Fit a new tank gauge
Measuring your boat’s water capacity is essential for any cruiser, yet is surprisingly hard to get right, discovers Roger Hughes
Measuring your boat’s water capacity is essential for any cruiser
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 freshwater 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.
A rigid plastic pipe mounts vertically inside the tanks from top to bottom and is connected to a gauge by a thin flexible nylon tube. As the water level varies in a tank, the air pressure changes in the tubes and is recorded on the gauge.
The pressure differences are microscopic, but enough to move the delicate watch-like mechanism in the gauge.
However, my system never worked properly from the day I bought the boat. The gauge frequently dropped to zero, sometimes immediately after a tank was filled to the top, or sometimes slowly over a few days. The whole thing was erratic and unreliable.
Since there are no electronics involved, there had to be an air leak somewhere in the connections or even in the gauge itself. I installed new tubing and even sealed the connections with epoxy, but the thing still didn’t work correctly. I even dismantled the gauges and examined the delicate mechanism in minute detail, even tightening the tiny diaphragm screws, all to no avail. I removed the complete assembly and immersed everything in a bathtub, expecting to see air bubbles to identify a leak. Finally, I even submerged the gauge itself, but still no air leaks were visible. The original manufacturers had long-since gone out of business and I could find no information anywhere on the internet.
I never did discover why the system kept failing, so finally decided to look for an alternative to replace the whole useless thing.
Sighting tube
The simplest method of reading tank contents is a transparent sighting-tube up the outside of the tank with no electrics or gauges that can go wrong.
I considered doing this, but both tanks are fully encapsulated by bulkheads at either end and it would be a major operation to drill a hole in the top and bottom of the heavy gauge steel tanks to install plumbing fittings and a clear sighting-tube. Even if I had managed to fit a tube on the outside of each tank, reading them would mean lifting the floorboards in the saloon every time I wanted to read the capacity.
The shape of the tanks meant that even a simple electrical swing-arm/float sender (available from any car-parts store) would not register accurately, because when
the arm is halfway along its ark and the gauge registers half, it will still not show the volume accurately.
I found a product which works on the same pneumatic principal as my old equipment, but from experience with my own air system I was concerned about leaks over time. I would also need two gauges, one for each tank and the cost for two tanks would be about $570 (£410).
Float system
I then found an electrically operated system, made by Wema/Kus USA in Fort Lauderdale, Florida – its fuel/water tank senders are available from UK chandlers. This employs a sealed vertical, stainlesssteel tube inside the tanks, but instead of working on air pressure it has a float which travels up and down the tube. As the float rises and lowers it activates electrical impulses in the tube which are read by a gauge. Luckily, the tube’s top flange fitting to the tank also matched the standard SAE (Society of American
Engineers) five-hole pattern in the top of my tanks, so I’d not need to drill any new mounting holes – or so I thought.
A major advantage of the Wema/Kus system is that the signals from the tank float can be custom calibrated up the height of the tube.
All the manufacturer needed from me was a sketch showing where the three water levels I wanted were on the tube. This would ensure the gauge would read the correct volume of water in the tank, throughout the whole range.
Kus also have a twin tank gauge on a fascia panel with a switch. When this is switched to one side the gauge reads from one tank, and when switched to the other it reads from the other tank. In the middle, the switch is off and no electric current is being used.
For this gauge to register accurately on twin tanks, both must be the same shape and capacity, which my tanks fortunately are. The gauge also lights up when reading a tank, with a choice between red and yellow illumination.
First test
The first test I therefore needed to conduct was to find out exactly how much water the tanks actually held. I drained both by running them dry through the boat’s sinks, then refilled them one at a time using a flow meter fitted to the water hose ($15 from Amazon, part number P3-P0550). The actual capacity is 160 Imperial gallons each side, a total of 320gal (1,455lt), eight more than the boatbuilder’s specification.
To find exactly how long the new tank sender tubes needed to be I removed the port side tube and measured the depth with a length of ¼in diameter wooden dowel. This measured 31in to the bottom of the tank. The tube would not go to the exact bottom of the tank due to the curvature, so I settled for 29in.
This was when I realised the original tubes could never have registered accurately even when they worked properly, because they were only 21in long and 8in too short to register from the bottom of the tank. I calculated this produced an error of about 25 gallons on each side.
Having determined the exact tank volume, I drained the tanks once more. I then ran 40gal into the tank, being one-quarter of the total capacity, then measured the water level with my wooden dipstick. I poured in another 40gal and measured where half full actually was, then another 40gal to bring the capacity to three quarters. I added these physical measurements on my sketch, which I emailed to the Kus engineers. I received my kit within two weeks.
Installing the new system
I drained the tanks once more, then lowered the port side tank tube through the hole in the top of the tank. I rotated it to mate with the holes and screwed five machine screws in the top, tightening them on the gasket. So far so good.
I’d assumed the two tank tubes would be the same, but as I inserted the starboard side tube an unforeseen problem arose. As I lowered it into the tank the end of the tube bottomed against the tapered side of the tank, and would not go further, which meant the top plate
‘Refilling the tanks with the flow meter it was gratifying to see the gauge registering correctly’
would not sit flat. I took some measurements with my dipstick and discovered the hole in the top of this tank was 8in further towards the hull causing the tube to bump against the side of the tank. There was nothing to be done except to cut a new mounting hole in the same place as the port side tank.
This meant first cutting out a square piece of the saloon floorboarding to gain access to the top of the tank. The float on the tube needed a 1½in diameter hole, but a hole-cutter would not even scratch the top of the 1/16in thick (16 SWG) stainless tank. I was also concerned that fillings from the drilling would fall into the water tank.
I positioned my vacuum nozzle close to the drill bit, and this captured most of the swarf. I drilled a series of 1/8in holes very close to each other in a 1½in circle then enlarged them with a 1/4in drill, which joined the holes together. I was then able to lift the centre piece out of the top of the tank. The resulting hole isn’t pretty, but it is covered by the sender top plate.
Success at last
This time the tube went all the way to the bottom of the tank and I then drilled and tapped the five securing holes. I used the original top plate and a home-made blank gasket to seal the old hole in the tank.
The gauge was easier to fit, but it did need a larger than the normal 2in round hole to mount it. A paper template is supplied to stick in place with tape, then the rounded corners can be drilled, and the spaces between them cut out with a keyhole saw. I wired the gauges through a spare contact breaker on the master distribution panel.
It was then just a matter of refilling the tanks with the flow-meter, in one-quarter increments, and checking the gauge registered correctly in both tanks. It was gratifying to see it stop at each incremental level.
Of course, it’s important always to be aware that the reading from any tank system, including even a simple sighting tube, will be slightly incorrect when a yacht is under sail and even slightly heeled. I suppose the only remedy for that would be gimballed tanks...
Kus make a similar system for fuel tanks, because the same problem exists on any yacht when diesel tanks are also formed to the shape of the hull. At least, when running an engine, the approximate consumption can be estimated by knowing the hours/fuel ratio – unlike freshwater consumption.
It is now a relief knowing Britannia has reliable water gauges, showing the accurate capacity for each tank. This enables me to maximise and enjoy the water usage to whatever length of passage being made.