenham said:

i take it you're a climber, if you're using that short form?  And British?  [as opposed to 'biners' on my side of the ocean]

British yes, hence the contraction to krabs, but a rubbish climber. I get pulled off the rock by gravity surges ?. I prefer to go in the opposite direction; caving - spelunking if you're on the other side of the pond.

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4 hours ago, Grebe said:

String to replace a link or a shackle?

 

Yes I suppose a bit of garden string would do the job. Thanks.

 

The carabina would be much the same as cutting through one side of the chain link (except that the link would actually break ?), which I may still do.

 

It seems to work ok with just one side chain intact. It falls off to the side and hangs there.

 

Thanks all.

Edited by eid
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1 hour ago, Jen-in-Wellies said:

British yes, hence the contraction to krabs, but a rubbish climber. I get pulled off the rock by gravity surges ?. I prefer to go in the opposite direction; caving - spelunking if you're on the other side of the pond.

caving... far too scary.  Remember, what goes up, must come down [perhaps not at the desired speed, RIP David Lama et al].  But what goes down, does not necessarily have to come back up...

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1 hour ago, eid said:

 

Yes I suppose a bit of garden string would do the job. Thanks.

 

The carabina would be much the same as cutting through one side of the chain link (except that the link would actually break ?), which I may still do.

 

It seems to work ok with just one side chain intact. It falls off to the side and hangs there.

 

Thanks all.

I thought they were designed to carry load, ether with a screw 55868.jpg

or the way the ends are cut

 Carabiner No Eye Diagram

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14 minutes ago, captain birdseye said:

Just cheap Krabs not the screw type, only lost a couple when the button on the bow has got caught and they gave out under the strain. So they do work as a weak link. They also make hanging the fender and adjusting the length of chain a doddle, far easier than a shackle

Like this:

https://eu.blackdiamondequipment.com/en_GB/hiking-accessories/micron-accessory-carabiner-BD800205_cfg.html

Sort of thing for hanging your keys off.

Not like this:

https://eu.blackdiamondequipment.com/en_GB/climbing-carabiners-quickdraws/positron-straight-BD210153_cfg.html#start=20

No screw gate, but still 25kN breaking load on axis with gate closed. Stronger than the fender chain it is linked to! Designed and tested to stop a falling lead climber.

 

Jen

Edited by Jen-in-Wellies
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8 minutes ago, Sea Dog said:

Well I'm in the camp that says keep it simple and just saw through one side of a link in the chain on one side of the fender. 2 minutes with a junior hacksaw and the job's done.

 

What about the top 2 though? If the fender gets stuck going up in a lock, the cut link at the side won't do a thing.

 

I am leaning towards cutting links though.

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20 minutes ago, eid said:

 

What about the top 2 though? If the fender gets stuck going up in a lock, the cut link at the side won't do a thing.

 

I am leaning towards cutting links though.

Pick a side and do a link in both the top and bottom chain. If it fouls going up or tries to hang up going down, the weak link will open out and the chain that side will part. The other side will hang on to your fender so you don't lose it.

 

Personally, I just have a button so I only have it secured by  upper chains hence it can ride up if it needs to. The chains are also short and attachment to the boat is by rope which is a weak link in itself. I prefer that solution as it's easier to replace the rope than a chain link. Your upper chain points are too close together for that solution though, but you can always use a bit of rope to replace the failed link if you need to.

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55 minutes ago, captain birdseye said:

2 x LARGE Carabina SNAP HOOK Clips - 10mm Dia x 100mm Long - HEAVY DUTY - Working Load 300kg - MEGA STRONG GALVANISED Locking CarabinerLike this

I see now. In the climbing world, that would be called a carbine hook, not used for safety critical loads and a very different thing from a carabiner. In the marine world it seems that carbine hook and carabiner are used interchangebly for the same thing, as in the pic above, with much lower strength than a proper climbing carabiner in the smaller sizes. See here and here. Hence my confusion.

