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How to choose the size of your watermaker / desalinator

The first question every inexperienced cruiser asks: “How much water will I use?

Well, I am sorry to report that there is no definitive answer. It really depends. Don't lose hope, as I will do my best to help you understand where your water demands will come from. Depending on your boat, you may have some or all of the following:

Water Consumption Systems:

  • Pressurized water system - Considered by old timers to be a luxury. Nowadays it is hard to find a suitable cruising vessel without pressure water.
  • Galley sink - Cooking, washing dishes, drinking, cleaning fish, washing bilge goo off hands, etc.
  • Head sink - Brushing teeth, shaving, washing bilge goo off hands, whatever it is that women do in there.
  • Head shower - The obvious, plus washing bilge goo out of your hair.
  • Fresh water toilet - A nice luxury, and depending on type, can consume a lot of water. VacuFlush is the best system for saving water.
  • Transom shower - Rinsing off after a swim.
  • Deck washdown - Helps get rid of grime and fish guts.
  • Ice maker - Another nice luxury. Everyone needs a cocktail after washing off the bilge goo!
  • Clothes washer - A really nice luxury. Beats washing by hand, or finding a clean laundromat in Nassau.
  • KIDS - It's great fun to have them on board, but they use up Everything!
  • Landlubber guests - They just don't get it. You'll cringe when they turn on the hot water faucet and let your precious water run down the drain, while they casually hold their finger in the stream waiting for the perfect temperature, and then say “Oh, do you have hot water?”

As you gain experience aboard your boat, you develop many techniques for conserving water.

Conservation Tips:

  • Don't try to make the water on your yacht flow like municipal water. The faster it comes out of the faucet, the faster it will get wasted. Install a modestly sized fresh water pump - your plumbing will thank you by lasting longer.
  • If you have dual galley sinks, use one for washing dishes, and the other for rinsing. Once the rinse water is too soapy, add a little more soap and use it for washing.
  • An even better way to save rinse water is to put your soapy dishes in a drain rack and spray them down using a pump sprayer.
  • Rinse your toothbrush in a glass of water.
  • Shave before your shower. By the time the sink has enough water for rinsing your blade, the hot water is ready for showering.
  • Navy showers! Get wet and turn off the water. Soap up and rinse. I can get squeeky clean with less than three gallons of water, without feeling like a camper.
  • Keep your hair short. Long hair takes a lot more water. You women out there, ignore this! We'll get you more water.
  • Wash the deck with seawater first, then a quick rinse with fresh water.
  • Train your kids to compete with one another to see who can use the least water and still get clean.
  • Educate your guests as soon as they step aboard. First, the safety rules and where the emergency equipment is located. Then, the conservation rules. Don't be shy about telling them how it all works. Your guests will appreciate the lesson in conservation, and perhaps take the lessons home with them. If your guests take well to the lessons, you won't mind having them back aboard.

Now that you have some tips on conserving water, here is how it worked out for us. For our first couple of weeks cruising full-time, our family of five (with kids between 6 and 11) used about 250-300 gallons of water per week. In friendly Carribean areas where you buy water by the gallon, this is no problem. You just pull up the fuel dock and top off your tanks. Don't forget to tip the attendant.

After getting some more experience, and developing some of the techniques listed here, we were able to lower our water consumption to about 150-200 gallons per week. We still showered every day, but lacking a clothes washer, we used on-shore laundry facilities. It became very clear that keeping clean bodies, clothes, bedding, equipment and boat was by far the largest demand on our water. Cooking and drinking demand very little water - less than 1/2 gallon per day per person (water isn't the only thing we drink).

Where to Get More Water:

When cruising in areas where you cannot just pull up to the dock and tank up, jerry jugs become the only way to replenish the vessel's potable water supply. This is back-breaking work, especially if your boat has a lot of freeboard. A five-gallon jerry jug filled to the brim weighs over 50 pounds. If you have to lug six of these every day, it gets old fast. You either stop being clean, or find another way.

You can save some water by bathing in seawater, then rinsing with fresh water. Make sure you use eco-friendly soaps, as there are very sensitive plants and creatures everywhere. By the way, even if you wash with fresh water, most likely it still drains overboard, so don't use anything that will harm the little fishies.

It is possible to catch rain water if your boat is equipped for that purpose. We tried rigging a tarp that we would deploy while it rained. The problem with the tarp is that it rarely just rains in our favorite cruising grounds. Rain comes in squalls. The tarp idea turned out to be a fiasco. The wind whipped the tarp all over the place, and not a drop of water funneled its way into our tanks.

If your boat is equipped for catching rain, you should be very cautious about drinking from your tanks. It is not so much that rain water falls through who-knows-what in the sky, but that it lands on your deck, which has who-knows-what on it. Any contamination on your deck ends up in your water tank, making your tank water useful only for cleaning. Buy bottled water for cooking and drinking.

We found that the best (although not the cheapest) way to keep up with our water needs was to install a water maker. This gave us freedom from hauling jerry jugs, and pulling up to docks for a meager $15 fuel purchase. It also gave us the freedom to cruise in areas where good fresh water is scarce, and perhaps not even available for a cruising yacht.

Sizing Your Desalinator:

The size you choose for your desalinator should be based on four major factors:

  • How much water you consume aboard your yacht
  • How much automation your watermaker will have
  • How you will power your watermaker
  • Your budget

Now that we have covered water consumption systems and some conservation tips, you should plan on using between 30 and 60 gallons of water per week per person. Consider up to 40 gallons more per person per week if you have an automatic clothes washer on board. So, a cruising couple with all the amenities aboard their boat will use about 200 gallons per week.

Most watermakers are sized in gallons per day (GPD), and it is simple math to divide by 24 to get the amount of fresh water produced per hour. Next, decide whether you will install a desalinator that can run unattended (requires automated shut-off), or whether you are willing to stay aboard while the machine is running.

Choosing how to power your desalinator can be a little more complicated. I discuss that in more detail on the Watermaker Power Options page.

Your estimated water usage, divided by the desalinator's output per hour, will give you an estimate of how many hours per week you will need to run your watermaker. As desalinator output capacity increases, so does the cost of the machine. Your budget will dictate how much water-making capacity you can afford. You may decide that a low-capacity desalinator with automated shut-off is better for you than a high-capacity machine that you must attend to. Your philosophy on K.I.S.S. weighs heavily here.

Happy cruising!
Related sites:
www.Battery-Load-Calculator.info Determine your optimum house battery bank size.
www.Marine-Alternators.info How to select the appropriate Marine Alternator
www.Marine-Generators.info AC power onboard your yacht is no longer a luxury!
www.Marine-Inverters.info Choosing the right DC-to-AC power inverter is critical to your happy cruising
www.Marine-Weather.info How to keep up with the marine weather forecast the easy way.