Galley sink won't drain unless we move the boat

[awaywego]

Is there some kind of pump to suck the water out of the galley sink? Our sink has two bowls that connect underneath with a T and then connect in a single line to a thru hull that is below the water line. If the boat is moving, the sink will drain. If not, it doesn't. Oddly enough, we're docked in a place where there are 6 knot currents between the tides but this doesn't create any flow. What's going on here and how do I get good sink drain flow?

[Mike OReilly]

Our sinks (double in the galley and single in the head). Both drain through thru-hulls just below the waterline. Our galley is pumped with an electric pump. Our head sink is pumped manually with a piston pump.

It's kinda a pita. I'd rather have simple gravity fed drain, but it works fine.

[awaywego]

That's interesting to hear you say that. Our head sink drains on its own perfectly. What is the difference I wonder. I guess back to the boat to do a close comparison.

[TurninTurtle]

sounds to me like the drain trap/loop to prevent water coming in is high enough that you need a bit of venturi effect suction to pull the water out.

[Mike OReilly]

It depends on where the waterline is verses the water in the sink. Our heavy displacement boat sits deep in the water. The bottom of our sinks are at or near the waterline, so they don't drain without assist.

I assume your galley sinks are deep, and sit down close to the waterline. It must be close though if you drain while underway. Does it work one one tack, but not the other? Or are you producing a pressure differential that is drawing the water out?

Your head sink is probably sufficiently above to produce a gravity flow.

[savoir]

Have you pulled apart the various pipes connecting the sinks to the hull to check for blockages ?

[fastinated]

Ours drains above the waterline. If there is any grease in what dishes you clean, it may solidify at the thru hull. Not sure if you can fill sinks with hot water and let them drain.

[JPA Cate]

Awaywego,

Another possibility is that the drain lines are partially clogged with grunge of either galley or marine nature, or a combination of the two. A simple way to partially clear the line is to make your own "snake" out of 7 x 7 rigging wire, using a piece you find on the ground from a rigger, or in a boatyard, or (ugh!) have to buy, and since it's flexible, use it to snake the drain. It will come up grungy or clean, but you shall have learned something. get it a little longer than the measured distance so that you have a little left over to struggle with. If the run is short enough, a heavy gauge wire coat hanger may do the trick, as well.

If the problem is that the sink drain is so close to the waterline that it only drains while you're in motion, then yes, it's being sucked out that way, and you might want to consider evacuation pumps like Mike has.

Ann

[darylat8750]

My thought is a Hunter is not likely to have the sink very close to the waterline. I'm in the sludge build up camp. If you have a shop vac available put the hose on the blow side and blow the drain pipe out. It may take several cycles of filling the drain with hot water and blow.... If that didn't work I would go for the wire snake.

[Kokanee]

It could also be an air lock issue. If the drain line is not direct, and there is any uphill travel in the hose, the lower part may be trapping some air. With air trapped in the hose, the water column won't have enough hydrostatic pressure to overcome the sea water pressure at the though hull. When the boat is underway, there may be enough agitation in the line to get rid of the air lock.

So ensure the drain hoses are as direct as possible without any rising sections. Also ensure your tee is as high as possible, preferably above the waterline.

[TurninTurtle]

Lets remember there might be a high "loop" in the drain to ensure no water flows into the boat regardless of heel angle and waves....

I wouldn't go altering the drain run (except maybe adding a pump) without very careful consideration.

[Stu Jackson]

Originally Posted by Mike OReilly

It depends on where the waterline is verses the water in the sink. Our heavy displacement boat sits deep in the water. The bottom of our sinks are at or near the waterline, so they don't drain without assist.

.............................

Your head sink is probably sufficiently above to produce a gravity flow.

Mike's right, nothing much more to be said, even with blockages, it wouldn't exhibit the issues described, it'd just take longer.

And you can find the waterline very simply.

[Terra Nova]

Originally Posted by TurninTurtle

...there might be a high "loop" in the drain to ensure no water flows into the boat regardless of heel angle and waves...

Originally Posted by TurninTurtle

sounds to me like the drain trap/loop to prevent water coming in is high enough that you need a bit of venturi effect suction to pull the water out.

There should be no such trap or loop.

[Stu Jackson]

Originally Posted by Terra Nova

There should be no such trap or loop.

Absolutely correct.

Traps are in houses and buildings, not on boats.

This issue is simple fluid dynamics.

Yeah, I know, most of us GLAZED OVER when THAT subject was raised in high school.

But like simple arithmetic that lets you balance your checkbook, it is pretty simple.

The air around us has PRESSURE which forces things down. If the bottom level of the sink is ABOVE the waterline of the boat, the sink will drain, because the atmosphere is pushing down on the water both inside and outside the boat.

Connect a clear hose to any thru hull, open the thru hull and hold the hose up vertically. Where the water stops IS your boat's waterline.

I gotta go back and balance my checkbook!

[Cheechako]

The closer the waterline is to the sink bottom the slower it drains. I worked on a powerboat once that the cockpit deck/drains were only ~1/2" above sea level. When it rained the cockpit would get up to a couple inches of water in it. on a dry day you could never get that cockpit floor totally dry. Sometimes the rain would build up faster than it would drain!
To solve your problem you may have to install a macerator pump in the sink drain... above the bottom of the sink level maybe...