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)
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.
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.
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