Page 204 - J. C. Turner - History and Science of Knots

Studies on the Behaviour of Knots 193

straight through the knot, with no bight in the load-bearing side of the knot;
this is confirmed by some early tests [3] showing that a Round Turn and Half
Hitch [5, #1834] and a Fisherman's Bend [5, #1840] were both stronger than
a Bowline [5, #1846] when used to fasten a manila rope to a standard pin.
Tables 2, 4 to 7 show that these are good generalisations, but with exceptions,
even within the few knots tested. The Bowline shares its basic structure with
the Sheetbend, but consistently has a higher efficiency (Tables 2, 4); the only
difference in practice would seem to be that in the Bowline the load is shared
between the legs of the loop, but in the Sheetbend that load is on one cord
only. I have seen no extensive accounts of just where the breaks occur in these
knots, and these results remain difficult to explain. My overall conclusion from
this work is that, if the data may be held to be reasonably reliable, then knot
efficiency is not simply a function of the knot, but rather a function of the
knot tied in that particular kind and size of rope. Therefore, whenever it is
important to choose a knot of high efficiency for any purpose, published figures
can only be used as a guide if the kinds and sizes of the rope to be used are
similar to those in the tests.
Mechanism of Knot Action: In 1864 the Alpine Club Special Committee
[1] claimed that the rope broke at he knot for two reasons: as they cross each
other, the parts of the knot are strained suddenly at the point of crossing, and
one of them is cut through; and the rope is so sharply bent that the outer
side of each curve is much more stretched than the inner, so that nearly all
the strain comes on only one side of the rope. One version or another of these
reasons are still stated from time to time. Day [16] commented unfavorably
on assertions that the breaking strength of a knot depends on the radius of
the sharpest curve within the knot and that the outside fibres on a curve are
the first to break. Budworth [11, p. 301 claimed that the sharper the curve
and the tighter the nip, the point of frictional pressure within the knot where
a sharp turn causes the parts to grip each other, then the more chance there
is that the rope will break. Blandford [9] stated that the strongest knots have
their parts taken in easy curves and use a minimum of them. Chisnall [15]
said that knots with greater bight radii tend to be stronger than knots with
sharp bends and angles. Ashley [5, #142, 143] quoted one of the `laws' of
knot strength as `the strength of a knot depends on the ease of its curves' and
compared the common method of breaking string by hand with the Bowline
Bend [5, #1455], believed to be one of the best of the hawser bends. Both
have two cords turning sharply 180° round each other, but one is thought to
be weak, the other strong; Ashley concluded that the `so- called law' did not
fit this particular case. An early test [3] of fastenings to a standard pin showed
that ordinary hitches, with a sliding knot on the standing part, broke where
the standing part met the pin; a Bowline broke at the inward part of the
turn of the knot; the strongest fastening was an eye splice. But none of those