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brain wrote:

...and I can't see any reason to go
through a tank at sea instead of flushing directly overboard.

Oh, reason is options, and simplicity.

"Options?" What options? It offers none...all the waste has to go
through the tank, no way to bypass it. It also means that waste has to
go uphill to the tank, leaving the head discharge hose full of standing
waste. So you have two hoses begging to become permeated. And in your
case, a ball valve that prob'ly won't work because you don't sail
outside 3 miles often enough to keep it from seizing.

A pump in one of the tank discharge lines, a y-valve in the head
discharge line...not much more complicated, and now you have options.

OK, this part I don't understand. Why would a second drain block the
vent(s)??

It wouldn't...one has nothing whatever to do with the other. But Murphy
was an optimist who prob'ly based his law on the typical boat owner's
approach to marine sanitation system maintenance. Tank vents,
especially on sailboats due to waste running out them when heeled, are
highly prone to clogging if owners don't backflush 'em regularly--which
few do (and when the tank pressurizes, it's last thing owners think
of...they think it's due to a clog in the discharge line, often with
disastrous results). Nor do many owners ever flush out their tanks to
eliminate sludge buildup. Worst case would be a blockage in both lines
occurring simultaneously...no escape for displaced air in either
direction--out the vent or down the discharge.

--
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/html/get_ri...oat_odors.html