Somewhere around day three of a passage, the watermaker becomes the most important piece of equipment on the boat. This is not an exaggeration. You can navigate without GPS. You can sail without an autopilot — badly, exhaustingly, but you can do it. Running out of water has a shorter fuse. At one gallon per person per day for minimum hydration, a crew of four burns through a 50-gallon tank in less than two weeks. Add cooking, washing, and some margin for seasickness, and that estimate gets very optimistic very fast.
A good watermaker ends the math. You produce water as you go. The tank becomes a buffer, not a budget.
How Reverse Osmosis Actually Works
Seawater contains roughly 35,000 parts per million of dissolved salts. Human kidneys can't process anything above about 1,000 ppm without drawing water from the body — drinking seawater accelerates dehydration rather than relieving it. Reverse osmosis solves this by forcing seawater through a semi-permeable membrane at high pressure. Water molecules pass through; dissolved salts, bacteria, and most contaminants do not.
The pressure required is significant — typically 800 to 1,000 pounds per square inch for seawater, compared to the 50–80 psi in your household plumbing. A high-pressure pump is the mechanical heart of the system and the component most likely to require eventual service. The pump draws seawater through a prefilter chain (more on this below), pressurizes it against the membrane, and the result exits in two streams: product water (freshwater, what you want) and reject water (concentrated brine, which goes overboard).
The reject ratio matters for planning. A typical cruising watermaker produces approximately one gallon of freshwater for every three to four gallons of seawater it processes. If you're making 6 gallons per hour, the pump is moving 18–24 gallons per hour of seawater. This is not waste in any meaningful sense — the concentrate goes back where it came from — but it affects how long you run the system relative to how much you produce.
Water quality from a properly functioning membrane runs 200–500 ppm total dissolved solids, well inside the 1,000 ppm threshold and typically lower than municipal tap water in many US cities. A handheld TDS meter (they cost about $15) lets you confirm your membrane is performing and catch a failing seal before the water starts tasting wrong.
Sizing: GPH, Daily Needs, Run Time
The starting point for sizing is your actual daily water consumption. One gallon per person per day is the number that gets used in emergency planning. Two gallons is comfortable living on a boat — cooking, drinking, occasional washing-up. Three gallons per person per day begins to approach the kind of consumption where you're doing laundry and not rationing showers.
For a crew of two living comfortably at 4 gallons per day total, a watermaker producing 3 gallons per hour covers the daily budget in 90 minutes of runtime. That's achievable on solar alone in the tropics. At 1.5 gallons per hour — a small 12V unit — you're looking at over 2.5 hours of daily runtime, which becomes a consideration when you're managing a battery bank.
The general rule: size the watermaker so you can cover daily needs in 1–2 hours of runtime at anchor, then let it run longer when the engine or generator is already on anyway. Running the unit while motoring costs you essentially nothing in extra fuel and keeps the tank full without thinking about it.
Brands
Spectra Watermakers makes the system most serious bluewater cruisers eventually end up with. Their Energy Recovery Technology (ERT) design recaptures energy from the high-pressure reject stream and uses it to assist the intake pump, dramatically reducing power consumption. The Newport 400 produces 1.6 gallons per hour on about 4 amps at 12V — roughly the draw of a single LED cabin light. It's quiet enough to run while people are sleeping below. It is also expensive, complicated to service yourself, and rewards careful installation. If you're doing the Pacific Circuit or spending seasons in remote anchorages, it justifies itself.
The Katadyn PowerSurvivor 40E is the other end of the spectrum: 40 psi input, 1.1 gallons per hour, about 4 amps at 12V. It's the smallest viable continuous-duty system and the one you'll see on boats that want a backup option or don't have space for a full installation. Output per hour is limited, but the power draw is similarly modest and the design is straightforward enough to service at anchor.
Village Marine and Horizon make AC-powered systems aimed at production boats and motoryachts — these are the units you'll find standard equipment on larger vessels. They move more water per hour but require generator or shore power to run, which limits usefulness for a sailboat at anchor far from a marina.
Rainman is worth knowing about as a portable rental or backup option. An Australian company, they make self-contained units powered by a small Honda generator — the whole thing fits in two cases and can be set up in 20 minutes. Some boatyards and cruising associations rent them by the week. If your main unit fails in a remote anchorage, knowing Rainman exists before you leave is valuable.
Installation
The high-pressure pump generates noise and vibration. Install it as far from the living space as physically possible — in a lazarette, a deep cockpit locker, or a dedicated machinery compartment — and mount it on vibration-isolating pads. Thin stainless tubing carries the high-pressure water; every fitting is a potential failure point, so minimize connections and inspect them regularly.
The prefilter chain is what protects the membrane from particulates and biological matter. A standard setup runs: coarse sediment filter (20 micron) to remove large particles, fine sediment filter (5 micron) for smaller particles, and a carbon block to remove chlorine and organic compounds. Filters are consumables. In coastal waters with high biological load — algae blooms, turbid estuaries, near river outflows — they can clog in hours. Change filters before passages and carry spares.
The intake seacock should be below the waterline and as far from the boat's wake and discharge outlets as possible. Running the unit in a marina with engine exhaust in the water, or near a sewage outfall, will foul prefilters and risk contaminating the membrane. When in doubt, wait until you're in clean water.
Maintenance: What Kills Membranes
The membrane is the expensive part — replacement runs $200 to $600 depending on the system — and it can last 3 to 7 years with correct care or fail in one season with neglect. Three things damage membranes: chlorine, biological fouling, and running dry.
Chlorine is the most common cause of membrane failure in boats that spend time at marinas. City water passed through a watermaker membrane without a carbon prefilter will oxidize and degrade the membrane in a single filling. A carbon block in the prefilter chain prevents this when running from seawater. But if you're filling your tank from a dock hose and then accidentally circulate that chlorinated water through the watermaker circuit — common on boats where the plumbing isn't clearly separated — you get the same result. Label your seacocks clearly.
Biological fouling happens when the system sits unused with trapped water in the membranes. Within a few days in warm water, microorganisms colonize the membrane surface and begin degrading the material. The solution is pickling: flushing the membrane with a preservative solution (sodium metabisulfite is standard) before storing the system for more than a few days. Most manufacturers supply pickling powder with the unit. The flush takes about 15 minutes and extends membrane life significantly.
If the boat is in active use, run the system every 5 days minimum — even if you don't need the water, divert it overboard — to flush biological buildup. Some cruisers set a weekly alarm specifically for this.
Troubleshooting
Low output is the most common complaint, and the first thing to check is always the prefilters. A clogged 5-micron filter looks exactly like a failing membrane from a pressure standpoint. Replace the filters, note the improvement (or lack thereof), and then consider whether the membrane needs cleaning. Membrane cleaning solutions — citric acid for mineral scaling, enzymatic cleaners for biological fouling — are worth carrying and will often restore 80–90% of a partially fouled membrane.
Salty product water (high TDS reading, or water that tastes like anything other than bland) points to either a failing membrane or a seal leak. First check all O-rings at the membrane housing — a single deteriorated O-ring allows salt water to bypass the membrane and enter the product line. Replace the O-ring set before condemning the membrane. If the O-rings look fine and TDS remains high, the membrane has reached end of life.
The cruisers who get the most from their watermakers treat it like engine maintenance: scheduled checks, filter logs, pickled when stored. The ones who buy a $3,000 unit and expect zero-maintenance operation are the same ones posting to forums about membrane failure two seasons in. The system does what you expect of it in proportion to what you put into it.