Rainman Watermaker Review 2025: 2 Membranes, 140L/Hour Output Tested

Thomas Flinskau
Sep 6
6 min read

Introduction: Why a Watermaker Matters for Cruisers

For liveaboard sailors and bluewater cruisers, fresh water is more than a convenience — it’s survival. Running out of drinking water on a passage or relying only on marina taps can quickly turn freedom into dependency. That’s why a watermaker often becomes one of the most valuable pieces of gear on a cruising sailboat.

In this review, we’re taking a deep dive into the Rainman 2-membrane watermaker system that we have onboard our sailboat "Prosperity" — a high-output desalination unit capable of producing 100–140 liters per hour. We’ll cover everything from performance and water quality to maintenance, costs, and who this system is best suited for.

Whether you’re considering your first watermaker or upgrading to more output, this guide will help you decide if the Rainman is right for your cruising lifestyle.

What Is the Rainman Watermaker?

The Rainman Philosophy — Simplicity & Reliability

The company’s approach is straightforward: build robust desalination units using industry-standard components like Dow FilmTec membranes, standard pumps, and pressure valves that can be serviced by the owner. Instead of locking you into proprietary spares or dealer-only servicing, Rainman empowers sailors to fix and maintain the unit at sea.

Rainman 2-Membrane Setup Explained

How the 2×40″ Membranes Work

The “2-membrane” Rainman uses two standard 40-inch seawater reverse osmosis (RO) membranes in parallel. That effectively doubles the surface area compared to a single-membrane unit, which in turn increases the freshwater output. More membrane area means less runtime to produce the same amount of water, a big benefit for families like ours or larger crews.

Output — 100–140 Liters per Hour

In real-world use, you can expect around 100–140 liters per hour (26–37 gallons per hour) depending on seawater temperature and salinity. That’s enough to keep a liveaboard family in showers, dishwashing, and plenty of drinking water without rationing.

Power Draw & Energy Requirements

Most 2-membrane Rainman systems are AC-powered, and that is also our unit, and draw about 1,250 watts when running. That’s manageable on shore power or with a small generator, but it’s a significant load if you’re relying only on batteries.

Since we have 400Ah LifFeP04 lithium batteries, and a 2kW inverter, we can easily run this from our batteries, that we charge from our solar panels. Although during winter we do need to run the generator as well.

For energy-conscious cruisers, Rainman also offers 12V and 24V DC units, but those typically produce far less water (26–55 L/hr).

Installation and Portability

On-Deck Portable Use

The classic Rainman setup is truly “plug and play.” You bring the case on deck, drop the feed hose overboard, run the brine discharge away, and route the freshwater hose into your tank. It’s simple, flexible, and requires no installation holes through the hull.

Semi-Permanent Installations in Lockers

Over time, many owners create a “semi-permanent locker” for the Rainman. This keeps the unit out of the way, reduces lifting, and allows for neater hose runs. Ventilation is key, since the pump generates heat and noise. A cockpit locker, lazarette, or even a spare head are common locations.

Modular systems

Installing a watermaker is, when you plan it proparly, is not too time consuming. And With the right planning, you can get the parts plsce in the most place and practic-efficient places onboard.

Handling Hoses, Cases, and Noise

The downside of portability is bulk. The Rainman cases are heavy, and carrying them up and down for every use can get old. The pump is also loud — not unbearable, but noisy enough that many cruisers prefer to run it on deck rather than inside the saloon.

Performance & Water Quality

Real-World Freshwater Output

Owners consistently report output close to the published 100–140 L/hr, provided the seawater is clean and the filters are fresh. We never run it in dirty marina water, and has always from 120-140L/hr. Cold or highly saline water reduces output, while warm tropical water increases it.

TDS Readings and Taste

The Rainman produces high-quality potable water. With new membranes, TDS (total dissolved solids) readings are well within safe drinking levels, often under 200 ppm. A handheld TDS meter is essential to monitor quality and confirm the system is running properly.

Our experience after half a year of use and 1 filter change, is that we always get levels under 200 ppm, and we are very happy with these results. And the taste has always been very good.

Factors That Affect Efficiency (Temperature, Salinity)

Warm, less salty water (e.g., tropics) = more output.
Cold, salty water (e.g., higher latitudes) = less output.
Dirty water near harbors = clogs filters faster and may require more maintenance.

Rainman Watermaker Maintenance & Care

Filters and Pre-Filters

The Rainman uses standard 5-micron pre-filters to keep sediment and debris out of the membranes. These should be checked often, especially after running in silty or plankton-rich water.

