Fitting a new water tank gauge

Measuring your boat’s water capacity is essential for any cruiser, yet is surprisingly hard to get right, discovers Roger Hughes

- Roger Hughes

Boats are likely to have many instruments aboard, and one of the most useful is that which indicates the exact amount of fresh water in the tanks, especially if the boat doesn’t have a watermaker. But accurate water gauge reading has been a problem on every yacht I have owned.

My 45ft schooner Britannia has two stainless steel fresh water tanks amidships, one on each side. Like most yachts the outer shape follows the curve of the hull and the tanks are therefore broadly triangular-shaped in cross-section, tapering to a point at the bottom.

This shape makes accurate calibration of water capacity difficult for most electronic measuring devices: when the device in the tank measures the water halfway down the vertical side of the tank, the actual capacity is much less than half – only about a third in Britannia’s case.

The remedy would have been quite simple, if somewhat crude, had my system actually worked. It would have just been a question of first determining the exact volume of water in the tanks by draining and refilling, using a flow meter. Then divide the total by four to give one-quarter, half, three-quarters and full. Then refill one quarter at a time, marking the gauge accordingly.

The original system was – note the past tense here – pneumatic (air) operated, which works on the pressure differential as the water level in the tanks varies. It is a simple concept having only two components, and no electrics other than illumination of the gauge. Just the job for a cruising yacht you might think.