Squeezing out fresh water

Modern watermakers range from hands-off automatic units to basic operator involved models

- WAYNE CANNING

While watermakers have become increasingly easier to use with each generation of models, this installed unit has a set of laminated instructions handy.

The term “watermaker” is a bit of a misnomer, as these units do not really make water but rather turn undrinkable salt water into drinkable fresh water. The correct name is “reverse-osmosis desalination system” or “RO system,” but let’s face it: That simply does not roll off the tongue quite as easily as “watermaker.”

Understanding the correct name helps understand what these systems really do and how they work. Watermakers take salt water and push it through membranes that only allow the smaller water molecules to pass through, blocking and separating the larger salt molecules. The result is fresh water from salt water. Sounds pretty simple, but of course nothing on a boat is that easy.

Membranes are a semipermeable material that, in a sense, filters on a molecular level. In order to do this, salt water is pushed against the membranes at high pressure. The pressure is required to squeeze the water molecules through the membrane material. This is why watermakers use an energy-intensive, high-pressure pump. The resulting water that gets through the membrane is called the product, while what is left over is called brine or reject.