Flushing and Pickling Explained

To prevent biological growth inside the membranes, the Rainman must be either flushed with freshwater after use or pickled with a preservative solution if it will sit unused. In warm climates, pickling may be necessary after only a few days of inactivity.

Onboard our sailboat "Prosperity", as a family of 5, we use a lot of water, making us running the watermaker 2-3 times a week. This makes it unnecessary to flush or pickling the watermaker.

Of course, if we plan to stay in a marina with high quality fresh water, or stay away from the sailboat for more a week or more, we will flush or do the pickling process of our watermaker system.

How to Avoid Biofouling

Neglecting flushing and pickling leads to biofouling — smelly, slimy membranes that are difficult to recover. With proper care, membranes can last 5–10 years; without care, they may fail in months.

Reliability and Repairability

Standard FilmTec Membranes

Rainman’s choice of industry-standard membranes is a huge advantage. Replacement elements are available worldwide, and you’re not locked into a single supplier.

Field Repairs with Basic Tools

The pumps, gauges, and valves are also non-proprietary. Many cruisers carry a small spares kit and have successfully repaired their Rainman at anchor with simple tools.

Long-Term Durability

With regular maintenance, owners report long service lives. High-pressure hoses and seals may need periodic replacement, but the overall system is designed to be rugged and user-serviceable.

Pros & Cons of the Rainman 2-Membrane System

What Owners Love

High output — 100–140 L/hr is necessary and enough for families.
Portable or modular — no complicated installation needed.
Repairable — parts are standard and widely available.
Durable — designed for long-term cruising.

Common Complaints and Drawbacks

Bulky and heavy (portable) — not fun to lift often.
Noisy — the pump makes a noticeable racket.
Requires maintenance — flushing and pickling are non-negotiable.
Not the cheapest — though cheaper than many dealer-installed systems.

Rainman Watermaker Price & Value for Cruisers

Initial Cost Breakdown

Depending on model and region, the Rainman 2-membrane systems typically cost €4,600–€6,800. That includes the pressure supply unit and membrane cases but not always installation kits or extras.

Ongoing Maintenance Costs

Budget for:

Pre-filters (~€5–10 each, replaced regularly).
Pickling chemicals and cleaning solutions.
Occasional hose or seal replacements.
Membrane replacements every 5–10 years (€300–500 each).

How It Compares to Other Brands

Compared to integrated systems, the Rainman is often cheaper, easier to maintain, and more portable. However, it lacks the quietness and “set-and-forget” automation of some high-end brands.

Who Is the Rainman watermaker 2-Membrane Best For?

Liveaboard Families

Perfect for families who need plenty of water for showers, dishes, and daily living. The high output reduces runtime and generator hours.

Offshore Bluewater Sailors

Ideal for passagemakers who want redundancy and the ability to service their system anywhere in the world.

Weekend Cruisers — Worth It?

Probably overkill. Occasional sailors might prefer marina water tanks or a smaller, single-membrane unit.

Final Verdict: Is the Rainman Watermaker Worth Buying?

The Rainman 2-membrane watermaker strikes an excellent balance between output, reliability, and serviceability. It’s not the lightest, quietest, or cheapest system, but it is one of the most practical and robust options for long-term cruisers.

If you’re a liveaboard sailor, a family cruising the world, or a bluewater adventurer who wants independence from dockside water hoses, the Rainman with two seawater membranes is absolutely worth considering.

Final score: 8.5/10 — A rugged, high-output watermaker designed for real cruising life.

❓ Rainman Watermaker FAQ

1. How much water does the Rainman 2-membrane system make? The 2×40″ Rainman watermaker typically produces 100–140 liters per hour (26–37 gallons per hour), depending on seawater temperature and salinity.

2. How much power does the Rainman watermaker use? The standard AC-powered 2-membrane system draws about 1,250 watts, making it suitable for shore power, small generators, or large inverter setups.

3. Do Rainman watermakers need pickling? Yes. If the system won’t be used for more than a few days (especially in warm climates), the membranes must be pickled with a preservative solution to prevent biofouling.

4. How long do Rainman membranes last? With proper care, the RO membranes can last 5–10 years. Without flushing or pickling, they can foul in just months.

5. Can I install the Rainman watermaker permanently? Yes. While designed as a portable system, many sailors build a semi-permanent locker installation, routing hoses neatly and reducing lifting.

6. How much does a Rainman 2-membrane watermaker cost? Depending on model and accessories, expect €4,600–€6,800 for the system, plus filters, chemicals, and spares.

7. Is the Rainman watermaker worth it for liveaboard sailors? For full-time cruisers and families, yes. The high output, portability, and repairability make it a solid long-term investment. For weekend sailors, a smaller unit or marina water may be more cost-effective.