Having just found out a week before my boat gets launched that the aluminum holding tank needs replacing I have some decisions to make and the one I've never seen discussed in 6 years of visiting the site is this : Does it matter if the outlet(pumpout fitting) is on the side at bottom or on the top with a "diptube" to bottom of tank? Why I ask is the fact that diesel tank hoses are all attached at the top because then if a hose fails the entire contents (supposedly) of the tank doesn't go into the boat. It seems reasonable then,to say that's the way it should always be with holding tanks also, but I don't know, having never had to replace one before. I think if I have my inlet and outlet on the side of my new poly tank I can get a slightly larger capacity tank in the boat . Thanks in advance, will be ordering a Ronco tank in a few days.
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Top is best choice
But you need at least 5" clearance above the tank to put all the fittings on the top of the tank. Fittings on the top not only prevent major spills, but actually increase the useable capacity of the tank...because once waste reaches the bottom of any fittings on the sides, it can start flowing back into the inlet hose and vent. And there's another consideration: when you spec your fittings, whether on the top or on a vertical surface, put them toward the centerline of the boat, not toward the hull. Reason: when you're heeled, waste in the tank either flows away from the fitting, or toward a hose that runs uphill. Either way, there's no way that waste can overflow out the vent or run back toward the toilet.
But you need at least 5" clearance above the tank to put all the fittings on the top of the tank. Fittings on the top not only prevent major spills, but actually increase the useable capacity of the tank...because once waste reaches the bottom of any fittings on the sides, it can start flowing back into the inlet hose and vent. And there's another consideration: when you spec your fittings, whether on the top or on a vertical surface, put them toward the centerline of the boat, not toward the hull. Reason: when you're heeled, waste in the tank either flows away from the fitting, or toward a hose that runs uphill. Either way, there's no way that waste can overflow out the vent or run back toward the toilet.
Advantage of the dip tube
I switched my pump out port from the bottom of the tank to the top with a dip tube because I had trouble getting the hose to attach without some seepage after a year of use.
I switched my pump out port from the bottom of the tank to the top with a dip tube because I had trouble getting the hose to attach without some seepage after a year of use.
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