Knots and rope strength

Hello all...lots of questions here... please help with as much as you
can...:-)

I'm curious as to whether anyone knows how much any given knot (eg
bowline, tarbuck, midshipman hitch) reduces the strength of a rope.
Can one tell by the number of wraps or loops or elbows or something?

I've only managed to find general rules of thumb floating around. It
seems to be that knots like the bowline reduce a rope strength by 50%.
Ok, so what happens if you have two bowlines in the same line? Two
bowlines and the line pressed against a rough angled surface? You get
what I mean...

Where can one find such informartion?

On Mon, 20 Dec 2004 05:50:14 +0800, wrote:

Hello all...lots of questions here... please help with as much as you
can...:-)

I'm curious as to whether anyone knows how much any given knot (eg
bowline, tarbuck, midshipman hitch) reduces the strength of a rope.
Can one tell by the number of wraps or loops or elbows or something?

I've only managed to find general rules of thumb floating around. It
seems to be that knots like the bowline reduce a rope strength by 50%.
Ok, so what happens if you have two bowlines in the same line? Two
bowlines and the line pressed against a rough angled surface? You get
what I mean...

Where can one find such informartion?

I forgot to ask...what exactly is the reason why a knot weakens a
rope?

I forgot to ask...what exactly is the reason why a knot weakens a
rope?

The reasons are not entirely clear, but I can give you an overview.
Look at the part of the knot that takes 100% of the applied load. It's
the part just outside of the knot. As soon as the rope enters the
knot, friction distributes the load away from that part of the rope.
Tests tend to confirm that the area just outside the knot is where a
knot will break.

Next, you see that at this critical location, there is almost always
some curvature that will cause some fibers to be streched more than
others. This causes the load distribution to be uneven, and reduces
its load-carrying capacity. Some fibers are underutilized, others are
overstressed and break.

Also, at this entry point, pressure is applied to the rope as it enters
the knot on its sides. When material is compressed from the sides, it
reduces the tension it can take, although I won't go so far as to say
how much influence this effect has. For more info, Google "Maximum
Distortion Energy Theory". It'll put you right to sleep.

Also, curvature at this entry point causes a little bit of shear
forces. Most materials aren't as strong in shear as they are in
tension. Again, I can't say how much influence this has.

What makes things so uncertain, is that there's not much incentive to
investigate this with advanced methods, and most advanced methods are
geared toward solid materials, not fiberous rope.

As for your other question, there is little reliable data on knot
strength. For any given knot, there are different bell-curve shaped
ranges based on rope material, contruction, age, etc. If you see a
chart that gives you a percentage for a certain knot as if applies in
every situation, it should cause you to raise an eyebrow.

As far as having two knots in series, just think of them like links of
a chain. It's only as strong as the weakest link. It may sound
unprecise, but figuring on a max of 50% reduction on rope strength for
just about any knot isn't a bad approach. The best approach is to use
a rope with a working load limit that is appropriate for the load
you're applying.
*
Cheers,
roo
--
See the Notable Knot Index for my real e-mail address.