We have valves at each water tank which is standard.  If you close both of them, no water goes into the system, and the freshwater pump runs because its starved of water.  Great.  That's how its supposed to be.

Now, the weirdness.  When we close the valve on only the starboard tank, and proceed to use the bow tank, our water level in the starboard tank goes down as well.  Not as much as the bow tank, but definitely is not staying at 100%.  How is this possible?  There is only one way for water to come out of the tank, and its through that valve that is closed.

I validated that the valves do close properly and is why the water pump runs dry if I close them both.

What am I missing?

That's odd. 

When the valve on my starboard tank is closed, the tank remains full as the forward tank is emptied. As expected. 

I have noticed that if I open the starboard tank valve, and close the forward tank valve, the water pump is starved. It appears on my boat that the starboard tank drains into the forward tank and then on to the fresh water pump. Not sure if your boat is set up the same way or whether it sheds light on your mystery. But I'd be curious if your water pump will be starved if the forward valve is closed and the starboard opened. 

I had thought that too, but the valves I turn off are AT the tank,  The tank has no other output other than the fill spout which goes to deck.

I am close to another fill-up and will try the reverse and see what happens.  

I have been trying to figure out which tanks drains into what tank.  It would make sense its the bow since its so close to the pump but yet from what you are saying its the opposite. 

I was trying to have the practice to drain one tank fully and then switch over to the other tank.  I cant have both valves open cause eventually we suck air when one is almost empty and the other isnt.

The tank valve  is about 2" away from the tank, so no possible alternate connection.  

When I look at the diagram on the Hanse manual, it doesnt show a second water line between the two tanks.  I guess I would need to expose the WHOLE tank and look around it to see if there is another possible connection between the two that is not documented.   I havent seen such a thing yet when poking around in the water system.  I will check the tanks again and see.  

And as far I can tell, the forward tank is the only tank with an outlet to the pump. 

If the forward tank is empty, the entire contents of the starboard tank will drain forward with the starboard tank valve open. 

Perhaps your starboard tank valve was not fully closed?

Interesting.  Where is the input to the forward tank from the starboard tank?  I am going to go look for that.  I am not seeing it on the diagram Hanse provides but it wouldn't surprise me if the diagram is wrong.

The way ours works is if you starve the bow tank and then open the starboard tank, there is a t fitting that empties the starboard tank into the bow tank. Prior to the pump. 

( we have a 505 slot #30)

-Chris

I spent the weekend looking for this water line that runs between the forward and starboard tank.  But, instead, I found that BOTH tanks water lines go to under the bathroom sink in the master cabin, where there is a t-connection right before the water pump.

It is odd that perhaps some models were plumbed differently.  On the freshwater diagram in the manual, I dont see a connection between the water tanks except at the junction before the water pump.

This really is a mystery.

Sigh.

My aft tank's blue water outlet line indeed terminates at a t-fitting just before the fresh water pump - as reported by Zara and Chillios and as shown in the schematics. And after closing the forward valve and opening the starboard valve, I get plenty of supply to the water pump. 

So I've been misinformed about how my system works - the starboard tank can drain forward to the forward tank - if both valves are open through this t fitting. Minor issue - but I don't like spreading misinformation. 

(No idea why once upon a time a couldn't pump water directly from the starboard tank). 

And no further clues as to Zara's mystery.

Gotcha.  Glad to see this.  

Now I am wondering if maybe these valves are not completely sealed somehow.  While both of them closed would starve the water pump, there is probably "some" water still going through the system, just not enough to keep pressure.  So, during normal operation, we will still possibly draw some water from the closed tank.  

I am not sure how else to explain what's happening.  

Currently, I just refilled my tanks and I only have the starboard tank open, and the forward tank closed.  I am going to test if the forward tank level goes down from 100% while drawing from the starboard tank.