How Reverse Osmosis Works on a Boat
Basics of turning seawater into fresh water on a yacht watermaker
A watermaker converts seawater into drinkable fresh water; the most common method in yachting is reverse osmosis (RO). Per our FAQ, the process relies on high pressure to separate salt and contaminants, collecting clean water (permeate).
This guide answers “where does the water come from?” before technical selection. For capacity and model direction see the capacity guide and which watermaker page; for care see the maintenance guide.
RO Path — 4 Main Blocks
- Intake + pre-filters
- High-pressure pump
- Membrane (TORAY / FILMTEC)
- Product water + brine reject
Normal vs Reverse Osmosis
In natural osmosis, water moves from low to high salt concentration. In a watermaker, the high-pressure pump reverses that direction: water molecules pass through a semi-permeable membrane; most salt and ions stay behind and leave as concentrated brine reject overboard.
| Stream | Content | Destination |
|---|---|---|
| Product water (permeate) | Low-TDS fresh water | Test outlet or water tank |
| Brine reject | Concentrated salty water | Reject line — usually overboard |
RO Steps in a Seacraft Watermaker
1 — Intake
The low-pressure pump draws seawater through pre-filters to the HP pump. Dry running damages the HP pump — low-pressure protection limits that risk.
2 — Pre-filters
Sediment and dolomite filters trap particles; carbon protects the path — spare parts catalogue. Dirty filters load the membrane early.
3 — High pressure
The high-pressure pump (triplex plunger, stainless manifold) feeds the membrane. Pressure varies by model and sea conditions; tracked on the digital display.
4 — Membrane + reject
TORAY / FILMTEC membranes — FAQ. Fresh water is collected; concentrated brine discharges overboard. The reject line must stay clear.
In RO, part of the feed becomes product water and the rest becomes reject; the reject ratio depends on model and sea conditions. Regular overboard discharge is critical for performance.
Pressure, Monitoring and Protection
| Feature | On Seacraft SCW series |
|---|---|
| Working pressure | Varies by model (e.g. SCW-30 product page lists 45/69 bar; SCW-150 lists 55 bar) |
| Low-pressure protection | Shutdown if pressure not reached within 30 seconds — LP pump protection |
| High-pressure protection | Automatic shutdown above 65 bar; sudden drop prevents flooding |
| Water quality | SCW-50 PRO continuous salinity monitoring; auto reject to sea if out of spec |
| Panel / test | SCW-30 test outlet; SCW-50 PRO pressure, flow, TDS and hour tracking |
Automatic TDS monitoring, brine reject valve and pressure protection are integrated in Seacraft automation. Post-production fresh-water flush is part of membrane care — flush guide.
What Affects RO Performance
Salinity and temperature
Lower Marmara salinity reduces membrane load; Aegean/Mediterranean water is saltier. Higher summer temperature can reduce nominal output — seawater salinity effect.
Filters and flush
Dirty pre-filters raise pressure and cut flow. Missed flush shortens membrane life — spare parts schedule.
Reject line
Blocked or closed reject valves raise pressure. Listed as a common mistake for first-time users — common mistakes.
Model capacity
Litres/hour is on the model label; daily need is calculated separately — 12V vs 230V, SCW-30 vs SCW-50.
When Does Product Water Go to the Tank?
SCW-50 PRO continuously measures product salinity; if out of spec it automatically rejects to sea via a solenoid valve, and directs acceptable water to the tank — product page description. SCW-30 has a panel test outlet; confirm quality before sending to the tank — water tank planning, pre-season checklist.
When TDS is low, Seacraft systems target near-drinking quality — see FAQ below.
Frequently Asked Questions
Related Guides
Seawater Salinity Effect · Watermaker Maintenance Guide · Which Watermaker · All Guides
Find the Right RO System at a Survey
Capacity, power source and tank plan reviewed together
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