What is the purpose of this plumbing setup? (3-way connection under the sink)

[jtknox]

This might be a dumb question but hey I gotta learn somehow.

I'm wondering what this 3-way connection is for (see photos)? Why does it need to drain into the bilge?

Attached are two photos from from under the sink on my Catalina 250, as well as a photo of the bilge. Under the sink there's this 3-way connection. There's one tube coming from down from the sink and another going overboard, to an outlet a couple fee above the waterline. Those make two make sense to me. The third tube that sticks straight down confuses me though. As far as I can tell, it leads into the bilge. However I have never noticed water go down it (thankfully). The bilge certainly isn't filling up while I'm doing dishes, nor does it smell like my dinner. I don't see any kind of lever on the 3-way connection it like I'm used to seeing on a Y-valve, though even if there was a lever I'm not sure why I would want to intentionally send water into the bilge.

So what is the purpose of that that third tube (leading into the bilge)?

And less important but also curious how does that 3-way connection piece work to seemingly allow water to pass into the overboard tube without going down into the bilge?

 

[dlochner]

It is a fairly common practice to route several drains through one through hull. In your case, I suspect there was an electric bilge pump or at least the intention to install an electric bilge pump and use the sink drain as the outlet. A confounding factor in your installation is the shallow bilge, it requires a really short pump. The only pump that I know of that is designed to work in shallow spaces is made by Whale.

https://defender.com/en_us/whale-supersub-smart-ic-automatic-bilge-pump

 

[Davidasailor26]

The only pump that I know of that is designed to work in shallow spaces is made by Whale.

Or a self-priming diaphragm pump that can be mounted remotely, like the Whale Gulper - https://defender.com/en_us/whale-gulper-320-high-capacity-bilge-pump

 

[dlochner]

Or a self-priming diaphragm pump that can be mounted remotely, like the Whale Gulper - https://defender.com/en_us/whale-gulper-320-high-capacity-bilge-pump

That is correct, and I have one in Second Star. I was suggesting a more frugal solution with the Super Sub pump. Both good pumps.

 

[Project_Mayhem]

Do you have a foot pump? I'm not entirely sure how it's configured on my C30 but when I tried using it, water came up through the sink drain. Your plumbing might be similar

 

[rgranger]

(snip). However I have never noticed water go down it (thankfully). The bilge certainly isn't filling up while I'm doing dishes, nor does it smell like my dinner.

I would guess there is a check valve someplace in the line.

and what is this other line (red arrow)?

 

[Scott T-Bird]

I don't see any sign of a check valve in the black y-fitting and I've never seen a fitting like that before. It would be helpful if you could describe the fittings and orientation in a bit more detail. It looks like you have the sink drain connecting to a white elbow that points down leading toward the black y-fitting. It looks like the black hose on the downhill side of the y-fitting goes to the thru hull to drain the sink. I suspect that the sink drain flows by gravity to the thru hull and doesn't back up into the bilge simply by the orientation of the y-fitting.

In the bilge, it looks like you have a larger hose end (at the bottom edge of the photo) that feeds a manual pump in the cockpit so that you can manually pump out the bilge in an emergency when you can't leave the helm. My Catalina has exactly the same type of manual bilge pump hose end at the aft end of the bilge. I suspect that you have an electric diaphragm bilge pump somewhere under a seat cushion that pumps water thru that y-fitting to the same thru hull as the sink. The diaphragm pump is probably typical of Catalina installations (mine is the same way) that runs a simple hose into the bilge without any screen or foot strainer. Catalina seems to use self-priming diaphragm pumps that are removed from the bilge as opposed to submersible pumps that are mounted inside the bilge and have to be submersed in order to pump out the bilge.

You should be able to locate the two separate bilge pumps (manual in the cockpit and electric diaphragm pump in the cabin) relatively easily if you have them. Assuming you have an electric bilge pump, pour water into the bilge and find out where it discharges. You may find it discharges at the same location as your sink drain. If not, then it must drain somewhere else and you still have a mystery about that hose that leads into the y-fitting.

Catalina does some funky plumbing it seems. If you have separate manual and electric bilge pumps (I have both) with separate hoses leading to separate thru hulls, that's a good thing. If your electric bilge pump goes thru a fitting that is common to the sink, that's a bit funky in my opinion, but I see no reason why it wouldn't work as long as the fitting is oriented the right way. If the fitting isn't properly oriented, I could see how the sink could be backing up into the bilge or vice versa as @Project_Mayhem experienced.

I have a thru hull that goes both ways. It serves the shower drain pump forcing water out (an anti-siphon fitting is required) and it feeds the pump for the anchor washdown. It used to also feed the toilet raw water supply until I changed the toilet to freshwater supply. I think it is a bit funky but it works! Every boat builder must have their quirks!

 

[Scott T-Bird]

I would guess there is a check valve someplace in the line.

and what is this other line (red arrow)?

View attachment 237744

That line feeds the thru hull. I really doubt that Catalina would install a check valve anywhere between a sink drain or a bilge pump and the respective thru hull. I suspect this works, assuming one of the feeding hoses is from the electric bilge pump, simply by gravity and the orientation of the y-fitting. Both input lines feed from above and the discharge line has to be pointed down.