Would it surprise you to know that over 70% of the earth’s surface is covered with water? In addition, over 95% of the Earth’s water is in the oceans. That means when you are in your boat you are floating on the largest water resource on the planet. The paradox is that despite this, you cannot drink the water.

Yet, thanks to the pioneering work of a French physicist over 250 years ago, reverse osmosis is able to convert salt water into drinking water. This is a game changer when it comes to cruising and planning your water supply.

Everyone that wants to be at sea and not be dependent on the shore for supplies like water, needs a saltwater filtration system. If you are thinking about buying your first system or replacing your existing system then there are important things you need to know. Read on to learn more.

Reverse Osmosis – What are We Talking About Here?

Understanding the basics of reverse osmosis will help you grasp several factors in choosing a saltwater filtration system. However, it is helpful to first consider, what is osmosis?

Osmosis is the chemical attraction between 2 fluids (separated by a semi-permeable membrane) that have a different concentration of molecules. The laws of physics (hydrostatics) dictate that the 2 fluids will seek to equalize the concentration of molecules. That means molecules from the fluid that is higher in concentration will be forced through the membrane into the fluid with a lower concentration. This is referred to as the osmotic pressure.

Reverse osmosis achieves the opposite. A fluid, low in concentration, can be further ‘filtered’ or forced using a hydrostatic pressure to push molecules across a membrane into the high concentration fluid.

The key here is the hydrostatic pressure applied to the fluid must be greater than osmotic pressure. When that happens, reverse osmosis follows. This is the basic science behind saltwater filtration systems.

Know Your Anatomy First

It is important to know the basic anatomy of your saltwater filtration system first before selecting a device. This will help you understand that the installation of a device is quite a bit of work and you want to make sure it is right.

Water Supply

You will need a supply of saltwater to enter the system from outside the boat. This then needs to feed into the filtration system and processed. The water storage tank then receives the drinkable water.


A low-pressure pump is needed to push water through the first part of the system. In particular, the water needs to be pushed through the particle filter.

A high-pressure pump is needed to feed the reverse osmosis membrane. This pump has to be high pressure as it is needed to create the necessary hydrostatic pressure. Natural osmotic pressure is around 24 bar. Therefore hydrostatic pressures between 40 to 70 bar are needed for seawater.


The particle filter will capture anything in seawater that you don’t want to drink, apart from the salt. Good filters will trap tiny molecules down to 5 microns.

The RO membrane is used in part to maintain the hydrostatic differential pressure across the 2 volumes of fluid. The high concentration fluid will become more and more concentrated as the salt levels in this volume of fluid builds-up. Eventually forming a sort of brine that is then returned to the sea.

A good strainer also needs to be fitted to the system. This serves the purpose of trapping medium to large debris that comes in with the seawater. Thought needs to be given to the location of the strainer. It is sensible to keep it away from electrics and it needs to be easily accessible so that it can be inspected and maintained.

Power Supply

Your power supply options can vary from the use of batteries, a generator or connect the supply of power from the boat’s engines. As a general rule, the more water supply you want or need, the larger the power supply needs to be.

AC power is the most likely supply offering 120 or 240 volts. This will suit most systems. The alternative is a generator that can deliver at least 3 to 4 kW. Lower power systems can get by on a DC supply driven by 12 or 24 volts. Engine driven systems are possible but that requires a bespoke system to be installed.

Of course, if you are relying on the engines, then you need to make sure they and the fuel system is in top condition. Check out this article here on fuel system maintenance.

Saltwater Filtration Systems – Final Considerations

The exact system you need will depend on the size of your boat and its design. Understanding the basics will go a long way to making the right decision. Here are a few final pointers to help you chose the right saltwater filtration system for your boat.

Try to keep the hose length that connects your system as short as possible. Longer hosing will just add work to your pumping systems and be less efficient.

Before you buy, make sure you calculate your daily water requirements based on the number of people on board your boat. A good rule is 2 to 3 gallons of water per day per person. That will include washing and drinking water. Expect to run your system for 2 to 3 hours per day.

Installing a new system is not a small job. It needs careful thought and planning. If you are unsure about anything or need further advice on which systems best suit your needs please contact us here.