How can I desalinate ocean water enough for cleaning or showering?

Question

I'm planning for a long cruise in my small yacht. The water tank in the yacht has enough clean water to drink, but not quite enough for showers, cleaning the dishes etc..

Desalinating ocean water during the cruise sounds like a great idea. And I can get by with only most of the salt gone for many of my uses (like showering, cleaning, toilet use..) . Is there a device that can remove salt from ocean waters? I have electricity or petrol available.

I know there are huge watermaking machines for big yachts like this one , but I need something smaller.

Answer 1

The problem is that desalinating water is hard; nearly desalinating it isn't much easier (it would be possible to design a bad reverse osmosis system but there isn't much market for that).

Instead if you need partially desalinated water you could get it by mixing drinking water with salt water. But there are better things to do.

Check what the toilet flushes with - I'm no yachtsman but I think it's seawater. So you can ignore that if I'm right. Cooking and drinking should be possible with less than 5 litres per person per day unless it's very hot (none of this boiling pasta in a vat business - the pan should be not much bigger than the food you're cooking, to save fuel as well).

Washing is harder to estimate. But here goes. When we had a touring caravan, a 40 litre container of water nearly did two quick showers. So there's a starting point for estimating how much you need for washing. But according to a sailor friend you wouldn't use fresh for all of that. Either get wet in the sea or with a bucket of seawater left in the sun, wash, then rinse with fresh water, which should be only a couple of litres. And that's when you need an all over wash - less than daily. After all, it's not antisocial like it would be in an office - you're all in the same boat.

As a further point of comparison, this question happened to pop backup just after my water bill came. At home, with one adult plus a child here half the time, we use about 70 litres per day - and that's with full domestic use, not efficiency-optimised boat gear, not saltwater toilets. I aim to be efficient, but don't take it to extremes

Answer 2

You linked to an industrial sized watermaker (75 gallons per hour) but they do come in smaller sizes for significantly less money.

But think hard about how much water you actually need. I'd consider your 80 L/day to be very generous. Ocean water at a normal level of salt is sufficient for showering or cleaning. If the salt remaining on a washed item or person is a problem, do a quick rinse or wipe down with fresh. That alone should cut your daily cleaning needs drastically. If your toilets require fresh water rather than salt or the same sort of chemical used in an RV, you should look into fixing the plumbing, because that's a huge waste of fresh water. Save the fresh water for drinking and cooking, and look at cleaning with it as a luxury.

For uses where slightly salty water is acceptable, but ocean water is too salty, you can simply dilute the salt water as needed with your supply of fresh, stretching the freshwater further.

Answer 3

The easiest way I've seen desalination happen is through evaporation. There are a few ways to do this. One way is to evaporate the water then feed it through a cooling coil (copper piping, helps if it's submerged in water). You could also try a modified version of a solar still.

I think by far the easiest way is to buy another water tank that you can fill before you leave.

Answer 4

You do not say where you sail. Bamboo jungle style. Take a number of 5 gallon buckets, and half-fill them with sea water. Put a 4 to 6 inch PVC pipe in the center to catch water and paint it black. Add a top of clear sheet plastic. Put a weight in center of the sheet so water runs down & drops into pvc pipe. Use a small rabbit pump to remove the water. You should get 1 quart of water per day per bucket in the tropics on average. 1 quart of water is enough for a sponge bath, or to do the dishes & such.

Also catch rain off your sail. For your clothes, put them in a net bag with rope & throw over the side. Seawater cleans clothes very well. If it rains, hang them out on the lines to rinse. Sail with the trade winds and you'll be in rainy season then.