When I first took an interest in this song after hearing it at Cropredy in 2011, I found this somewhere about how the song came to be:
"Irving Johnson was born in Massachusetts on Independence Day 1905 and dreamed of the sea and sailing upon it. And for a farm boy, he didn't do badly over the next 85 years. Clearly and totally a man's man, his watchword was wade in, do it, challenge yourself and reap the rewards of immersing yourself in life. His home-made movie "Around Cape Horn"(1929 and onward) with his own narration added decades later caught Ralph McTell's attention and the virility of his description is freely used verbatim in the lyrics
The Peking is now afloat in Mystic Seaport, Connecticut and spent some time in the Thames estuary as a Royal Navy training ship, temporarily renamed HMS Arethusa. Amazingly, one of Ralph's uncles served on her as a boy sailor! Johnson himself is commemorated world-wide as the father of youth sail-training and two beautiful ships bearing his and Exy. his wife's, names sail his beloved Pacific to this day. SN

I don't know who SN is, but all credit to him for the information.

Look up Irving Johnson and the Peking on t' internet and you will find that he (Johnson) sailed round the Cape Horn on the Peking in 1929 under the captaincy of Jürgen Jürs and produved a film of the trip.