Sue, your shutoff/drain valves really interest me. But I'm having a tough time visualizing how they work. Maybe that has something to do with how differently the plumbing works in a 31 (yours) vs. a 27 (mine). So I hope you don't mind a few questions.
Are you shutting off the water as it comes to the water pump? Would the theory be that it would mostly drain back into the holding tank, so that there wouldn't be that much in the line?
Then, what about the drains? I currently "drain" my system by pumping the water out through the faucets. So my drains are the hull outlets for the sink drains. I'm particularly unsure of how a drain in the middle of the line would work. What kind of fitting is it and where in the line does it go? Where does the water drain (the bilge?)?
I ask because your system seems good for anyone who keeps their boat in the water and uses it through the winter (Pac NW boaters, like you, for example). I run my boat through December in the Portland Christmas Ships. For the first time this year, I'm opting to leave the fresh water tank full. I have a bilge heater, and an enclosure around the cockpit, as well as space heaters on board. During Christmas ships, I have a Honda generator on board, so I can keep the board warm even in a power outage. I'm not overly concerned about freezing lines with my set up. But the lines running close to the hull seem vulnerable to outside freezing air temps, and the interior heat may not be a big help keeping them warm. It seems like your setup permits draining the lines to the faucets if needed, without draining the tank itself. Which would make the system significantly freezeproof for a lot of NW winter boating.
If you can offer any other details, that would be great. Thanks!
Gini