Build and Install a 40 Gallon an Hour Watermaker For Less Than $4,000

7 Jun

completed pump mount

There are so many choices when deciding on a water making system to install on a cruising sailboat. When deep in research for which reverse osmosis device was right for our boat, I saw two main schools of thought in terms of system design. First was the low output, low draw DC watermakers that output 1-5 gallons an hour. Second was the high draw high output models that utilize 110 volt electric motors or engine driven pumps.

First, a brief primer on watermakers for the uninitiated. Essentially you take seawater, clean it of critters and other suspended particles down to about 5 microns. That now cleanish seawater is sent to a pump of some sort that puts it under extreme pressure to the tune of about 800-950 PSI. Here’s where the magic happens: the now extremely high strung water comes from the pump and enters the RO membrane. The membrane blocks salt and lets some water through. The water that gets through the membrane is the “product water” and is diverted to the ships tanks for hour long showers and slip and slides on the deck. The slightly more salty water, called “brine”  that doesn’t make it through the membrane is diverted overboard.

It's a hell of a thing, that final load of gear shipped onto the boat before heading off for a long trip.

It’s a hell of a thing: that final load of gear shipped onto the boat before heading off for a long trip.

I could spend a whole series of posts debating the advantages and detractions of the various watermaker systems. I wanted a lot of water and I wanted the system to be as cheap and robust as possible. By robust I mean I wanted it to be made out of non-proprietary parts and simple enough so any part could be taken apart and fixed by me when out cruising. So far the system that I ended up with has exceeded all my expectations. Hell yeah.

Specifically, I wanted a 40 gallon an hour, engine driven watermaker utilizing two 2.5”X40” pressure vessels. There are a few choices that are sold as complete units or kits. The one I was closest to buying is marketed by Rich Boren who runs Cruise RO. It’s a nice kit that has everything I wanted. But –  I figured out if I bought all the components I could still save $2,000. Problem is, sourcing some of these parts is pretty hard if you’re not hip to the high pressure plumping and pump trade.

Enter eBay. My new friend Stuart assembles all the parts that someone that was building their own system would need and sells the collection at a slight markup. All the parts are quality and non-proprietary.  Here’s the spec list I went with this item on ebay. I spoke with him on the phone to change the pump to one with a electromagnetic clutch and add the freshwater flush option, then placed the order.

The package arrived and all the world rejoiced. Now to figure out how to shoehorn it into some unused recess of the boat. The hardest part of the system to find a place for is the pair of 2.5″X40” pressure vessels. These are mounted parallel to each other and need to be able to be removed for service. My original plan was to mount them under the sink where the old hot water heater had been but it became apparent there would not be nearly enough space. I changed the install location to under the port settee. There was just enough space to mount both vessels.

pump mount mockup

The next major hurtle was to engineer and construct the mount that the pump would sit on and attach by belt to the power take off pulley on the engine. It had to be adjustable to install and tension the belt. I mocked the whole thing up with cardboard and hot glue then ordered some 1/4X4” steel. I reproduced the mockup in steel and tack welded it together. Amazingly, the trial fit went perfectly and I got my dad to weld it all up properly. This was as far as the install went in the USA. We had to get the boat down to Norfolk for the start of the Caribbean 1500 and this was not an essential system. The fabrication was the only part I really needed shore side support for so I planned to resume the install in the Bahamas. In part II of this post we will finish the installation of the 40 gallon an hour, $4,000 watermaker on the hook in stormy Marsh Harbor, Bahamas.


IMG_2424welding the mount milling the slots

This article was syndicated from Cruising – Beautiful Crazy Happiness


  1. Rafael Haciski

    A thought occurred to me – there are plenty of electric power washers, generating 1,500-2,000 PSI. Why not use those as the basis for the system?

  2. Pingback: 40 Gallon an Hour Watermaker For Less Than $4,000 Part II – Beautiful Crazy Happiness

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