Quick Facts
| Four Components | High-pressure pump, membrane housing, pre-filter train, product/brine plumbing |
| Pre-filter Sequence | 50-micron sediment → 5-micron sediment → activated carbon block |
| Product Water Target | <500 ppm (ideally <200 ppm for cooking and coffee quality) |
| Electrical | Dedicated circuit; 30A fuse for most DC pump installations |
| Intake Location | Seacock below waterline, away from exhaust outlets |
| First Run Note | Initial product water often tastes of glycerine — discard first 30–60 minutes |
A watermaker installation done well is invisible — the system fits into available spaces, runs quietly, and requires no special procedures to operate. A watermaker installation done badly is a source of persistent problems: low pressure from an improperly sized intake line, membrane fouling from a skipped carbon filter, electrical faults from a shared circuit with the autopilot. The difference between the two is almost always in the planning, not the execution.
This guide walks through a complete installation on a cruising sailboat, covering each of the four main components, the plumbing decisions, electrical requirements, and what commissioning looks like. It assumes a DC unit in the 12 to 20 litre per hour class — the most common type installed on passage-making boats in the 36 to 45 foot range.
The Four Components
Every marine reverse osmosis watermaker, regardless of brand or output, consists of four functional components. Understanding each one individually makes the installation logic much clearer.
The high-pressure pump is the heart of the system. It takes seawater at ambient pressure from the pre-filter train and compresses it to 800 to 1,000 psi — enough to force water molecules through the membrane while rejecting dissolved salts. In DC systems, this pump is driven by a 12V or 24V motor; in energy-recovery designs like Spectra's Clark Pump, an additional piston recaptures energy from the exiting brine stream and uses it to pre-pressurize incoming water. The pump assembly is the largest and heaviest component, typically weighing 3 to 8 kilograms for a DC unit.
The membrane housing contains the rolled semi-permeable membrane — typically a 2540 format (2.5 inches diameter, 40 inches long) for smaller DC units, or a 4040 for higher-output systems. Water enters one end under pressure, product water exits through a central tube in the middle of the roll, and brine exits from the far end. The housing itself is a sealed tube, usually fibreglass or stainless, with end caps and O-ring seals that must be inspected at every membrane change.
The pre-filter train is the upstream protection system. It removes the particulate and chemical contaminants that would foul or destroy the membrane. In a correctly specified installation, the pre-filter train is sized to handle the full flow rate of the pump without creating a pressure drop that starves the membrane.
The product and brine plumbing routes clean water from the membrane to the water tank and brine back to the sea. These two circuits must never cross-contaminate, and both require careful routing to prevent siphoning, backflow, and air locks.
Intake Location: Getting This Right First
The seawater intake is the beginning of everything, and its location determines what quality of raw water your membrane sees. The requirements are straightforward but non-negotiable.
The intake must be below the waterline at all points of sail — if the boat heels significantly, an intake that is below the waterline at rest may come out of the water at 25 degrees of heel, letting the pump run dry. For most fin-keel sloops, an intake on the hull near the keel, forward of the propeller shaft, satisfies this requirement at any reasonable angle of heel.
Distance from the engine exhaust is critical. Diesel exhaust outlets discharge a mix of water and combustion gases; the exhaust plume can extend several metres downwind of the boat when the engine is running. An intake directly adjacent to the exhaust outlet will draw contaminated water into the pre-filter train — carbon filters will not remove diesel combustion products, and the membrane will be damaged. Standard guidance is a minimum of 1.5 metres separation, preferably on the opposite side of the hull from the exhaust.
A dedicated seacock for the watermaker — separate from the engine raw water intake, the AC seacock, and any other through-hull — is strongly preferred. It allows the watermaker to be isolated quickly if contaminated water is suspected, without affecting the engine cooling circuit.
The Pre-Filter Train in Detail
The pre-filter train runs between the seacock and the high-pressure pump. The correct sequence is: 50-micron sediment filter, then 5-micron sediment filter, then activated carbon block. Each stage serves a distinct purpose.
The 50-micron sediment filter catches large particles — sand, shell fragments, marine growth — that would rapidly block the finer downstream filters. In clean offshore water this filter changes slowly; in harbours with disturbed sediment it can block within hours. It is the cheapest and easiest filter to change, and it extends the life of everything downstream.
