Venting sink drain

[Statistical]

Originally Posted by Nicholson58

Several points we discovered living aboard for eight years.

The typical sink drain on a boat, 1/2” or so is too small. It should be the same as a household drain. 1-1/2 inch.

We replaced our overly complicated, always plugged drain system with a single straight shot to a 1” through hull fitting and valve. Typical fittings and hose leave a step at each joint. The step traps small particles especially coffee grounds. We use a strainer at the sink drain to capture stuff but I must dive with a long bottle brush every couple months.

Occasional bleach in the drain helps.

Agreed especially for the galley sink. Was thankfully surprised our "new to us" Hunter 340 has a 1 1/4" galley drain straight to its own 1 1/4" thruhull. Not sure if that is original or a prior owner got annoyed enough to switch it but thankful either way.

[rslifkin]

I also agree that large sink drains are very beneficial. The hand wash sink on deck on my boat only has a 3/4" hose, but it's got a good 8 - 9 feet of vertical drop to the through hull so it works ok. Interior sinks all have big hoses and have never been slow to drain or clog prone as a result (although mine all drain above waterline). 1.25" hose for the head sinks and 1.5" for the galley sink (and that was factory plumbing on this boat).

[Billberner]

My apologies for writing first and rechecking the system later.

There is no vented loop in the system.

I love Ms Hall’s idea as well as the air admittance valve.

Thanks to all!

[Turtle Blues]

Performance demonstration in plumbing class a hundred years ago showed that a 1/4" line provided enough air for a 1-1/2" gravity line to drain unrestricted. Codes requiring full size vents prevent other problems from arising. If it's not draining with open air behind it the problem is restriction or the actual static water level in relation to the bottom of the sink.

[cory_reinking]

Originally Posted by GordMay

You might consider installing an Air Admittance Valve [AAV], as high up, under the sink, as possible.
An air admittance valve [AKA: AAV, or auto vent] is a one-way, gravity-operated device, attached to a fixture [sink] drain line. It has a mechanism that opens up, to admit room air into the drain, under the force of the negative pressure, caused by water flowing through the drain. When the sink is drained [no flow], gravity closes the valve.

I don’t see how venting the sink drain will improve the flow of waste water through the system. As mentioned by someone else, the vent is to prevent the water in the p-trap from being syphoned out, which would allow air/odor to make its way from the waste tank back up through the drain and into your living space. So the vent is a good thing if you have a p-trap, but it won’t likely improve flow in any way I can see.

[wingssail]

Galley sink drains can be problematic, especially if there are T's in the drain line and small diameter hose or pipe is used.

The issue, in my experience is clogs.

Other than using much larger hose or pipe which I just don't want to tackle, I made sure that the drain hose is a direct shot from the sink to the through hull fitting. All the T's (there are four) come in at a right angle.

I use a straightened wire coat hanger with a small spiral at the end which I can push and rotate down the drain and clear the blockage. This is fast and easy and restores quick flow out the drain.

[sailcrazy]

Our center-mount double sink in the galley has the 2 drains connected by a straight section, then drains vertically to a 1 1/2 " bronze flapper check valve, and then to the dedicated thru-hull. As far as I can tell, it's a factory install. Every time we sail, we need to plunge the drains to get them to drain....and then they work just fine until the next sail. We keep a small plunger just below the galley sink, and only us it for the sinks. In 25 years of sailing this boat, we've never had any backflow/siphon action/seawater coming into the sink(s).