Marine Watermaker Knowledge Base
Practical technical information for planning, building, installing, maintaining, repairing and upgrading marine reverse osmosis systems.
Knowledge Base Topics
Getting Started
Start here if you are planning a new marine watermaker, evaluating a DIY system or trying to understand which components are required.
A marine watermaker is a reverse osmosis system designed to convert seawater into freshwater. Seawater is filtered and pressurised before passing through a semi-permeable membrane, which separates freshwater from salts and other dissolved substances.
Yes. A conventional marine watermaker can be assembled from individual components including a seawater intake, feed pump, prefilters, high-pressure pump, membrane pressure vessel, regulating valve, instruments and freshwater diversion system. Correct component sizing and safe high-pressure installation are essential.
Selecting the right watermaker is essential for ensuring comfort and freshwater independence during your voyages. The ideal system should match your crew’s daily consumption, preferred operating time, available power, installation space and maintenance requirements.
💧 1. Assess Your Daily Water Needs
Start by estimating how much freshwater your crew normally uses each day.
-
Basic consumption: Drinking and cooking typically require around
6–8 litres (1.5–2 gallons) per person per day. -
Moderate usage: Including showers and dishwashing, allow approximately
20–30 litres (5–8 gallons) per person per day. -
High consumption: Laundry, frequent showers or deck washing may increase demand to
40–60 litres (10–15 gallons) per person per day.
120 litres (32 gallons) of freshwater per day.
⏳ 2. Determine Your Preferred Production Time
Decide how many hours per day you want the watermaker to operate. The same daily requirement can be met by a larger unit running for a short time or by a smaller unit operating for several hours.
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Short operating periods: To produce 120 litres in two hours, you need a system rated at approximately
60 litres per hour. -
Longer operating periods: A system producing around
20 litres per hour can meet the same requirement when operated for approximately six hours.
🔋 3. Evaluate the Available Power
Marine watermakers can be powered in several different ways.
- 12 V or 24 V DC systems: Suitable for boats with adequate battery capacity, alternator charging or solar generation. They are commonly used for lower and medium production rates.
- Energy recovery systems: Designed to reduce electrical consumption and particularly useful where battery power is limited.
- 110 V or 230 V AC systems: Normally powered by an onboard generator or a suitably sized inverter. These systems can support larger motors and higher production rates.
- Engine-driven systems: Use mechanical power from the main engine and can provide high output, but normally operate only while the engine is running.
📐 4. Consider the Installation Space
Watermakers are available as modular systems or as complete frame-mounted units.
- Modular systems: Pumps, filters, pressure vessels and controls can be installed separately. This provides greater flexibility in machinery spaces with an irregular shape.
- Compact frame-mounted units: These may appear easier to install, but the complete assembly can be bulky and difficult to move through narrow lockers or access openings.
🧰 5. Factor in Maintenance and Reliability
A well-designed system should be straightforward to service, particularly when cruising far from specialised technicians.
- Pre-filtration: Protects the feed pump, high-pressure pump and membrane from suspended particles.
- Membrane care: Regular freshwater flushing, correct preservation and occasional chemical cleaning help maintain performance.
- Service access: Filter housings, pump heads, valves and electrical components should be easy to reach.
- Spare parts: Systems using widely available, non-proprietary components are generally easier to repair while cruising.
💶 6. Consider the Overall Budget
The cost of a pre-assembled marine watermaker depends on production capacity, motor type, control system, automation level and installation equipment.
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Entry-level systems: Smaller systems producing approximately 20–30 litres per hour may begin around
€5,000–€7,000. -
Medium-capacity systems: Units producing approximately 60–100 litres per hour may cost around
€10,000–€15,000. -
High-capacity or highly automated systems: Systems exceeding 100 litres per hour may cost
€20,000 or more.
Prices vary considerably depending on configuration, accessories and installation requirements. A DIY or modular system can provide greater flexibility, allowing components and automation levels to be selected individually.
📊 Quick Capacity Guide
| Crew size | Estimated daily need | Preferred run time | Suggested output |
|---|---|---|---|
| 2 people | 60–120 litres | 2–4 hours | 15–30 litres/hour |
| 4 people | 120–240 litres | 2–4 hours | 30–60 litres/hour |
| 6 people | 180–360 litres | 2–4 hours | 45–90 litres/hour |
It is the system that can reliably meet your daily freshwater needs while remaining compatible with your boat’s energy supply, available space, maintenance routine and cruising plans.
A manual system is simpler, easier to understand and generally easier to repair. A semi-automatic or automatic system can manage flushing, product-water diversion, alarms and operating sequences. The right choice depends on installation complexity, budget and the level of onboard automation required.
A manual system is simpler, easier to understand and generally easier to repair. A semi-automatic or automatic system can manage flushing, product-water diversion, alarms and operating sequences. The right choice depends on installation complexity, budget and the level of onboard automation required.
How a Watermaker Works
These questions explain the basic hydraulic and physical principles behind seawater reverse osmosis.
The high-pressure pump raises seawater pressure above its natural osmotic pressure. This forces part of the water through the membrane while most salts and dissolved substances remain in the concentrated brine stream and are discharged overboard.
Feed water is the seawater entering the system. Product water, also called permeate, is the freshwater that passes through the membrane. Brine, also called concentrate or reject water, contains the salts and water that do not pass through the membrane.
Seawater contains a high concentration of dissolved salts. The system must overcome the osmotic pressure of seawater before freshwater can pass through the membrane. Marine systems therefore normally operate at considerably higher pressure than freshwater or brackish-water RO systems.
Production depends mainly on membrane size, feed-water temperature, salinity, operating pressure, membrane condition and the available feed flow. Published membrane production figures normally refer to standard laboratory conditions and must be corrected for real operating conditions.
Planning Your System
Before selecting individual components, define the required freshwater capacity, available electrical supply, installation space and intended operating conditions.
Estimate the normal daily consumption of the crew and decide how many hours per day the watermaker should operate. A smaller system running for longer periods may be sufficient for modest consumption, while larger crews or shorter operating windows require greater hourly production.
The choice depends on the boat’s electrical system, required production and available power. Low-voltage DC systems are practical for smaller capacities, while higher-output conventional systems are often better suited to AC motors powered by shore power, a generator or an appropriately sized inverter.
The required space depends on whether the system is modular or frame-mounted. A modular installation allows pumps, filters, vessels and controls to be distributed around available machinery spaces, but all serviceable components must remain accessible.
A conventional system is mechanically straightforward and usually easier to understand, maintain and source parts for. Energy recovery systems reduce electrical consumption but use more specialised hydraulic components and may require a more specific installation and maintenance approach.