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Old 22-11-2019, 13:01   #1
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Sizing a Watermaker

Another watermaker thread veered a bit, and I had questions on sizing a watermaker that I thought warranted a new thread.

My wife and I will be leaving next year for an open-ended cruise from California to Florida. Trying to decide on a watermaker as part of a refit. Our boat has 200g freshwater capacity which lasts us about 10-14 days, so we go through about 15-20 gals/day of water which we can easily stretch if needed. We have a 6kw generator, and enough solar/battery to be off-grid (e.g. no engine or genny) unless we need A/C or run a Watermaker.

Simplicity and off-the-shelf components of a system such as these systems from Cruise RO is compelling for me. Note the 20gpd and 30gpd use same motor, only difference is a second membrane/vessel ($800 uplift), which means there is no power penalty for the increased production.

Is bigger always better? Is there a recommended run-cycle for a watermaker? I would have thought bigger is better - shorter run time. But maybe not - some systems (such as household A/C) do not work well when over-sized. And as someone pointed out on the other thread, the flushing cycle wastes same amount of water regardless of cycle time. 20gph, 30gph, or 40gph?

General rule of when to start refilling tanks? As mentioned, we have 200g tankage which lasts 10-14 days. All things being equal, is there a recommendation on when to run the watermaker? Keep the tanks topped-up? Wait until they are down, say 1/4 full? Doesn't matter, just fill them opportunistically when you happen to have excess power (engine running with HO alternator, for example)?

Soft-Starts?. I'm guessing the startup amperage (LRA) on a 9-amp/120VAC pump is in the neighborhood of 25-amps. Are soft-starts possible/common?

Thanks in advance -


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Old 22-11-2019, 18:53   #2
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Re: Sizing a Watermaker

Having abundant fresh water on the ocean is a luxury that's easy to get used to. So bear that in mind when figuring a system. Also membranes are rated at 77įF. Colder water - less water made. I use a lot more water in hot weather - more showers, but less laundry. I have a watermaker similar to CruiseRO's that makes 40gph+. I have a bigger pressure pump because it doubles as a pressure washer and could make 80gph with more membranes.

In salt water I make water every other day or third day at the most. Whether I need it or not. Otherwise you get growth in the membranes. At the end of watermaking, I fill a 5 gallon bucket for flushing. So the whole system sits in fresh water. And I could go more than 3 days sitting. But if the system is going to sit, you need to pickle it.

On the ocean I keep my tanks full. I tank 450 gallons. In the event of a watermaker failure, I got a couple weeks to sort it out. And should I develop a coolant leak, I have plenty of replacement water.

Making water cruising becomes a routine that I time with laundry, the water heaters and charging the inverter batteries. When I can, I pull into fresh water and make water there. I don't have to flush the watermaker, my engines have been flushed with fresh, and sitting a few days kills the bottom growth.

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Old 22-11-2019, 19:39   #3
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Re: Sizing a Watermaker

You want the 30 over the 20. As you said there is no penalty just 50% more water. Also there is redundancy as in if for some reason a membrane goes bad you can isolate the bad one and your back to a 20.

The 40 is a larger pump and motor. Each membrane can make 20. The 30 is pump limited but that pump is about all a Honda can handle, increase the pump size and you get 40, but itís more than a Honda 2000 can reliably handle.

Overwhelming majority of the water makers Rich sells is the 30, call him, he can explain and doesnít mind.
Be sure to get lots of extra filters before you go, I will clean the 20, but not the 5 myself, others clean them several times and that may be a mistake.
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Old 23-11-2019, 03:44   #4
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Re: Sizing a Watermaker

Thanks for responses. I'm looking at Cruise RO for two reasons. First, simplicity that comes with off the shelf parts - pump head and fittings carry lifetime warranty. Second, Rich. A fellow cruiser, he totally gets it. And he's responsive as hell. He looks pretty healthy so lifetime warranty should last a while

Question. Auto-flush timer and solenoid is a $500 option. Although it adda complexity, seems like a good idea. Thoughts?
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