How weak is a bowline?

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mick

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I've just been reading on a caving site that a bowline can reduce the strength of rope by up to 50%. Is this really true, and if a particular thickness of rope is recommended for genoa sheets has allowance been made for weakening by use of a knot?
 
I think any time you put a knot in a line it reduces its effective strength to half. That's just a rule-of-thumb mind you - it would take a destructive test to get a definitive answer.
 
There was a good YM article on this some time ago.
The figures for the bowline look about right. A surprise item was the strength of the fisherman's bend for joining two ropes.
This chart shows some test results
image008.gif


Here the bowline gives 67% of the line strength.
 
A bowline reduces the breaking strength by about 40% apparently.

Depending on your level of geekery, you could find this interesting. /forums/images/graemlins/wink.gif
 
Reduction to 50% has been demonstrated, but it is generally to about 60-65% .
However, even the best knots reduce strength to about 85%, so for the convenience of untying without tools we are really only losing about 20-25%: and as 'sailorman' points out, a polyester rope just thick enough to handle well usually has ample strength.
Having said that, I did once have one break on my previous boat (trimaran).
We were close-reaching in fitful light winds when an extreme blast struck: the lee bow dived, the crew pulled the genoa sheet free of its jam-cleat but it remained tight on the winch barrel while the bow came up again and we rocketed off in a cloud of spray: then the sheet parted at the winch. After dropping the sail, we found the remainder of the sheet pressure-welded to the (tufnol) winch barrel.
NOTE: the bowline was OK!
 
Re Saltwater_gypsy.
I don't think you would use a fisherman's bend to join two ropes. Traditionally it's used for making a line fast to an anchor or similar.
A sheet bend, or better, a double sheet bend is a much better option.
 
[ QUOTE ]
A surprise item was the strength of the fisherman's bend for joining two ropes.

[/ QUOTE ]

Well it might do for joining them but you'd wish you hadn't used it when it came to separating them. /forums/images/graemlins/confused.gif
 
[ QUOTE ]
A surprise item was the strength of the fisherman's bend for joining two ropes.
This chart shows some test results


[/ QUOTE ]

In the chart it shows the results for a 'fishermans knot' not a fishermans bend.
Two totally different knots.
As mentioned earlier the 'fisherman's bend' is used for attaching a rope to an anchor, but once it has been put under strain you usually have to cut it off because it's practically impossible to undo. And it is not actually a 'bend', it is a hitch, because it's tied around something.
A 'Fisherman's knot' is used for joining to lines of the same diameter together. Traditionally used by fishermen for joining their fishing gut together after it had parted, but it is also used in the form of a double fisherman's knot in the climbing game for joining two ends of the same short length of rope together to make round slings, or for securing the tail of a bowline around the standing part of the knot for added knot security, as well as a few other uses. To add to the comfusion a fisherman's knot is actually a 'bend' because it's a 'rope' (cordage) tied to another piece of 'rope' (cordage). CONFUSED!
 
I would suggest that the limitation of the bowline in use as a jib sheet is really a question of the surface area of the eyelet in the sail that you have attached it to.
Presumably the graphs given would be for a bowling around a large area like a drum so the limit of strength is where the rope holds itself. ie the rope tends to want to cut it self on itself in the knot itself.

But if your rope in the bowline is pulling on a small area eyelet this could act like a very blunt knife to cut the rope. It is most likely the rope could cut in half on the eyelet or ring than on itself. (a much large area or blunter cutting surface)

So if it is rope strength you are worried about you need a thimble at the sail clew to increse area where load is applied to the rope. If you have a big thimble then indeed the rope limitation is its tendency to cut itself on itself. ie strength reduced by 40%.
Thats my theory olewill
 
Thanks Spinney I stand corrected.
Although BENDS are usually the KNOTS used to join two lengthts of rope together, in some cases such as the Fishermans BEND (it is a variation on two turns and half hitches) it is used as a KNOT to attach anchors to warps etc.
The Fishermans KNOT is however the BEND used to KNOT two lengths of rope together and is the KNOT which is remarkably strong (perhaps 75% of rope strength). I agree that it can be a difficult BEND to untie (impossible).
Well I hope that clears up this KNOTTY question and resolves any CONFUSION!!!!!.
/forums/images/graemlins/crazy.gif /forums/images/graemlins/crazy.gif /forums/images/graemlins/crazy.gif /forums/images/graemlins/crazy.gif
 
SailingTV did a feature a while ago on splicing a rope. They then tested it to destruction on a hydraulic rig with a gauge. This was followed by doing the same to the same rope with a bowline in it. It broke at about half the force of the splice. Not sure if this is viewable from their archives or not.

It does make you think though.
 
William H has illustrated a good point, basic for the performance of any knot; the tightness of the radius of the most highly loaded turn in the knot.
Another basic factor is the elasticity of the rope. A highly elastic one (eg. nylon) allows the fibres round the outside of a turn to stretch and spread the load to share it with those nearer the centre. In a very inelastic rope (eg. kevlar), the load is concentrated in the outer fibres, which are then overstressed and break.
Which is why such low-stretch ropes need spliced eyes with adequate thimbles, and should not be knotted. I abandoned kevlar reefing pennants for this reason. Those of us who prefer to bend on our sheets should therefore ask our sailmakers to put good thick cringles in the clews, and make it standard practice to vary the size of our bowline loops to spread the points of maximum stress; and end-for end the sheets regularly; and buy them over-length in the first place so we can chop off the ends when they have ''had it''.
I've been getting 20-25 K miles from a pair of jib sheets.
 
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