Holding Tank Won't Completely Pump Out

[Dean Strong]

I've been treating the 12 gallon holding tank in my new-to-me '90 Hunter 30 with KO, and went to pump and rinse the tank several times. By holding a big flashlight on the tank, I can see quite clearly how far it pumps out...and after about emptying it 2/3rd,the pump station at my marina doesn't develop enough suction. You can see the contents in the site glass move up the hose, then drop back down in rapid cycles after the tank is about 2/3rd empty. Any ideas?

 

[Debra Blatnik]

Try a pump-out adaptor?

There may be other problems, since the pump can get the first third with no problem.BoatUS sells "Sea Land Noz - All Pump Out Adapters" for 10 bucks. It is supposed to aid in creating enough of a vacuum to correct these problems. I haven't used one - maybe someone else has - but is seems like a cheap enough experiment.Can get out beyond the 3 mile limit and pump overboard?

 

[Peggie Hall/Head Mistress]

How far from the bottom of the tank...

Is the discharge fitting? The pumpout will lose its prime as soon as the contents level drops below the top of the discharge fitting, causing it to start sucking air...doing exactly what you describe. Typically, that leaves about an inch in the bottom of the tank, but if the discharge fitting is higher, that's as much as the pumpout will get. So that's the first thing to check. If the pumpout isn't emptying the tank to the top of the discharge fitting, something else is going on...but if you see that the flow is "breaking up" when you look at the sight glass, that almost certainly has to be what's happening.