We had a Shannon 28 set up exactly like your system. If you're off shore you can pump straight overboard
, by passing the tank. If you're on shore and pump into the tank, I'm assuming you're close to a pumpout. if you need to pump-out of the tank off shore, I'd go with the diaphragm pump. The ideal would be an electric pump
with a manual override.
A simple option that we used on the Shannon, although a little clunky, was a manual diaphragm pump an a board with two hoses. When I had to I'd simply haul out
the pump and hoses on deck
, screw in the hose to the deck
fitting and the other overboard. Simply pump. Clean out the hoses and store. Never had a problem with it working or smells. Again, it's a simple option.