Even a half-full holding tank can
run back toward the head when the boat is heeled if the inlet is next the to hull. When the boat leans, it's like tipping the tank to pour the contents out. It'll also cause it to spill out the vent if the vent is on the "downhill" side when you're heeled. Even if the fittings are toward the centerline, sailing with the tank overboard discharge seacock open can flood the tank with seawater, overflowing it out the vent and back toward the head. It can be prevented when a tank is installed by making sure all the inlet and vent fittings are toward the centerline so that tank contents run away from them one tack, the hoses run uphill on the other...and by keeping the seacock closed at ALL times except when actually dumping the tank. The solution is a) keep the seacock closed except when dumping the tank, and b) a vented loop in the head discharge line that's at least 8-12" higher than the tank at any angle of heel. The best place to put it is immediately after the toilet, mounted on the bulkhead behind the toilet. Come straight up from the toilet (if your head discharge fitting is a straight fitting, call the mfr for a 90 degree replacment) over the loop and back down to another 90 degree fitting and then on to the tank.
