strange head sink issue on Pacific Seacraft 34

[sh3rbet_]

my dad's got a strange issue on his Pacific Seacraft 34, and we're hoping someone here has some ideas.
Here's what happened, when we're sailing on a port tack, we've noticed that fresh water starts to back up into the head sink.

It's not a huge amount, but it's enough to be a problem. What's really weird is that it doesn't happen on a starboard tack, and the galley sink is completely unaffected.
We've already checked to make sure the seacock for the head sink is closed, and we've confirmed the water is fresh, so it's not a saltwater issue. We're stumped.

Any ideas on what could be causing this? Has anyone seen something similar on their boat? Thanks for any help!

 

[folded&hung]

Yeah, based on info we have right now, I believe it's a venting issue or a low loop somewhere that's letting water shift when you're heeled over. Maybe check the freshwater lines and any shared vent hoses near the head sink. Something might be backfeeding.

 

[littlewarrior]

Yeah, based on info we have right now, I believe it's a venting issue or a low loop somewhere that's letting water shift when you're heeled over. Maybe check the freshwater lines and any shared vent hoses near the head sink. Something might be backfeeding.

Agreed. If it only happens on a port tack and it's fresh water, it's likely a vent or plumbing line near the head sink is positioned in a way that allows water to backflow when the boat leans. Could also be a missing or failed check valve. And yeah checking the freshwater lines is a good idea, especially ones that might dip below water level when the boat heels.