Getting Fresh Water While Cruising on Your Sailboat

UPDATE 2/3/2017: I’ve been fielding quite a few questions about what I ended up doing.  I did get the Katadyn 40E, and finally published some thoughts I wrote down while using it.  Read them here.

I learned a lot during my first year cruising on a small sailboat.

One of the big ones: it sucks taking the dinghy to shore with empty 5-gallon jerricans, finding a reliable source of fresh water to fill them up, take the dinghy back out to the boat, lift the cans onto the boat, fill up the water tank, and repeat until my 30-gallon water tank is full.

Most small sailboats have small water tanks, and this would normally require staying somewhat close to fresh water sources.

In Arthur’s Town, Cat Island, Bahamas, I had to walk 1/2 mile one-way carrying ten gallons of water by hand – about 83 pounds.

And don’t think that fresh water is free either.  The Dinghy Dock in Culebra, for example, charges 25 cents per gallon (as of 2013).

I don’t always have the luxury of tying up to a dock with fresh water because 1) I like the remote places and 2) my engine isn’t always working for docking maneuvers.  Don’t get me started on that last point.

I tried making a rain catcher.  This was an epic failure, probably because I made it in a hurry with only materials I had readily available on my boat.  Still, it didn’t rain enough to keep up with my 1-2 gallon per day consumption.

Last option – the expensive one – a reverse osmosis marine desalination system.

Boat Desalination Systems

Watermakers work through a process called reverse osmosis.

Salt water is forced towards a membrane at very high pressure.  The high pressure and permeability of the membrane allows only the pure H2O to pass through, but 99% of salt, dirt, viruses, bacteria, etc are left behind.

The process is not very efficient; about 90% of the saltwater taken in by the watermaker is wasted and discharged back overboard.

Reverse Osmosis Schematic

Reverse Osmosis. (A) Pressure is applied to the (B) seawater, (C) salt and other contaminants cannot pass through the (D) membrane, leaving (E) potable water. Diagram Wikimedia Commons, Colby Fisher.

A few different methods may be used for creating the high pressure needed to pass water through the membrane.

  • Engine driven
  • AC or DC electric motor driven
  • Hand pumping

A somewhat new technology called “hydraulic amplification” has really made these systems more efficient.  Think of it as a low-gearing system like on cars or bikes.  Low water pressure is amplified into high water pressure by varying pumping cylinder diameters.

Desalination systems do require maintenance.  Proper maintenance will ensure your system will work for years without the need to replace parts.  Forgetting a maintenance item will almost guarantee needing to replace the membrane prematurely or not having water when you need it most.

Maintenance items require replacing seals, oiling pumps, replacing prefilters, flushing the system, and something called “pickling” where the membrane is flushed with a biocide.  Especially in hot tropical climates, nasty growth can accumulate inside the system and will dramatically reduce efficiency, even after only a few days of non-use.

Watermakers for Small Sailboats

I’m only going to focus on sailboat watermakers for small boats – for those of us with small cruisers, 25-35 feet, maybe only two or three people, no generators, and none of the fancy systems like dishwashers and washing machines that larger cruising boats may have.  Those systems are a completely different (expensive) animal!

AC-driven watermakers are out.  These systems provide very high output but at very high energy cost.  You will need a high-capacity generator for these systems, and many small cruisers don’t have room for these (or enjoy the noise they make).

engine

Most small boats have small engine spaces, with no room for extra pulleys, pumps, and hoses.

Engine-driven watermakers are probably out too.  In order to install an engine-driven watermaker, you’ll need to put an extra pulley on your engine shaft, then have the room inside your engine compartment for the pump and other components.  The engine spaces (I can’t call them “rooms”) on most small boats won’t allow for this.

Manually operated pumps may seem like a novel idea at first, but you’ll quickly tire of them.  They’re good to have for emergencies but not to use on a daily basis.  The KATADYN Survivor 35, for example, is only two feet long by six inches around, costs $2,300, and produces just over one gallon per hour.  That’s a lot of pumping to get your daily water!

That leaves DC-powered watermakers probably the best watermakers for small sailboats.  But which one?

