Recent UK radio 4 quiz program "Quote, Unquote" claimed Ewan McColl said it. I somehow doubt he was the source, Stan Hugill always said it though he may have had the number different. Masefield would have said it but in print? Captain Whall &/or R.R.Terry may have put pen to paper on this.
One quote I have come up with is William Main Doerflinger "Songs of the Sailor and Lumbermen" page 1 (paper back reprint) - "A shanty" shellbacks say "is worth a hand on the rope" - and I don't doubt it was in the 1951 edition (titled "Shantymen and Shantyboys")
as this program only really responds to written evidence anyone with knowledge of this kind of phrase in print earlier than 1951?
and if we all sang it together would it make it easier?