Holding Tank Wont Pump from deck fitting

[drpaoletta]

I'm having issues pumping the sanitary tank from the deck fitting with vacuum equipment available at CA docks. I can pump the tank overboard using the macerator pump, the discharge line does not have a selector valve (deck/overboard) and I am able to add water to the tank through the deck fitting.
- It occurs to me that the vacuum is trying to pull fluid through the pump discharge up to the deck fitting - do I need to run the macerator pump while sucking through the deck fitting? I'm stumped.

 

[Peggie Hall HeadMistress]

You posted the same question on Cruisersforum.com...since not everyone here is also a member there, I'll repeat my answer here:

Your tank vent is blocked, causing the pumpout to pull a vacuum that prevents anything from being pulled out of the tank. It's a common problem on Catalinas because Catalina doesn't (or didn't when your boat was built) run the vent to a thru-hull, they run it into a rail stanchion that has only a few very small holes on the aft side of it...and even dust and pollen can plug up those holes. If you're lucky, a previous owner has corrected that by rerouting the vent line into a thru-hull.

Whatever the situation with the vent, it's blocked. The most common locations for vent line blockages on a sailboat are the previously discussed vent thru-hull and the connection to the tank--both the tank vent fitting and that end of the vent line. Clean all of it out using a screwdriver blade, ice pick or whatever works and you should be able to pump out.

 

[Capt.JDavis]

Agreed. Very good advice by Peggie...

 

[Capt jgw]

Or it could be a slight leak in the pump out line, allowing air to pump instead of the tank contents. Does the dock pump sound like its sucking air, or vacuum locked? If nothings moving at all, it's probably the vent as mentioned. Usually caused by a chunk of TP being forced up the hose when the tank gets full. Prevention is to always pump out before its full, but that's hard to do.
On one of my previous boats the vent fitting was behind a linen cabinet that had to be disassembled to get to the fitting. 62 screws and 17 pieces of wood. I could have killed the guy who designed that.

 

[Sailvayu]

If you can pump it out using the macerator it may not be the vent but rather a air leak into the deck line. I have found that some dock pumps can be a bit weak as well. Maybe try another dock. When trying to pump the tank see if you can see the top of the tank, is it collapsing a bit? Pull the pump out fitting off the deck fitting while pumping, does it seem like there is a vacuum there? If so could be the vent, if not may be something else. These are clues to what is going on. If there is no vacuum in the tank or at the fitting there may be a leak somewhere else letting air in. Go below while pumping and listen for a hissing of air. Like most things you just have to use logic and follow the clues one at a time. Never just assume it is just one problem you have to follow the clues till you find the problem.