 

Jen

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In my experience its really hard to get this right. At 71 foot we have to drive right in to the end of locks so sometimes (very occasionally) do touch a bit harder than intended, and getting a link to fail when needed but not fail in normal use is difficult. I found cables ties failed much too often. Tried shackles with either the pin or the arch cut half way through but never sure if they would fail when required. Currently have a chain link cut all the way through on one side. This starts to open up after a few bumps and I pull it back into shape every month or two using a little portable vice. Feel happy with this as I am confident it will likely fail if required.

 

.................Dave

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11 hours ago, dmr said:

In my experience its really hard to get this right. At 71 foot we have to drive right in to the end of locks so sometimes (very occasionally) do touch a bit harder than intended, and getting a link to fail when needed but not fail in normal use is difficult. I found cables ties failed much too often. Tried shackles with either the pin or the arch cut half way through but never sure if they would fail when required. Currently have a chain link cut all the way through on one side. This starts to open up after a few bumps and I pull it back into shape every month or two using a little portable vice. Feel happy with this as I am confident it will likely fail if required.

 

.................Dave

Cable ties tend to go brittle with age, particularly if subjected to UV.  Strong is the weakened chain link or rope link that can hang up a (say) 17 tonne boat, so I'd suggest you needn't be too afraid that such a measure won't fail when called upon.

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Cables ties failed very quickly, but they had been in the cupboard for ages so might have degraded. A cut through chain link fails by opening up due to steel bending, and steel does bend easily. My concern about a half cut through shackle (or chain link) is that failure requires the steel to actually break under tension and steel is much much stronger in this mode. After a good bump I have seen a shackle totally deformed but still not failed.

 

...............Dave

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1 minute ago, dmr said:

My concern about a half cut through shackle (or chain link) is that failure requires the steel to actually break under tension and steel is much much stronger in this mode.

Try cutting through the curve of the link, not the straight bit.  The link only then needs to bend a bit, not shear the other side of the link.  If that would give too easily, try partially cutting through the other side of the link to weaken it further.

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I wonder how much weight you actually need to hold on the bow of a 57 foot modern Narrowboat to put the back end under far enough to flood, I bet its not much with boats that have engine vents below the gunwale maybe 10 inches above the water line

 

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16 hours ago, Sea Dog said:

Well I'm in the camp that says keep it simple and just saw through one side of a link in the chain on one side of the fender. 2 minutes with a junior hacksaw and the job's done.

 

I keep it simpler than that. Top chains only, so the fender can slide up if it gets stuck when descending and a cut link to break if the fender gets stuck when ascending a lock 

 

20190125_112241.jpg

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47 minutes ago, cuthound said:

 

I keep it simpler than that. Top chains only, so the fender can slide up if it gets stuck when descending and a cut link to break if the fender gets stuck when ascending a lock 

 

20190125_112241.jpg

I have a similar arrangement, but with the the link half sawn through on one side. This isn't enough to get the chain to break, as I found when the bow fender got stuck going up a Calder & Hebble lock. I'll be completing the saw cut as you've done to weaken them further.

 

1 hour ago, ditchcrawler said:

I wonder how much weight you actually need to hold on the bow of a 57 foot modern Narrowboat to put the back end under far enough to flood, I bet its not much with boats that have engine vents below the gunwale maybe 10 inches above the water line

 

Holding down the bow will at some point flood the well deck, then the cabin through the doors on most modern narrowboats before the engine vents at the back get close to the water.

Some quick calculations,

Not the bow, but the stern. A 57' boat is about 17m. If held down at one end, effectively half that length is held down, so 8.5m (think triangles). The boat is 2.1m wide, ignoring the pointy bits at each end. 10" to an open engine vent is 0.25m. The volume from 8.5x2.1*0.25 is 4.5m3, so 4.5 tons of upthrust from the extra water displaced. Quiet a lot of force, enough to break a pair of weakened chains, but three, or four complete 6mm chains (official breaking load 1.6 tons each, so very close) could well survive and cause the boat to sink.

 

Jen

Edited by Jen-in-Wellies
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