Holding tank overflow into bilge?

countrygent5201 · #1 (**permalink**) 12-01-2009

A week or so back I motored my Hunter 28.5 over to the head holding tank pump-out station because I figured with a capacity of just 12 gallons it had to be pretty close to full. The guy operating the pump said nothing came through the hose, so the tank must be empty. I knew this wasn't possible, so today I decided to make sure the tank was full by pumping it full. Big mistake. My boat has an electric toilet, and my plan was to pump water through the commode into the tank until it backed up into the toilet bowl, then take it back to the pump-out station. The tank NEVER filled, but every time I stopped pumping water into it I could hear water running under the deck. THEN I heard the bilge pump kick in! For the next 10 minutes, a stream of blue water spewed out of my boat into my (and surrounding) slips. Luckily, it was rather chilly here, so no one noticed (I hope). I immediately motored back to the pump-out station, only to be greeted with a sign saying that the marina office was closed on Monday and Tuesday, and power to the pump had been shut off. I puttered about in the cove until I was certain the bilge was empty, then back to the slip. There was still a half-inch or so of the foul blue stuff in the bilge which had to be scooped and sponged out. By hand. My question is: Is it normal for an overfilled holding tank to discharge into the bilge? I'm not talking about the engine bilge, but the small bilge under the salon deck. I noticed there is a small (3/4" dia.) flexible tube that goes into that bilge, but don't know where it originates. Any help would be appreciated.

n0w0rries · #2 (**permalink**) 12-01-2009

Typically you got a holding tank and it's plumbed intake, discharge, vent.

You've got a leak somewhere. Inspect the tank and hoses.

countrygent5201 · #3 (**permalink**) 12-01-2009

Thanks. I found a diagram of the plumbing system and it shows a hose going from the holding tank into the bilge. I wonder if that could be a vent line -- the 3/4" flex tube I mentioned? Since I was running water in pretty fast, is it possible that I pushed some out through a vent line before it could back up in the toilet bowl?

mitiempo · #4 (**permalink**) 12-02-2009

I`m pretty sure that no boat was designed and built with a holding tank venting into the bilge - we would have heard about it by now.

But I would love to see that diagram. The vent should go constantly upwards with no dips to the vent which is usually near the top of the hull. As messy as it will be you should trace all the lines and check every hose clamp while you`re there. Follow them from tank to wherever they go. I am a firm believer than when you get a boat you should explore every hose or wire run and installation to find out where they go and what the previous owner has changed - it pays off in less problems. Besides you`ll be down there cleaning anyway.

n0w0rries · #5 (**permalink**) 12-02-2009

Your holding tank and fuel tanks should vent outside the hull. If it's not that way, you should fix it!

Whenever you flush the toilet, you push waste water in to the holding tank, and that displaces foul smelling air that goes out the vent line. I wouldn't want that nasty air in my boat.

I also wouldn't fill my holding tank until it overflowed, as bad things can happen. At least you found out now, and not in the middle of a long stay onboard--wondering why your boat smells like an outhouse.