Pump out black tanks?

[Hobiehobie]

I have always used “out to sea” emptying of black water tanks. But yesterday needed to get marina to do a pump out. They tried but said nothing came out. I am wondering if my vent pipe is blocked and all they were doing was creating a vacuum.

But on investigation it appears my deck fitting leads to a hose that goes to the top of the tank on my Hylas 54. So how on earth is the pump out vacuum supposed to suck out the fetid contents if it is trying to do so from the top of a vented tank!? Am I missing something obvious here?

[rslifkin]

Many tanks pump out via a dip tube within the tank. Mine is plumbed that way for both overboard and deck fitting pumpout. The advantage is no hose connected at the bottom that's always full of waste (being that all fittings are on top) so it's harder to end up with a failure that leads to a bilge full of sewage.



As far as your failure to pump, does the macerator draw from a different fitting on the tank? And could you tell if the pumpout was just building vacuum or if it was drawing air?

[Hobiehobie]

Yes the macerator that pumps
Overboard pumps from the bottom via separate tube. I can’t work out if the tank is vented and the deck tube just enters the top of the tank how it could ever pump out the contents no matter how much vacuum was applied. All it would do would be draw air in through the vent pipe.

[rslifkin]

The fitting at the top likely has a tube within the tank that runs down to about an inch above the bottom. If it doesn't, either the tube has broken off inside the tank or someone has changed the plumbing previously (and done it incorrectly).

[Bill O]

Originally Posted by rslifkin

The fitting at the top likely has a tube within the tank that runs down to about an inch above the bottom. If it doesn't, either the tube has broken off inside the tank or someone has changed the plumbing previously (and done it incorrectly).

^^^^ this!

You'll need to take the tank fitting off to see what is going on. Never a nice job and unfortunately on top of that, you are full.

Good luck.

[Shrew]

Check to see if there is a Y Valve between the tank and the discharge macerator.

Having a deck plate pump out and a macerator discharge would require two separate discharge lines on the bottom or lower side of the holding tank. Otherwise you would have a single discharge line from the tank to a Diverter valve. The diverter valve would go to either the macerator or the pumpout deckplate.

[peghall]

I strongly suspect that there was a metal pickup tube inside the tank that has failed, eaten through by urine, which is highly corrosive. It may only have holes in it or it could be eaten through enough to fall off. It's a common problem on boats old enough for the industry to use metal "diptubes" and often metal tanks too.

If your tank has an inspection port, you can see whether there's a tube or any part of one in the tank...if not, you'll have to remove the threaded discharge fitting, which will require disconnecting the hose.

Replacing the pickup tube is a fairly easy job: First, go out to sea to dump and thoroughly rinse out the tank. You don't have to fill it, use the washdown pump to add about 6" of water to the tank via the deck pumpout fitting...dump that and repeat till you're dumping clean water.

Next, measure the depth of the tank from the pumpout fitting to the bottom, and measure the inside diameter of the threaded hose fitting. Then buy a piece of PVC pipe that has an OD (outside diameter) to fit into that fitting, long enough to reach the bottom plus enough more to go into the fitting ..you should be able to find it in the plumbing department at any good hardware or big box store. You'll use pvc cement to "glue" the connection. Cut the other end of the pipe at about a 25 degree angle (any shallower will trap TP or solids under it, any sharper will leave too much in the tank). Insert the tube, thread in the fitting, put the hose back on it and you're done.

Hopefully you have enough clearance above the tank to insert the tube without having to remove the tank.

--Peggie

[rwaldrop13]

Originally Posted by rslifkin

The fitting at the top likely has a tube within the tank that runs down to about an inch above the bottom. If it doesn't, either the tube has broken off inside the tank or someone has changed the plumbing previously (and done it incorrectly).

We actually had to open the tank-top fitting when we took possession of Pau Hana to look at this. Hatteras originally built our boat with a bronze pickup tube in the holding tank, but in the 45 years between then and now, someone epoxied a PVC liner/pickup tube into the tank fitting. I've actually had more issues with the tank vent than the pickup tube.

Interesting note to add however... Our area has a vendor who, while not specializing in marine plumbing, is absolutely the best in that regard. They told me to drop a quarter to half a cup of Cascade dishwasher powder into the tank mixed well in fresh water after every pumpout. This has served to break up/desludge the tank, and ensure my dockside pumpouts go smoothly. So I pass that along, since the Cascade (or the generic, doesn't matter) is $3-5 for a 3lb box.

[Hobiehobie]

Thanks. I have inspected the tank previously and ther is not pipe going into the bottom. I will empty rinse and fit a new one. Thanks so much for your expertise all!!! Anyone want to help?

[OS2Dude]

Does your 'Direct To Sea' have a Y valve or something similar? You may need to change the valve position to access the tank contents.

[rwaldrop13]

Originally Posted by OS2Dude

Does your 'Direct To Sea' have a Y valve or something similar? You may need to change the valve position to access the tank contents.

Most US jurisdictions including the USCG will expect your Y-valve to be secured in the closed (dump to tank) position whenever you're in coastal and Inland waters. Just FYI

[Bowdrie]

Originally Posted by Hobiehobie

But on investigation it appears my deck fitting leads to a hose that goes to the top of the tank on my Hylas 54.

Assuming there IS a dip tube.
Just curious, how long is the hose? does it have good routing? could a hose clamp be loose? could the hose have a small leak/split/crack, that would ruin any vacuum from sucking out the contents?
Perhaps at some point their was a Tee at the bottom, one branch leading to a overboard discharge pump, the other branch to a/the deck fitting.

[Mike Banks]

The outflow pipe is supposed to exit either from the bottom of the tank, but more likely through a pipe reaching almost to the bottom of the tank passing through the tank top. Before emptying such a tank it is often a good idea to pump a little fresh water and detergent down the outlet pipe to ensure the bottom is clear, and always pass wastes to the tank through a macerator, and always have a detergent/sterilant added to the tank via heads, which will help prevent the coagulation of wastes on the tank bottom. At sea, the ships motion will help prevent coagulation because waste tanks have anti-surge baffles reaching only about two thirds to three quarters of the way down from the tank top, unlike water tankage where the tanks tend to be larger and the baffles go right to the bottom of the tank and have only limber holes.

Never use a water tank as a waste tank unless it has been modified for wastes--they will always be blocking the limber holes and discharge pipe, and it is necessary for some surge to occur in waste tanks, and some tanks have pumps or impellers to keep waste sludge circulating.