The Basic Scheme

The High-Pressure Pump

The critical question still was the pump: I already had a few offers for plunger pumps of around 1200 USD when I occasionally read in a sailing newsgroup (I have become member in dozens of newsgroups during this research) that someone used a standard low-cost ordinary high-pressure washer from Kärcher for 70 USD without problems. The idea attracted me immediately: 70 instead of 1500 motor included! So, I tried to find out why not everyone is doing this and what would be the disadvantages.

Very obviously, the ordinary high-pressure washers use the same plunger pumps, however the material of the pumps is different: The watermaker pumps are made from stainless steel and / or seawater resistant bronze, whereas the "normal" pumps are made from brass and aluminum, to explain it the simple way.

Kärcher Pressure Washer

Now, the sailor who experimented with the Kärcher wrote that especially the very cheap washer pumps are made mainly from plastic, so the corrosion problem should not be one. I did not disassemble one to find out but I thought I should give it a try. If such a pump only would last for 6 - 8 weeks’ holidays I could buy a new one every season for ten years and still save half the money of a CAT pump or something similar. Further, I said to myself, as I am not bluewater sailing the worst-case scenario is that the pump refuses to work after a certain number of days and I will be where we were before: canisters and long, long arms ...

Another limitation of such a pump is the flow rate. The maximum pressure (100 bar or 1450 psi) is far beyond what we need and will cause no trouble, but the maximum flow rate is stated as 340 liters per hour or 90 gph. This equals 5,5 liters per minute or 1.5 gpm. When actually running, I estimated that it probably would be rather 4 liters or 1 gpm.

With these figures the calculation goes like this:

Nominal pars of the membrane: 

Maximum feed rate: 23 lpm / 6 gpm
a) Maximum permeate rate at water temperature of 25°C / 77°F 108 lph / 28.5 gph 
b) Maximum permeate rate at a water temperature of 20°C / 68°F 90 lph / 24.5 gph
c) Maximum permeate rate at a water temperature of 15°C / 59°F 76 lph / 20 gph

With the Kaercher Pump installed the results should be:

Maximum feed rate of the Kärcher pump: 4 lpm / 1 gpm
a) 19 lph / 5 gph
b) 16 lph / 4 gph
c) 13 lph / 3.5 gph

This means: for our “spring trip” in April/May (sea still cold) we should expect about 13 liter or 3.5 gph, for our summer trip (water warmer) we should expect about 19 liter or 5 gph fresh water with the Kärcher pump. The watermakers “from the shelf” with their expensive plunger pumps and the same membrane produce twice as much water the brochures say. But anyhow, still not bad for that money.

So, I went to the Home Depot, bought one of those 59.95 Euro Kärchers, opened it and took off the pump/motor-unit. I connected it to the referring hoses (again: various different threads), turned on the water and electricity – and them bones walked!

The “test drive” at home with homemade seawater proved that the system worked. The first glass of “permeate” as the fresh water coming from the membrane is called tested like – water! Now the final question was: would the system work on board under real life conditions?

Fresh Water Made by Dido for the First Time

For our annual “boys’ trip” we went to the Croatian Adriatic as usual. Dido was lifted into the water and we left the harbor without a drop of fresh water in the tanks, fully relying on the watermaker (well, we knew the other day we would be in a harbor where we could get water – just in case). After we were a bit away from the shore I turned on the autopilot and then the watermaker. I adjusted the pressure to 55 bar / 800 psi by means of the needle valve and controlling it by the pressure gauge, and the water started to flow. We had to fill up the tank in the bow and an additional 30 l / 8 gall. canister, all together about 95 l or 25 gall.

It took about 3 hrs to get them full. So, the production rate was far above expected. But why so? Assumingly the flow rate of the Kärcher is actually higher than expected when operated below pressure-washing pressure of 100 bar / 1450 psi. Water temperature was as expected – cold, and the salinity in the Mediterranean is anyway higher than anywhere else (3.6%). So, the only remaining variable is the flow rate of the pump.

I was surprised to the positive. Still the question how long the Kärcher would do service was bothering me a bit. We were 8 days on the water, and during this time the watermaker was running each day for about an hour. During this hour, it supplied us with about 25 to 30 liters (6.5 to 8 gall).

Although the Kärcher is built in somewhere in the bilge one cannot ignore it. It is noisy. With a few further measures it will certainly be more silent. But anyway, this is a price that we are willing to pay for an incredible increase in comfort: washing dishes with fresh water, showers that deserve this name instead of a seeping hose on the stern platform, shaving every morning, no more long arms due to heavy water canisters, no more searching for places where one can do some laundry, …