Wayne.B wrote:


I'm guessing it might be extra water tankage.

Of course. :-)

IMO:
This is a large cask for gathering water from streams, rivers, or
sources ashore that aren't equipped with a hose or to which the boat
cannot sail. Would be handy on a long voyage.

There is a good sized shoreboat available (see empty chocks to port of
the cask).
The fitting on the top allows the cask to be loaded into or out of the
shoreboat using a block and boom. The removeable cap on end of the
barrel to starboard in the photo allows water to be poured in. We can
assume there is also a spigot somewhere to release water, but that it
doesn't show in this photo.