How does a boat watermaker work, and what maintenance does it need?

A boat watermaker uses reverse osmosis to pump seawater through a membrane under high pressure, collecting fresh water and discharging concentrated brine overboard. Maintenance centres on keeping intake filters clean, flushing regularly with chlorine-free fresh water, checking for leaks and preserving the system when left unused.

From seawater to fresh water

Seawater enters through a seacock and coarse strainer, then passes through sediment prefilters before reaching the pressure pump and membrane. Many installations use a 20-micron cartridge followed by a 5-micron cartridge; some need only one. Use the grades specified for your machine.

The membrane separates water from most dissolved salts, typically at 55 to 65 bar, roughly 800 to 950 psi. Fresh water, called permeate or product water, passes through the membrane. The remaining seawater carries the rejected salt overboard. Energy-recovery systems, including Spectra machines with a Clark Pump, recover pressure energy to reduce electrical consumption.

A salinity sensor checks the product water. On automatic installations, a diverting valve sends unsuitable water overboard and admits acceptable water to the tanks. Initial output normally goes to waste until the reading stabilises. Some simpler systems require manual testing and diversion.

Output depends on seawater temperature, salinity, filter condition and available power. Colder water normally means fewer litres per hour. Compare performance with the manufacturer's temperature-corrected figures before assuming the membrane needs replacing.

Before each run

Check the intake and discharge valves first. A closed intake can starve the pump; a closed brine outlet can prevent correct operation.

1. Choose clean seawater away from sewage outlets, fuel slicks, muddy anchorages and busy harbours. Avoid brackish water unless your model's instructions specifically cover it.
2. Check the seawater strainer, prefilter condition indicator and hoses for restrictions, leaks or chafing.
3. Confirm adequate battery charge or the required AC supply. DC watermakers can place a substantial continuous load on the battery bank.
4. Follow the prescribed starting sequence, then check feed pressure, operating pressure, brine flow and product-water salinity.
5. Record output and salinity once stable. These readings give you a baseline for fault-finding.

A low salinity reading does not prove the tanks and pipework are hygienic. Contamination can occur after the membrane.

Routine cleaning and flushing

Inspect and clean the coarse strainer as needed, checking more often in water carrying weed or sediment. Replace sediment cartridges when the condition indicator or pressure drop calls for it, rather than relying solely on a calendar. In dirty water, cartridges can block within hours.

Close the intake seacock before opening the strainer. Isolate power and release system pressure before loosening filters or pressure fittings.