Looking at the National Archives seaman's records throws up 2 records for
John Boyd in Manchester one
born 23rd September 1891 and the other also on Ancestry for
22nd May 1891 so the plot thickens.
Have you checked 1939 Register for John born dob 23 9 1891 he has a wife.
On his seaman’s record his occupation fitter & turner
Found this record no date of birth given
John Boyd 20 prisoner admitted 27 Nov 1911 in Cork Ireland for stealing goods
Ht 5 6, brown hair & Eyes , complexion fresh occ soldier
Tattoo heart & flower ? Left forearm pierced heart . Death before dishonour back left wrist
Mother named as Mary , address Hamilton Street Manchester