bathroom sink can flood cabin?

[InetRoadkill]

I'm curious about bathroom sinks in a sailboat. Mine is located next to the port side hull wall. It's rather apparent that the sink will be below the water line when heeled which would make for an unpleasant situation if the drain seacock is left open.The alternative is to drain into a holding tank. But that would likely be a source of odors. Is there a better way?

[alan white]

A Whale foot pump to drain the sink, and a vented loop higher than the heeled waterline should solve the problem. This also allows the through-hull to be above the resting waterline, so the threat of sinking from sea cock or hose failure is diminished when the boat is sitting upright.
It's not as convenient as gravity drainage, but safer in cases where back-siphoning is possible.

[InetRoadkill]

I cheated. I decided to drain the sink into the toilet bowl. It has a pump and a check valve.

[marshmat]

I wouldn't trust a check valve on its own, without a vented loop....

[PAR]

Put a trap in it, then down to a tank, or a vented loop to an over the side drain. The trap prevents odors, the loop doesn't need one, because it drains completely.

[InetRoadkill]

I can't make it drain over the side. The sink is too low to the water line. Any drain low enough to allow the sink to drain outside would flood the interior when heeled. A foot pump and check valve would work with that arrangement, but the plumbing for the head is already messy enough without adding more pumps and valving.

For now, I've decided to drain the sink into the toilet bowl and using its pump to flush the sink water into the holding tank from there.

The head arrangement looks something like this:

[PAR]

With a vented loop, it will not flood the boat, assuming the loop is tall enough. If routing the sink as you have, to the tank, put a standard "J" trap in the drain, to prevent obnoxious gases from coming out of the tank.

[InetRoadkill]

The sink drains into the toilet bowl which is open to room air. A trap would do nothing.