Galvanised shackles as used on
smacks from SWALE MALDON WHITSTABLE FAVERSHAM BOSTON LYNN ROWHEDGE
MERSEA TOLLESBURY BRIGHTLINGSEA
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In
E.L. Delmar-Morgan's book 'Cruising Yacht Equipment &
Navigation (1960) he has an interesting chapter on shackles
which he sub-titles 'One of the Cinderellas of yacht equipment'.
He fitted new rigging screws and galvanised shackles to
his 10 ton yacht and it appeared obvious to him that the
shackles were the weakest part. His choice of shackle
size was probably governed by the old problem that while
the pin may fit in one end, the head won't pass through
the other and a smaller shackle has to be used.
He
went to his riggers and borrowed their test equipment
and put a number of shackles, selected at random from
the stores, through their paces. He photographed the results
and they are copied below, loosing resolution on the way
I'm afraid but enough to get the picture.
He
loaded the shackle till it was difficult or impossible
to remove the pin and that reading is given in the row
headed 'Distort' which indicated a top-end working load
even though the shackle had distorted somewhat. Then tested
to destruction. One of the 3/8" bow shackles broke
at a low figure of 1828 Kg due to a flaw which goes to
show the need for a healthy safety margin. (I have converted
his cwt. results to Kg. for our younger readers, but for
those not bi-lingual, call a 1000Kg a ton. I've also added
stainless steel breaking loads for forged shackles as
a modern comparison)
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PHOTOGRAPHS
OF SHACKLES TESTED TO DESTRUCTION. (loads in kg.)
An interesting
feature that emerged was that the shackles all broke in different
places indicating that the proportions and manufacture are both
satisfactory.
He also
experimented with shackles with the pins loose and found that
in all cases the shackles broke either in the threaded portion
of the pin or the threaded portion of the body and with a load
roughly half that shown above; proof that mousing a pin after
screwing it in to the end of it's thread is a good idea.
BS 825 for
galvanised shackles gives Safe Working Loads about half those
above but the tests are for the load to be in the middle of
the pin and tested for snatch loading as found in hoisting tackle.
Mr. Delmar-Morgan doesn't say if the loads in his tests were
spread over the full width of the pins as is the case of figures
given for stainless shackles today (breaking loads shown above
as a comparison) which is a pity. In practice, most shackles
on a boat are cocked over with the load on one side of the pin
which indicates that it would be prudent to err towards the
BS 825 advice of a SWL.
Another
source of information gives breaking loads for galvanised shackles
as half those shown in the above experiment! Clearly, without
the full information on how the tests were carried out and the
quality of the shackles tested it is difficult to make a comparison.
In conclusion
a safety factor between 8 and 10:1 should be used for galvanised
shackles unless you pay stainless prices and buy them tested
to a proof load which is twice the Safe Working Load.
Roger Walker
From an
article first published in the Colne Smack Preservation Society's
Journal Issue No.21
Thanks to
E.L. Delmar-Morgan for the information.
7/10/1999
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