If you check marine catalogs, you'll find that the "standard location" for most off- the-shelf tanks and OEM tanks on production boats is all fittings in one end of the tank, inlet high, discharge low, and vent high in a corner for as long as holding tanks have been required (which has been 40 years now). Discharge on top with a dip-tube is actually a fairly recent innovation (I first head of it and began recommending it in the mid '90s) seldom used by OEMs 'cuz it costs a bit more and because additional clearance of up to 5" above the tank is needed for hose fittings--space that isn''t always available.
On top with a diptube is my favorite way to do it when there is enough clearance above the tank 'cuz it eliminates standing sewage in the tank discharge line...and whether the discharge fitting can go on the top or not, I also recommend putting TWO discharge fittings in a tank that's plumbed to dump at sea 'cuz it eliminates the need for a y-valve in the pumpout line.
--Peggie
Peggy
The only time I’ve seen (in recent production boats) drains at the bottom are on Beneteaus, jeanneaus, where they have gravity dump tanks. These are typically prettty small tanks compared to what you can fit under a floor or berth.
These tanks are found a lot on charter boats, and they instruct the “captain”, to open the lever when the tank guage reads full.
Well, guess what happens? They just open it, and leave it that way for the whole charter, irrespective if the boat is anchored in a tranquil bay, or offshore.
And the charter based love this, because the boats comeback with empty tanks, and no macerator pumps to ever repair.
On my CS 33, the drain was at the tank bottom. Complete disaster. (I won’t elaborate)
Thanks for your informative posts!!