The 5-micron sediment filter removes finer particulate that passed the first stage. Together, the two sediment stages bring the water to a clarity that the carbon block and membrane can handle without overloading. A single 5-micron filter without the 50-micron pre-stage will block quickly and is a false economy.
The activated carbon block is the chemical protection stage. Carbon adsorbs chlorine — the primary membrane killer — along with many organic compounds, hydrocarbons, and compounds responsible for taste and odour. A good 5-micron carbon block rated for chlorine removal is adequate for most open-ocean and anchorage conditions. The carbon block must be changed on schedule; a saturated carbon block allows chlorine and organic contaminants to pass through to the membrane.
All pre-filter housings should be sized to handle the full pump flow rate without restricting flow. Undersized housings create a low-pressure zone upstream of the pump — the pump cavitates, output falls, and the pump sustains damage over time. Filter housings rated to twice the pump's nominal flow rate are a worthwhile margin.
Membrane Housing Orientation
The membrane housing can be mounted vertically (feed water entering at the bottom) or horizontally. Vertical is preferred because air bubbles — which are always present when the system starts and any time the intake comes out of the water — rise naturally to the top and exit through the brine outlet rather than accumulating against the membrane. Horizontal mounting is entirely acceptable and more common in practice because of space constraints; the key requirement is that the housing is properly supported along its full length and that the end caps are accessible for membrane changes.
The membrane must be installed in the housing in the correct orientation — the product water tube exits toward the product end cap, not the feed end. Installing a membrane backward passes high-pressure feed water against the backing layer of the membrane, which will destroy it. All manufacturer instructions mark the feed end clearly; verify orientation before closing the housing.
Product Water Testing with a TDS Meter
A TDS (total dissolved solids) meter is an essential commissioning and monitoring tool costing $10 to $30. It measures the dissolved salt content of product water in parts per million and tells you immediately whether the membrane is performing correctly.
Seawater runs approximately 35,000 ppm. A healthy membrane operating at correct pressure should produce water in the 100 to 500 ppm range. Water below 500 ppm is safe for drinking; below 200 ppm it tastes genuinely good and is suitable for cooking where water quality affects flavour — coffee, pasta, sourdough starter. Water above 500 ppm from a correctly pressurised system indicates a membrane in decline, a damaged O-ring allowing brine to bypass into the product stream, or insufficient operating pressure.
Test product water TDS at every commissioning run and record the result with a date. The trend over time is more useful than any individual reading — a membrane that produces 150 ppm in year one and 350 ppm in year three is aging normally; the same membrane jumping from 200 ppm to 800 ppm in two weeks has a problem.
Electrical Requirements
The high-pressure pump is the largest sustained electrical load on most cruising boats outside the autopilot and refrigeration compressor. A DC unit drawing 6 to 8 amps for two hours per day is a manageable load, but it must be wired correctly to avoid voltage drop and circuit problems.
The watermaker requires a dedicated circuit from the battery bank or distribution panel — it should not share a circuit with the autopilot, navigation instruments, or any other load that could cause a voltage drop that affects pump performance. Wire sizing must be appropriate for the current draw and cable run length; undersized wire causes voltage drop, which reduces pump pressure and membrane output. A 30-amp fuse or circuit breaker is standard for most DC installations; verify against the manufacturer's specification for your specific unit.
An anti-siphon valve on the product water line prevents siphoning of tank water back through the membrane if the system is off and the tank is above the membrane housing. Many watermaker installations are damaged or contaminated by reverse siphoning from the tank; the anti-siphon valve is cheap insurance.
The Flush Valve and Bypass Valve
Two additional valves complete a properly specified installation. The flush valve routes fresh water from the tank back through the membrane housing for flushing — this is used after every operational run to displace salt water from the membrane before it dries or concentrates. On systems with an automated flush controller, this happens automatically; on manual systems, the operator opens the flush valve and runs fresh water through for two to five minutes after shutdown.
The bypass valve on the brine side allows the operator to divert brine product directly back overboard without routing through the system — used during commissioning and testing to avoid routing untested or off-specification water into the drinking tank. During the membrane break-in period and any time TDS is elevated, run brine-side bypass until the product meets the target specification.
