Post by newsI've never seen what, if any, cargo those merchant ships carried back to
the US, Canada or wherever they originated from after they unloaded in
Britian
Not much, because Britain was not exporting much after 1940 -- only
such exceptionally high-value as Scotch whisky, cf.
https://en.wikipedia.org/wiki/Whisky_Galore_%28novel%29
Returning freighters in ballast also carried occasional passengers
and one-off shipments, e.g. German aircraft for examination in
N.America, early British jet engines for testing etc. It was a
small convenience that returning empty freighters could steam
a little faster than when loaded: but many tramp steamers of
the 1930s could not make more than 12 knots.
--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)