Brian Whatcott wrote:

"Heads" for pooping in are often plural: the heads of small barrels
or buckets can also be plural, but the head of a vessel is invariably
singular, is it not?

Today we tend to think of the head as just one device...two heads as two
devices. But that wasn't always true. So I think you're confusing the
multiple holes in the board mounted on the the head of the ship with
multiple toilets--multiple separate devices. Think instead in terms of
an outhouse...it may be a 2-, 3- or even 4-holer...but it's still just
one (singular) outhouse. So too was the board with multiple holes
mounted on the bow--or at the head of--the ship.

Btw...Sir Thomas Crapper didn't invent the flush toilet, either.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1