While sailing my Tanton 39 Lunacy south to the Caribbean a few seasons ago my crew and I noticed there was some sandy grit in the fresh water running out the taps when the boat was bouncing around in rough weather. The operative theory at the time was that this crud was just what it appeared to be: sand. We reckoned at some point in the boat's long cruising career, which included a westabout circumnavigation, some sandy water had come aboard in some remote tropical port. The sand had then settled to the bottom of the tank, where it sat until it got stirred up by the boat’s motion.
The following summer, after moving the boat north again, I decided to open up all three water tanks and try to remove the sand. Getting into the water tanks was easy. The inspection plates are under the settees and nav seat in the main saloon, with access hatches right over them.
I was a bit mystified by what I found inside. The walls of the aluminum tanks were bright and clean, but there was indeed a build-up of crud at the bottom of each V-shaped tank--a bit of brown schmutz, plus an accumulation of what appeared to be sand. But was it sand??? Though the tank walls looked good, I wondered if this "sand" was the result of some sort of corrosion.
Cleaning out the tanks was easy. They are well designed, with the lowest point in each tank right under its inspection plate, so flushing out the channel under the baffles and then vacuuming out all the crud was quick work. I hoped this would solve the problem, but a little over a year later, when I opened the tanks again, I found more sand inside.
On doing some research I learned that my sand is the result of the tank interiors coming in contact with chlorinated water. I have never treated the water on Lunacy with chlorine, as many cruisers do, because I have always assumed that chlorine cannot be good for aluminum. But many public water supplies these days are treated with chlorine, and this water, if left for prolonged periods in aluminum tanks, will cause deposits of aluminum oxide to form.
To solve this problem, I’ve put together a filter kit that cost all of $50 with bits and pieces I found at Home Depot. A stock home water-filter housing and some charcoal filters for removing chlorine, plus some adapters that allow me to plumb in a dock hose, should be all I need to keep that damn sand out of my tanks in the future.
If you have aluminum water tanks on your boat, you’d do well to put together such a rig yourself and run all the water you’re taking aboard through it before it hits the tanks. If you need to treat the water on your boat to keep things from growing in it, you should use peroxide rather than chlorine. Peroxide has a longer retention time and will not attack aluminum.