Sink drain air lock

[Davidasailor26]

The sink in our head is very slow to drain. Until today I assumed it was just because the sink isn't much above the waterline. But then today when it was hardly draining I closed the sea cock for a few seconds. While the seacock was closed a large amount of air bubbles up the drain. Then when I opened the seacock the sink drained very fast. Is there anything that can be done to prevent the airlock other than closing the seacock? Attached is a picture of the drain line (black hose) from the sink to the seacock. The other line that T's into it is from the shower sump pump.

 

[Gunni]

Valve off the shower sump pump hose. Keep it off when using the sink. Next time you haul the boat run a brush up inside the thru-hull. I find my through hulls develop a significant amount of bio- growth. Found a herring in one!

 

[JamesG161]

Is there anything that can be done to prevent the airlock other than closing the seacock?

I see no potential for an "air lock".
I am guessing that you might have had say "hair mat" that when you closed the sea cock you severed the mat and it drained rapidly. Air bubbles from the empty shower sump line, equalizing.

Tip: To find your water water line under the sink, pour several quarts of very hot water down the drain. After a few minutes the hot water filled hose will be warmer than the air filled hose. Mark it with red tape. Of course that is the static water line.
Jim...

 

[Davidasailor26]

Thanks for the ideas Gunni and James. I don't think the issue is with anything clogging the drain, because it flowed fast as could be after the bubbles were cleared. Also, it's not like it was just a clog that I cleared, because it was repeatable. Even after getting it to flow fast, once the sink drained completely I turned the faucet back on, and it drained slowly again until I closed the seacock and let the bubbles percolate.

 

[JamesG161]

I closed the seacock and let the bubbles percolate.

I agree with Gunni post #2 then. You are building water level in the hose to over come the back pressure due to a restriction. If you can block the sump line then a sink "plumber friend" might cure the flow restriction.

My last wild azz guess... you could be back flowing to the sump and then creating a siphon back with you open the sea cock. When the siphon breaks with air from the sump, you get the bubbles.
Jim...

 

[Stu Jackson]

If you can block the sump line then a sink "plumber friend" might cure the flow restriction.

Bad idea. Take the bloody hose off and LOOK.

you could be back flowing to the sump and then creating a siphon back with you open the sea cock. When the siphon breaks with air from the sump, you get the bubbles.

James, How is a siphon in any part of this picture? The sink goes straight down, the shower sump is below it and straight off 90 degrees from the seacock.

 

[Windborne]

Why would you have a seacock on it if its above the waterline? Regardless, you should exercise your seacocks anyway.

 

[Gunni]

I suspect that diaphragm shower pump has the ability to blow more air pressure into your sink drain hose than the head of whatever water you have in the sink can overcome. And there is not much head to begin with. When you shut off the through hull that compressed air escapes up the sink hose, bubbles out, and releases the pressure. Start by valving off the shower pump hose when not in use. And yes, the sink head is not that high above waterline, so it will not drain as fast as the plumbing stack back at your house. In fact the lower 1 ft or so is full of seawater while on her lines.

 

[JamesG161]

Take the bloody hose off and LOOK.

Good idea and check the boat water level too when you disconnect it from the sink.

James, How is a siphon in any part of this picture?

I said a "wild azz guess" because I don't have x-ray vision on what the sump relative level is to the boat water level.
You must know this boat, as I don't.

Other than as Stu said, "check the line" for a block, because AIR will rise in the line pictured, no air trap seen.

Hmmm I wonder where the air could be coming from?
Jim...

 

[Newport Bob]

Dave,
My 373 has the same grey plastic " pan" with a right angle hose connect at the sink. At the thruhull end is a T above the thru hull, but installed in the T orientation, with a right angle fitting on each "wing" facing up, to which the two drain hoses are attached.
I believe my slow drain is due to the pan at the sink - right angle vs straight down like a normal sink. Won't see any bubbles, just like a normal sink, but flow is slowed as air escapes.
If your shower line off the T is horizontal then the large amt of bubbles also make sense - water level equalizing between the two lines. With my plumbing if I close the thruhull then run the water I get air bubbles as the hose fills.