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Fuel and Water Filters: Simple Insurance Policies

By Bill Sandifer

Article taken from Good Old Boat magazine: Volume 2, Number 2, March/April 1999.

Picture a hot, windless Sunday afternoon as you power home on a
glassy sea. Suddenly your engine slows and stops or overheats. Today
of all days! You really did not need this, and it could have all been

Fuel filter cross-section diagram

How? By installing and maintaining filters to clean the fuel
and water systems you and your boat need to operate successfully in a
water environment. Filters come in many types and sizes and are
custom-designed to serve a specific purpose. Many sailors tend to
ignore the mechanical side of their vessels and assume the attitude
that, “It’s a sailboat; it should sail, right?” Well yes, but the
wind does not always blow in the desired direction or with the
desired velocity. In times of need, our mechanical friends on board
make the difference between a reasonable end to a cruise, no matter
how long or short, and a long wait on a hot and windless sea.

Filters fall into three groups. Required, for fuel and engine
cooling water. Desirable, for engine oil, potable water,
refrigeration cooling, and seawater uses. Cosmetic, for air and sound
filtration. Let’s take a detailed look at each type available for
today’s vessels.

Pre-tank filters

Fuel filters can be defined as pre-tank, primary, and secondary. A
pre-tank filter would be a funnel type that provides basic filtering
of the fuel as it is poured or pumped into the tank. This type of
filter is very basic but very valuable. They range from a plastic
funnel with a screen in the bottom to catch dirt, leaves, and large
contaminants such as bits of plastic, to the more sophisticated Baja
filter. The Baja filters are aluminum funnels designed for cruisers
who travel remote cruising grounds such as the Sea of Cortes,
adjacent to Baja California in Mexico, where fuel is scarce and
supplied in used 55-gallon drums of dubious origin.

Baja filters have two extremely fine stainless steel mesh
screens to trap fine particulate matter (sand, dust, etc.) and a
water-resistant filter to keep out a large majority of water that may
be present in the fuel. The filters are really designed for diesel
fuel but will assist in filtering gasoline. The Baja filter protects
your tank from water-loving bacteria and helps prolong the useful
life of the onboard primary and secondary filters.

Primary filters

Primary filters are the off-engine filters, usually added as
after-market equipment to your fuel system. Their manufacturers have
names like Racor, Fram, Sierra, and Groco. The filters come in single
or multiple units, in-line or independent mount, spin-on element or

Filters are sized in accordance with the projected fuel flow
per hour required by a specific size engine. Diesel engines, because
they return unused fuel to the tank, will have a larger flow rate in
gallons per hour (gph) than equivalent gasoline engines but will
consume less fuel per hour. The filter must be sized for the flow
rate, not the consumption rate, of the engine. The gasoline engine
either burns or discharges as unburned all fuel fed into it.

A rule of thumb for sizing filters for gasoline engines is 10
percent of maximum horsepower equals gallons per hour (gph). My 30-hp
Atomic 4 gasoline engine has a potential maximum gph of 10% x 30 hp =
3 gph. I don’t think the engine will ever burn this amount as it
never runs at peak power but, theoretically, it could. My primary
filter is a Racor 200 series turbine with a 15 gph flow rate.
Overkill? Sure but it works all season long.

A diesel engine flow rate is horsepower x 18% = gph, thus a
30-hp diesel would have a theoretical flow rate of 5.4 gph.

A filter that is oversize for the projected gph will work and
last longer than an exact gph filter. A filter that is too small
(less than the calculated potential gph) may restrict fuel flow and
cause engine performance problems. With filters, bigger – within
reason – is better.

The newest type of fuel filters is the spin-on canister type
which looks like the familiar spin-on oil filter we use for our
automobiles. They are very differenton the inside, however. Spin-on
filters are commonly chosen for gasoline engines, while spin-on and
turbine type filters are commonly used on diesel engines. The larger
diesels usually use turbine type filters. Pure fuel is more critical
to diesel engine operation as the injectors of a diesel are
particularly sensitive to particulates in the fuel.

Spin-on elements are easier to change out than turbine units,
and when you change a spin-on element, you renew the whole filter. In
turbine units, you can change the paper element and still have
particles and water in the filter if you do not completely
disassemble and clean all parts of the unit. Some of the fine
contaminants in the fuel of my Atomic 4 adhered so tenaciously to the
turbine vanes they required scraping to remove. Is it any wonder we
need good filters when the contaminants in our fuel harden up like

Filters come as filters alone or as a combination filter and
water separator. Three-stage filters have a turbine section for large
particles or gross amounts of water, a coalescing ring to trap the
remaining water, and a micron element to remove fine particles.
Filters are classified by microns. Different engines have different
micron requirements. Racor’s standard filter size is 2 microns for
their spin-on type filter/water separators. Their turbine type has
elements that can be interchanged between 2, 10, and 30 microns. A
common combination is a 10-micron primary filter and a 2-micron
secondary (on-engine) filter.