Commissioning: The First Run
New membranes are shipped in a storage solution — typically glycerine — that keeps them moist and prevents biological growth during transit. The first operational run will produce water that tastes strongly of glycerine, which is harmless but unpleasant. Discard the first 30 to 60 minutes of product water by routing it overboard through the bypass valve.
Begin the commissioning run at low pressure — most systems allow you to start below full operating pressure and ramp up gradually. Check all connections for leaks at low pressure before going to full operating pressure. The high-pressure side of the system runs at 800 to 1,000 psi; a loose fitting at that pressure is a serious safety hazard. Inspect every connection methodically before and during the first full-pressure run.
Once product water is running clear and the TDS meter shows a stable reading in the target range, open the valve to the water tank and record the commissioning TDS as your baseline for future comparison.
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Frequently Asked Questions
What are the four components of every marine reverse osmosis watermaker? Every marine watermaker regardless of brand consists of: a high-pressure pump that compresses seawater to 800 to 1,000 psi, a membrane housing containing the semi-permeable membrane through which product water passes, a pre-filter train that removes particulate and chemical contaminants that would foul the membrane, and product and brine plumbing that routes clean water to the tank and brine back to the sea. The high-pressure pump is the largest and heaviest component, typically weighing 3 to 8 kilograms for a DC unit.
What is the correct pre-filter sequence and why does order matter? The correct sequence is: 50-micron sediment filter first, then 5-micron sediment filter, then activated carbon block. The 50-micron filter catches large particles — sand, shell fragments, marine growth — that would rapidly block finer downstream filters. The 5-micron filter removes finer particulate that passed the first stage. The activated carbon block adsorbs chlorine — the primary membrane killer — along with organic compounds and hydrocarbons. A single 5-micron filter without the 50-micron pre-stage will block quickly and is a false economy; skipping the carbon block allows chlorine to pass through and damage the membrane.
Where must the seawater intake seacock be located and what must it be kept away from? The intake must be below the waterline at all points of sail — an intake that comes out of the water at 25 degrees of heel allows the pump to run dry. For most fin-keel sloops, a position on the hull near the keel forward of the propeller shaft works at any reasonable angle of heel. The minimum separation from the engine exhaust outlet is 1.5 metres — diesel exhaust plumes extending several metres can contaminate the intake with combustion products that carbon filters cannot remove and the membrane cannot tolerate. A dedicated seacock separate from the engine raw water intake is strongly preferred.
What does TDS testing tell you and what readings indicate a healthy membrane? A TDS (total dissolved solids) meter measuring dissolved salt content in parts per million is an essential commissioning and monitoring tool costing $10 to $30. Seawater runs approximately 35,000 ppm. A healthy membrane operating at correct pressure should produce water in the 100 to 500 ppm range. Water below 500 ppm is safe for drinking; below 200 ppm is suitable for cooking and coffee quality. Water above 500 ppm indicates a membrane in decline, a damaged O-ring allowing brine to bypass into the product stream, or insufficient operating pressure. The trend over time is more useful than any individual reading.
What electrical requirements apply to a marine watermaker installation? The watermaker requires a dedicated circuit from the battery bank or distribution panel — it should not share a circuit with the autopilot, navigation instruments, or any other load that could cause voltage drop affecting pump performance. Wire sizing must be appropriate for the current draw and cable run length; undersized wire reduces pump pressure and membrane output. A 30-amp fuse or circuit breaker is standard for most DC installations. An anti-siphon valve on the product water line prevents tank water from siphoning back through the membrane when the system is off and the tank is above the housing.
Why must the first 30 to 60 minutes of product water be discarded on a new watermaker commissioning run? New membranes are shipped in glycerine storage solution that keeps them moist during transit and prevents biological growth. The first operational run will produce water tasting strongly of glycerine — harmless but unpleasant — until the storage solution is flushed out. Route the first 30 to 60 minutes through the bypass valve overboard rather than into the drinking tank. Begin the commissioning run at low pressure, check all connections for leaks before going to full 800 to 1,000 psi operating pressure — a loose fitting at that pressure is a serious safety hazard — and record the commissioning TDS reading as your baseline for future comparison.