One of the instruments I find most useful on a boat is the gauge that indicates the amount of fresh water in the tanks. If there is one thing you need to know when out cruising, especially if you don’t have a watermaker, it is an accurate indication of how much drinking water you have.
Over the six years I have owned my Down East 45 brigantine schooner, Britannia, I have expended endless hours messing about trying to get the twin water-tank gauges to register correctly.
Britannia has two stainless-steel water tanks amidships, one on each side. The shape follows the curve of the hull and is therefore somewhat triangular in cross section, tapering to a point at the base. This makes accurate calibration of any type of measuring system difficult because when the water is halfway down the vertical side of the tank, the actual capacity is much less than half full — only about one-third full in my tanks.
I considered installing a transparent sighting tube up the outside of the tank, but since my tanks are enclosed by bulkheads on all sides, I couldn’t easily drill a hole in the bottom of them to install plumbing fittings and a clear plastic tube. Even if I had managed to fit a sighting tube in each tank, I still would’ve had to physically mark the tube at various capacity levels. To read it would also have required lifting the floorboards on each side every time.