Pure Water & Clean Energy
Bringing Clean Water To Every Corner On Earth
Pure Water & Clean Energy
Bringing Clean Water To Every Corner On Earth
Sizing seawater RO for vessels: demand, runtime, power budget and recovery at sea.
A marine watermaker is a seawater reverse-osmosis system that has to work in the worst conditions RO ever sees: 35,000 mg/L feed, a moving platform, a limited power budget, and often weeks of idle time between runs. Sizing one starts with demand and ends with the power budget — and on most vessels the power budget is what actually decides the answer.
Consumption aboard varies enormously with vessel type and whether the water is metered, so treat these as planning brackets and confirm against the vessel's own logs where they exist:
| Vessel type | Typical per person per day | Driver |
|---|---|---|
| Working crew, water-conscious | 10–20 gal (40–75 L) | Galley, short showers, no laundry |
| Commercial vessel with laundry | 25–40 gal (95–150 L) | Laundry and galley dominate |
| Passenger, yacht or cruise | 50–100+ gal (190–380+ L) | Guest expectations, deck wash |
Add deck wash-down, engine-room make-up, and any process demand separately — on fishing vessels, ice and hold wash frequently exceed domestic use.
Do not buy a watermaker whose rated output equals daily demand. That would require running 24 hours a day with no margin for fouling, cold feed water, or a fault. Size so the unit meets demand in a runtime window you actually want to run it in — usually whenever the generator is already loaded for other reasons.
Required capacity = daily demand ÷ target daily runtime hours × 24
An eight-person crew at 25 gal/person/day needs 200 gal/day. Targeting 8 hours of runtime gives 200 ÷ 8 × 24 = 600 GPD rated capacity. That is the sizing calculation, and it is why a 500 GPD unit suits a crew that a naive reading of "200 gallons a day" would put on a 200 GPD machine.
Rated output is quoted at a reference condition — commonly 25 °C feed at 35,000 mg/L. Cold water cuts membrane flux by roughly 3% per °C below that reference, so the same unit in 10 °C North Atlantic water produces on the order of 60–70% of its nameplate. Size for the coldest water the vessel will work in, not the brochure figure.
This is where marine sizing diverges sharply from shore plant. Small watermakers usually run without energy recovery, and pay for it:
| Configuration | Specific energy | Typical application |
|---|---|---|
| Small SWRO, no energy recovery | ~8–15 kWh/m³ | Under ~1,000 GPD, 12/24 VDC or small AC |
| SWRO with energy recovery | ~3–5 kWh/m³ | Larger vessels, continuous duty |
| Large plant with modern ERD | ~2.5–3.5 kWh/m³ | Shore and large offshore installations |
The reason is recovery. Small marine units typically run 8–12% recovery — they discharge most of the pressurised feed overboard — against 35–45% on plant-scale SWRO with energy recovery. Low recovery is deliberate: it keeps concentration polarisation and scaling risk low with minimal pretreatment and no antiscalant dosing, which is exactly the right trade on a vessel with no operator. But it means the high-pressure pump does a great deal of work on water that goes straight back to the sea, and on anything above roughly 1,000 GPD an energy recovery device starts to pay for itself in fuel.
Open-ocean feed is the easiest water SWRO ever sees — low turbidity, stable salinity, biologically quiet. Harbours are the opposite, and the difference dictates the operating rule:
More marine membranes die of neglect between voyages than of use. Two defences matter:
Seawater plus a moving deck is an unusually hostile combination. Specify 316L stainless as a minimum for wetted metal, duplex stainless for high-pressure components on larger systems, and non-metallic piping or FRP vessels where practical. Ordinary 304 stainless will pit in seawater service and is a false economy on a system meant to last the life of the vessel.
Sizing a watermaker for a vessel? Send us crew or passenger count, target runtime, available power, and the coldest water you expect to work in, and our engineers will size the system. Contact ForeverPure or call +1-408-969-2688.
Work out daily demand, then divide by the runtime hours you want and scale to 24 hours: required capacity equals daily demand divided by target runtime hours times 24. An eight-person crew using 25 gallons each needs 200 gallons per day; at an 8 hour target runtime that calls for a 600 GPD unit, not a 200 GPD one.
Water-conscious working crew typically use 10 to 20 gallons (40 to 75 litres) per person per day. Commercial vessels with laundry run 25 to 40 gallons. Passenger vessels and yachts run 50 to 100 gallons or more per person. Deck wash-down and process water are additional.
Small seawater units without energy recovery use roughly 8 to 15 kWh per cubic metre of fresh water. Adding an energy recovery device brings that to about 3 to 5 kWh/m³. Large shore plant with modern energy recovery reaches 2.5 to 3.5 kWh/m³.
Small marine SWRO typically recovers only 8 to 12% of the feed, against 35 to 45% on plant-scale systems. Low recovery keeps concentration polarisation and scaling risk low without antiscalant dosing or close operator attention, which suits an unattended system at sea. The cost is energy, since most of the pressurised feed is discharged overboard.
No. Fuel and oil in harbour water will foul RO membranes irreversibly, and no practical shipboard pretreatment removes hydrocarbons. Make water offshore in clean, open water only.
Membrane flux falls by roughly 3% per degree Celsius below the rating reference, commonly 25 degrees C. In 10 degree C water a unit produces on the order of 60 to 70% of its rated output, so size against the coldest water the vessel will work in.