If we use a Baja filter when we fill up the tank and still
have water in the tank, where does the water come from? Since boat
fuel tanks are vented, regular air interchange between the atmosphere
in the tank and the external atmosphere takes place. A cycle is
established in each tank when the air heats and expands during the
day, and excess air is expelled through the vent line. When the
ambient air cools, the air in the tank contracts and sucks in
external air to equalize the pressure. The air that is sucked in is
cool and damp, bringing moisture into the tank. This moisture
condenses on the exposed interior of the tank forming droplets which
fall into the fuel and settle on the tank bottom in the form of water.

If we never introduced water from contaminated fuel into the
tank, we would still have some water in the tank from the
condensation process. A good way to reduce this air interchange is to
keep the tank topped up with fuel, thus limiting the air space
available in the tank and minimizing atmospheric condensation.

In an effort to keep operating even with a plugged fuel
filter, boats with larger engines may have multiple primary filters
piped in such a manner that one filter may be used in the system
while the other filter is being cleaned. This setup is more common to
power boats and trawlers than to sailboats, but has definite merit.
Multiple filters can be piped together as Primary A, Primary B, and
so on. Finer and finer filtration is possible as the fuel passes
through each stage. Usually single filters are designed to clean the
fuel down to 2 to 10 microns (a micron is one one-thousandth of a
millimeter). Filters are manufactured down to two microns so a
multiple filter system could commence with a Primary A at 30 microns,
Primary B at 10 microns, and a Primary C at 2 microns. At 2 microns,
the fuel is very, very clean. Commercial firms that advertise that
they “polish” your fuel use this multiple filter approach plus a
centrifuge for a complete cleaning.

When considering a multiple filter system, remember that two
filters piped in parallel to a common manifold will have a combined
gph of the capacity of both filters, e.g., two 60-gph units will
equal 120 gph. Three 60 gph filters piped in series (Primary A, B,
and C), will have the gph of the single unit which is 60 gph.

Before we leave the primary filter discussion, let’s talk
about filter maintenance. The best way to determine the state of
cleanliness of a primary filter is to install a vacuum gauge on the
discharge side of the filter. The gauge shows how hard the engine is
having to “suck” to pull fuel through the filter. The higher the
vacuum, the dirtier the filter and the greater the need to replace
the element and clean the filter unit.

Racor makes a vacuum gauge that replaces the tee handle on
the top of their turbine filter. This makes for a very neat
installation. Individual vacuum gauges on single or multiple filters
may be teed into the discharge line of each filter to reveal the
state of the filtering element. The other ways to determine when to
change a filter (other than vacuum gauges) are more subjective. A
good method is to rely on running hours to set a time to change
filter elements. This can be anywhere from 50-hour intervals to 200
hours depending on how careful you are in providing clean fuel. This
is where the Baja filter will help extend the life of the primary and
secondary filters.

If you do not use a pre-tank filter and put a load of
contaminated fuel in the tank, a brand new filter may only last five
minutes. Use a pre-tank filter or know, for sure, you are pumping or
loading clean fuel. When I was a kid and worked at a fuel dock in
Oyster Bay, New York, Gulf Oil provided off-pump, in-line gasoline
filters in an effort to assure clean, waterless fuel. One of my daily
jobs was to check the large storage tanks with a long rectangular
wooden combination fuel gauge smeared with water finder paste to see
how much fuel we had and if it had any water in it. Some ports today
are not as careful about providing clean fuel. Even fuel purchased at
the local gas station may not have an in-line filter and may give you
a good dose of water.

Secondary filters

Pre-tank fuel filter location drawing

Assuring clean fuel to start with is your best guarantee of
trouble-free engine performance. After the fuel has passed through
the pre-tank and primary filter stages, it flows to the engine and
the secondary filter. The secondary stage may be as simple as a
screen in the intake line of a gasoline carburetor or another
canister-type filter mounted directly on a diesel engine. The
secondary filter on my Atomic 4, which is equipped with an electric
fuel pump, is in the bottom of the fuel pump itself and is a fine
nylon screen on a round plastic frame. I recently received a
communication from Don Moyer of Moyer Marine recommending the
addition of an in-line filter between the fuel pump and the
carburetor. Compared to a 10-micron primary filter, my screen is
pretty coarse, but then this is a gasoline engine, not a diesel.

The engine manufacturer normally provides the secondary fuel
filter, sized to the engine. Other than carrying a spare element or
having a screen you can clean, there is little to be done with the
secondary element. If the primary filter system is efficient in
cleaning the fuel, the secondary system should be trouble-free except
for an annual maintenance. Remember, clean fuel is the lifeblood of
your engine. Take care to purchase a high quality marine (not
automotive) type primary fuel filter and learn how and when to
maintain it. You’ll really be glad you did when you are powering home
over a hot or cold windless sea.

