......The boat has forward and aft fresh water tanks. I believe the two way tap in the port side bathroom cabinet (in the photo) controls the water flow from the tanks.
Q1. Does one position drain only the forward tank and the other drain only the aft tank and
Q2. if so which way is which?
Q3. Unfortunately none of the clean or dirty water plumbing system is labelled on my boat....
Hi Simmo
I've slightly rearranged your query in the quote above in order to answer it, and in answering it I will also refer to your picture where needed.
In answer to Q1. Yes, one position will drain only the forward tank while the other will drain only the aft tank.
Regarding Q2, with the handle in the vertical position as shown in your photo, water is being drawn from the left blue water pipe. You will have to follow that specific blue water pipe to see which tank it is connected to in order to determine whether it is the forward or aft tank that is being pumped.
As for point 3, actually there is labelling or rather marking already on the valve shown in your photo which indicates what is happening, but only if you understand what that marking is telling you.
Look closely at the picture and on the valve handle you will see an "L" shape marked on it in black. That "L" shape indicates the channel within the valve which will allow water flow through it. In your photo that channel is shown connecting between the left blue pipe and the valve outlet leading to the filter above left and onwards to the pump. If you then turn your valve handle clockwise to the horizontal position, the "L" shape would now indicate connection between the right hand blue pipe and the valve outlet to the filter and on. Your only remaining problem is where does the left blue pipe come from, and where does the right blue pipe come from. Well the answer is that one comes from the forward tank while the other is from your aft tank, but in order to determine which is which, all you need to do is to follow one or other pipe to see which tank it comes from, and it's odds on likely that if one leads forward that it will be the pipe from the forward tank, and the other then must be from the aft tank.