Technical Information

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Reverse Osmosis

Al these units work on the same principle, reverse osmosis. Osmosis Ė passage of a solvent (H2O, water molecules) through a semipermiable partition into a more concentrated solution (saltwater). Not what we want to have fresh water dilute the salty seawater. So the process we want is reverse osmosis where the solvent (fresh water) goes from the salty side to the less salty or fresh water side. To persuade the water molecules to do this takes pressure, about 750psi for seawater. So the basic pieces we need are a semipermiable partition (membrane) and something to create 750psi pressure. Sounded simple enough, so the choice was obvious, weíre going to build our own. Parts needed include:

  1. Membrane
  2. Pump

Doing some research based on the magazine article it turns out that membranes (the semipermiable partition) come in several standard sizes, the size determining the amount of fresh water produced. The smallest is 2.5"x14" and produces approximately 4gph. The largest is 4"x40" and produces approximately 62gph. The one I chose is 2.5"x40" and produces about 20gph.

The trick, of course, is to find a space to put a 40" long pressure vessel. We have that space under the V-berth, not very accessible, but it shouldnít need any maintenance. Once the membrane has been selected, itís time to pick the other major component, the pump to generate the pressure. The membrane is designed to pass a maximum of 12% of the raw water (seawater) to the product (fresh water) side. Running the membrane at a 10% flow rate, we need a pump that will flow 20 gph x 10, or 200 gph, or roughly 3.5 gpm. The units change back and forth, the membranes are actually rated in gallons per day (gpd) and the pumps are rated in gallons per minute (gpm).

High Pressure Pump

So we need a pump that delivers 3.5 gpm at 750 psi. These types of pumps are used a lot in carwash installations and are fairly common and readily available. CAT Pumps makes a line of pumps that meet these requirements, 3.5 gpm at a maximum pressure of 1200 psi. So now we have what it takes to make 20 gph of fresh water. Well, almost. The pump needs something to turn it. At 3.5 gpm and 750 psi that comes to about 1.75HP.

Motor & Pressure Regulator Valve

So one additional item we need is a 2HP electric motor. I read the specs but until it came in I didnít realize how heavy 45lbs really is. That sucker is going to need a sturdy mounting. Of course the membrane needs to be put into some kind of container, so one more additional item we need is a pressure vessel. And we canít just run the output of the pump through the pressure vessel because it would just come out the other side and there would be no 750 psi pressure. So we need one more item, a back pressure regulator valve. I guess you can see where this is going, our two basic pieces to make water have turned into quite a collection of hardware.

Feed Pump & High Pressure Hose

In addition to the items already mentioned, a complete system also needs a source of seawater which requires a through hull and strainer. The high pressure pump likes to have a positive pressure on the suction side so we also need a feed pump. The membrane and high pressure pump donít like dirty water, so we need some pre filters, 10 and 5 microns. We also need some miscellaneous valves and hoses. We need to be able to divert the product water overboard when we first start to make water. We also need to be able to divert the feed pump from the seawater source to a source of fresh water in order to flush the membrane before shutting the unit down. These function can all be done with plastic hoses. The connections from the high pressure pump to the membrane and the membrane to the back pressure valve need to be made with high pressure hose, I used 3000psi rated air brake hose. This particular hose can be had with reusable brass fittings.

Flow Meter

One additional item I added is a flow meter for the product water. The membrane will pass product

water based on two parameters, pressure and salinity. Thereís a third one, feed water temperature, but itís not that critical unless there are major changes in feed water temperature. As salinity decreases, product flow will increase for a given pressure. So what happens if you want to make water using brackish water, if you maintain the 750 psi pressure on the membrane, the product flow will increase beyond the rating of the membrane and it will be damaged. By having a flow meter and adjustable back pressure valve I can adjust the back pressure until I get 20 gph no matter what the salinity of the feed water is.

Pulsation Damper

Another item I added is a pulsation damper. The output of the high pressure pump contains  pressure pulsations which cause hoses and fittings to vibrate. The pulsation damper eliminates these pressure spikes and really quiets things down. Not essential, but should prolong the life of the high pressure components of the system.

UV Sterilizer

The membranes will filter out almost anything in the feed water, including most viruses and bacteria. But Fran was concerned about drinking water that came from the ocean around us, no telling what some of those fish do in the water. Can you imagine a whaleÖÖ.never mind. So we added a UV sterilizer to kill anything that might be left after passing through the membrane.


With some effort and contortion, this equipment was all mounted under the V-berth, replacing some small drawers. Problem was, we really didnít want to run it in the nasty water in the marina. The membranes do not like oil in the feed water. So we never got to test our brand new watermaker until we left for our cruise.


We started our cruise by going to Offats Bayou, just off Galveston Island, to try to unwind after the last hectic weeks getting ready. Even though the water was somewhat muddy and brackish, I decided it was time to try the watermaker while we still had some access to marine and hardware stores. I had wired the motor for 220Vac operation to reduce the current draw. Well it turned out that our genset was wired with both 110Vac outputs wired in parallel. So the first redesign was to rewire the genset and the Ďshore off gensetí selector switch. I hadnít wanted to put another hole in the bottom of the boat so I had spliced into the engine cooling water through-hull for the watermaker feed water. The thinking was that we would never run the engine and the watermaker at the same time. Turned out that the long run from the through-hull to the feed pump was too much and the feed pump couldnít keep up with high pressure pump. In other words the suction pressure was going negative, something the high pressure pump doesnít like.


So the next redesign was to use the A/C water and its intake and strainer as the feed pump. This made the high pressure pump happy and after running for about an hour to flush the preservative out of the membrane, WE MADE WATER!!!! This lasted for about another hour when the pre filters plugged to the point that once again the feed pump couldnít keep up with the high pressure pump. Given the muddy water this was to be expected. After rinsing the pre filter we ran for another hour and then pickled the membrane since we didnít know when we would get to run the unit again. From what Iíve gathered from others, if the membrane is flushed good after making water, it doesnít require pickling if it's run every week or two.

Pickling, for those unfamiliar with the term, flushes and fills and the membrane with a biocide to kill anything living that's left on the membrane. By thoroughly flushing the membrane after every use and using it periodically, pickling should not be required unless the unit will not be used for an extended period of time - more than a week or two. The problem with most 12Vdc watermakers is that they make so little water to start with that you donít want to use a lot of that water to flush the membrane and hence they need to be pickled if not used every few days. As I said earlier, this is information Iíve gathered from various sources on the internet and may or may not be totally correct. I guess weíll find out.

Up to this point, the watermaker has allowed us to stay on the hook for two weeks, without having to go in someplace or haul water.  We've even been able to run our new washing machine, which is another story.