Choosing an Electrical Watermaker for Small Sailboats

First determine your water needs

If you schlepped water to your boat in jerricans, you’re probably like me and very frugal with your water.  You dribble water out of your foot pump to fresh-water rinse your dishes and brush your teeth.  You take a sponge bath once a week.  You wash your clothes, by hand, once a month.  Doing this, you got away with one gallon per person per day.

But now that we’re going to have a watermaker we can be more comfortable!  Now we can rinse down with fresh water after each dip in the ocean, rinse our anchor chain with fresh water before stowing it in the locker, rinse our decks and sails with fresh water after a passage, take real showers on a daily basis, etc.  It’s a good idea to double or even triple your previous usage.

That means we’re up to three gallons per person per day.

Three people aboard your boat will mean nine gallons per day.  Round up to ten.

Now for our choices of watermakers for small cruising sailboats:

This is the smallest, cheapest, lowest-current-draw watermaker I’ve found.  They’ve been around for over a decade and have had all the bugs worked out and are highly reviewed.

The KATADYN PowerSurvivor 40E has an output of 1.5 gallons per hour at the cost of only 4 amps per hour on a 12VDC system.  It isn’t modular, but it is small at 16.5″ long, 15.5″ wide, and 6.8″ tall.

If it’s just you on the boat, this means you’ll be running this system for 2 hours and need 8 amps.  Three people and you’re up to a long 6.5 hours of operation and 26 amps.

This pump does come with a handle for manual operation in emergencies.

KATADYN also makes the PowerSurvivor 80E, which may be the better option for more than one person.  It produces 4 gallons per hour at the cost of 8 amps and $4,995.00.  It’s modular and actually smaller than the 40E, if you want to spare the extra $900 and 4 amps.

Spectra Watermakers Ventura 150 | Marine Warehouse $6,150.00

Ventura 150The Ventura 150 outputs 6.3 gallons per hour at the cost of 9 amps.  Doing the math, you can get the same amount of water as the PowerSurvivor 40E but for almost half the power consumption.  The drawback, however, is the price and size.

It is modular, unlike the KATADYN, meaning you can squeeze the components where you can fit them.  The pressure vessel module is roughly 27″ x 9″ x 12″ (already bigger than the KATADYN); the feed pump 6″ x 12″ x 15″; the pre-filter 6″ x 12″ x 7″.

If you have the space on your boat and can dish out the extra cash, your three people can get their water at the cost of 14 amps, compared to the PowerSurvivor 40E’s 26 amps.

The system is available with a full electronic MPC control unit for an extra $2,000.

Horizon Reverse Osmosis and Sea Recovery also make modular watermakers but they’re slightly larger than what we’ve seen here – the pressure vessels are in the 2 to 4-foot range with an array of strainers, filter, pre-filters, and pumps.  They output in the 8GPH range but operate around 25-30 amps, with startup draws of up to 90 amps.

These draws are typically too much for the battery banks that small sailboats have and will quickly deplete the batteries.  The systems also cost in the $10,000-$12,000 range.

Final Comparisons

There are stories out there about the KATADYN units being much louder than the Spectra units, and stories of the Spectra units failing quite a bit.  Spectra seems to have worked all these bugs out, and the KATADYN doesn’t bother me at all.

Then again, even though the Spectra is modular, I don’t know where I’d find room for the larger components.

I like the idea of only drawing 4 amps from my batteries while using a watermaker.  My DC refrigerator draws between 7-9 amps and puts quite the load on the batteries.  I’d hate to put another 9 amps on them to make water.

It’s better on your batteries to draw fewer amps for a longer time than it is to draw a larger load for a shorter time.

And since it’s just me (usually?) I can manage 4 amps for 2 hours.  If you have more people, that means you have a slightly bigger boat with more room for components, and more options for energy, so the Spectra may suit you well.

I’ll also re-engineer my rain catcher to hopefully augment my watermaker, which I now see as a necessity.  I’ll keep you updated on this!

Well, what did I end up doing?  I installed the Katadyn 40E.  Here’s some thoughts on it in Reviewing the Katadyn 40E While Cruising the Caribbean.

 

Anyone have stories on either watermaker?

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