I have a boat with 2 each 380hp 454s, a 7Kw Westerbeke genset, and air
conditioning. (no consumption jokes, please)

I run this in Fl. salt water. Because of the waterlift muffler systems
on the engines and the gen, I can't use the Mercruiser fresh flush
inlets on the engines. They need to be run, and fresh water must
replace salt water at the pump intakes. Same for the air conditioner
pump.

I'm thinking about rigging a water tank in the engine room that would
have hoses attached and then have them Y into the water pickups,
probably after the sea strainers, with a Y valve, or gate valves.

I would only run one engine at a time, and only long enough for it to
push fresh through the system. Outside of just trying it with a
bucket, or something, what's the easiest way to calculate the
anticipated water consumption for this engine at say, 900 RPM, and for
the Westerbeke? The A/C will be a sub set, for sure.

The tank could be refilled between engine flushes, but I'd rather get
one that could accomodate everything. Another angle would be to tap
into the ship's 70 gal fresh water tank, but that's across the vessel,
and the runs would be long.