I think you're all missing the important question... WHEN was 'Blow the man down' written?

It seems that 'speaking tube' is the landlubbers version whereas mariners, ancient or otherwise, called it the 'voice pipe'. Wikipediment has this (amongst other stuff) to say about voice pipes: "About 1780, one captain removed a canvass voice pipe installed by an imaginative midshipman saying he was sure the topmen would "use it for an improper purpose"[1]."

So... we have a documented early dating of 1780, about 20 years before the first commercial steam boat was built. HMS Belfast built in 1954 had voice pipes in it, so that's some 170 years of use.
Here's some more info... and this is about the Black Ball Line mentioned previously.

This seems to indicate that 'Blow the man down' comes from the early 1800's, 20 years before our documented dating of the voice pipe.

Any advance?

LTS