Catching Water

In this day of cruising with watermakers, the art of catching rain water is slowly being lost. But watermakers take power, are maintenance-prone, and no matter how good a job they do, there is just nothing like a fresh glass of freshly caught rainwater.

Aboard BEOWULF we now have two Village Marine “Little Wonders”. These crank out between 16 and 18 gallons an hour, and we typically run the waternaker one to two hours per day. That’s not too hard to take. But consider what we can do with a single, moderate squall.

Catching rainwater

Often the combination of deck drains and holes in the toe rail make it impractical to catch water off the deck. That was the situation we found with INTERMEZZO. In this case, you can use various combinations of awnings to catch your rainwater. The key is to take this into account when having your awnings built.

In the photo above you can see a hose (arrow) pulling down from the awning. An awning catchment system like this can be very effective.

It is not unusual for a squall to drop a 1/4″ (6mm) of water within 15 or 20 minutes-and often a lot more. If you multiply your length times the beam, and then take 70% of this number to allow for the pointy ends, you quickly see there is a lot of surface area to act as catchment. In our case, 78′ x 16.25′ = 127 square feet. 70% of this is 887 square feet. Now, to calculate how much water you can catch for every quarter of an inch which falls, divide the surface area by 48. In our case 887/48 = 18.47 cubic feet. There are roughly eight US gallons of water in each cubic foot of water-so for every quarter of an inch which falls we collect 147.7 gallons of water!

What this means in the real world is that now that the trades have arrived (I’m writing this in the British Virgin Islands in December), and we get hit by a couple of squalls a day, we basically don’t need our watermakers. And years ago, when we were cruising without a watermaker, we went a whole year without ever taking water from shore-we simply relied on the rain water we caught aboard INTERMEZZO.

Let’s check the numbers for an average 40-footer, assuming a beam of 13′. 40 x 13 = 520 x .7 = 364

364/48=7.58 cubic feet x 8 = 60.6 gallons per 1/4″ of rainfall!

So what’s involved in rainwater catchment? The easiest way is to catch water directly off the deck with your fresh water fills. On our boats, we always put the fresh water fills at the low spot on the deck, so they are easy to fill with run off. Usually we use the deck drains, and then install one valve to shut off the drain, and another to open the tank fills. This way one set of plumbing fittings does two jobs. If you have separate deck drains and tank fills, the drains will need to be plugged, along with any openings in the toe rail.

For more on catching rain water, see: Offshore Cruising Encyclopedia page 368

Posted by Steve Dashew  (February 28, 2001)

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