Choosing the right watermaker for you!

Its extremely difficult to make an informed decision about which type of watermaker to buy without having the opportunity to “test drive” all of the choices over an extended period of time. While asking a fellow cruiser for advice is a good place to start, everyone's needs are different. Chances are that they only have experience with one type of system - so although they may be happy with their system, they have nothing to compare to. There are basically three types of Watermakers to choose from: 12V systems, 110V systems, and engine driven systems; each with advantages and disadvantages.

Before looking at each of these systems, let’s have a basic overview of how watermakers work. Seawater is pumped by a high pressure pump through a membrane that only allows fresh water to permeate the membrane film. Salt, bacteria, minerals etc. molecules are too big to pass through the membrane, and are flushed overboard. Ideally, about 10 to 20% of the sea water is turned into fresh water, with the other 80 to 90 % of the brine being flushed overboard. The higher the pressure (within the limits of the membranes specs), the greater the water output and water quality. The closer to the 10% product water ratio you can get, the longer the membrane life. The key to maximizing water output is to be able to move a lot of water, at high pressure through a large membrane. Watermaker output can almost be doubled by adding a second membrane in series with the primary membrane. Water temperature and water chemistry also affect water output. Membrane manufacturers rate their membranes in US gallons per hour at 77 degrees Fahrenheight, warmer water will increase output while colder water will decrease output. Variations within membrane specs and water chemistry can result in 20% to 30% plus or minus output from the rated output.

Armed with a basic understanding of how watermakers work, let’s look at the advantages and disadvantages of the 3 types of systems.

12V systems; Advantages:

  1. Can be relatively easy to install by the boat owner
  2. Could potentially  make water even if the engine/genset failed if there is an alternative battery charging source (wind, solar)

12V systems; Disadvantages:

  1. Low water output; 12V pumps can not pump a lot of water or produce much pressure. This results in high product water ratios (inadequate flushing) and faster salt build up in the membranes which leads to shorter membrane life.
  2. Product water tends to have a higher salt content.
  3. The low water output means long running times to fulfill the water needs, which results in already overburdened batteries needing to be charged with the main engine or genset for long periods of time to make a small amount of water.
  4. 12V motors and high pressure pumps tend to be inefficient, expensive and unreliable.

110V systems; Advantages:

  1. Large pumps produce high pressure and large water volumes ensuring good product ratios (good flushing) and excellent water quality.
  2. Relatively easy to install by owner.
  3. Pumps and motors are very reliable, efficient and inexpensive.
  4. High water output means shorter running times and plentiful water.

110V systems; Disadvantages:

  1. Need to have a genset or large (2000w) inverter with a high output alternator(s) capable of producing at least 180 Amps at relatively low RPM.
  2. If the genset or inverter fails, you can’t make water. If you have both a genset and an inverter you have some built in redundancy.

Engine Driven systems; Advantages:

  1. Main engine can easily drive a large capacity high pressure pump.
  2. Has the fewest number of parts resulting in excellent reliability (fewer things to go wrong), lower cost and the highest water output with excellent water quality.
  3. Minimum running time, make water while underway, or while charging batteries without robbing precious amps from the battery bank.

Engine Driven systems; Disadvantages:

  1. Mounting the high pressure pump to the engine via a V belt and 12V clutch requires some mechanical skills (would be similar in difficulty to mounting a second alternator).
  2. If the main engine fails, you don’t make water.

Other things to consider when shopping for a watermaker are:

  1. Does the system use commonly available components available all over the world, or does it have a lot of proprietary components.
  2. Does the system use standard or proprietary membranes.
  3. Is the system easily serviced by the boat owner.
  4. Remember to calculate the water output in gallons per hour, rather than the gallons per day the manufacturers like to use.

As you can see there are many things to consider when deciding which type of watermaker will best suit your needs, and fit the mechanical systems on your particular boat. The people who seem to be the most satisfied with their watermakers are the people who can make the greatest quantity of water in the shortest period of time.