Engine overheating

In our second problem scenario, the engine overheats. The probable
cause is trash in the seawater intake or strainer or a failed water
pump impeller. Most sailboat engines are seawater cooled, either
directly by seawater circulation or indirectly through a heat
exchanger. The seawater enters the hull through a through-hull intake
with a perforated round bronze screen over the outside or a
rectangular finned through-hull. It is possible for the screen or fin
to plug up with foreign debris or marine growth, but if the engine
was running cool when you left the mooring and suddenly overheats,
the problem is probably elsewhere.

I once chartered a sailboat in the Bahamas. I left the dock
under power, and within five minutes the engine overheated. The
problem was marine growth on the seawater strainer. The boat had not
been properly maintained. On my own Pearson, I routinely check all
the through-hulls with a mask and snorkel. Zebra mussels, barnacles,
and even oysters love the cozy atmosphere of a through-hull

If it is not the through-hull, then where? The next step is
the seawater strainer. This strainer should be installed in the
seawater intake line between the seacock and the downstream
distribution. I say downstream distribution because it is possible to
use the seawater intake for more than one purpose, but that’s fodder
for another article. The seawater strainer should be as large as
practical. The larger it is, the more trash it can hold before
becoming clogged. Groco makes a fine line of bronze and Plexiglas
seawater strainers. Some other manufacturers are Puritan, Par, Vetus,
and Forespar.

A pre-sail checkout should include making sure the seawater
strainer is clean. A few minutes’ work will assure a trouble-free
trip. I realize it is a pain to crawl into the bilge to check the
seawater filter, but it is worth doing. I know it’s time to diet when
I cannot easily climb into the cockpit seat lockers to check out the
seawater filter. (Something about the ratio of sun to beer intake,
according to my wife.)

Finally, if it is neither of the above, check the seawater
pump impeller. Old impellers tend to throw off their blades, which
then get caught in the engine cooling system and block the water
flow. The only defense against this is to replace the impeller
annually. Globe/Barco impellers are made of niprene, which is an
elastomer combining properties of rubber, nitrile, viton, and
neoprene. The impellers are self-lubricating. They are used by the
U.S. Navy and Coast Guard and are sold by marine outlets. Carry at
least one spare impeller, if not two.

If you change your impeller every season, or at least inspect
it, you will know which tools are required and how much time it will
take. On some engines, it is fast and easy, and on some it takes
several hours. (Editor’s note: We remove ours during winter layup and
lubricate the housing with Vaseline or water pump grease when we
reinstall it.)

Water filters

Racor Water filter

Next let’s look at filters we need for our health and well being.
Consumable water is a requirement for all manner of life on earth,
sailors included. The concept of fresh water in our homes and on our
boats is sometimes a misnomer. Scientists studying pollution
worldwide are coming to the conclusion that water quality and
quantity around the world is in serious decline and will constitute a
major problem in the 21st century for the developed and developing
countries of the world. Bacteria and toxins, along with chemical and
hydrocarbon contaminants, endanger all of our earth’s water.
Antiquated water treatment facilities are ill-equipped to handle
today’s level and type of pollutants. Many of the new breeds of bug,
especially cysts like Cryptosporidia cannot be effectively removed
from the water supply. If this is true of the municipal water supply
of the United States, consider the out islands and other remote
locations. Even rainwater can pick up contaminants from the
atmosphere on its way to the earth.

Now consider the water on board our good old boats. It may be
anything but pure and fresh. It may taste of the bilge, smell of
fiberglass, look like mildew, and carry particles of unknown origin.
We need to take care of this most precious commodity. Remember, a
person can live without food for more than 30 days but cannot live
even five days without clean water.

We can have clear, sweet, clean fresh water on our boats
through the care of our freshwater tanks and the use of filters to
cleanse the water of many of its impurities prior to use.
Pre-tank water filters are the equivalent of the Baja filter,
which is used as we fill our fuel tanks. The least we can do in this
regard is to use a funnel with a fine mesh screen to remove any
solids that may be present in the water and, of course, to carefully
select the source of our water in the first place. The best we can do
is to use a pre-filter such as General Ecology’s Dockside Pre-Filter
to keep dirt and sediment out of our freshwater tanks.

Letting the water in the fill hose run for a short while will go a
long way toward assuring that we are getting fresh water from the
supply, rather than the water in the hose. Water that has been
sitting in plastic hoses will usually add an unpleasant taste to your
water supply unless you have a special potable water hose made to
eliminate the problem. Even if you do use a special hose, unless you
can hook up to the hard piping of the source there may still be
conventional plastic hose between your hose and the water source with
the associated taste problem.

Water will also contain dissolved chemicals and contaminants
that cannot be seen, tasted, or smelled but are not good for humans.
These can be parasitic cysts, solvents, and other nasty critters and
substances. Pre-filters will not remove these contaminants. Our water
tanks provide an almost ideal breeding environment for these nasty
substances to grow and multiply. Fungi, Giardia cysts, amoebic cysts,
microscopic worms, larvae, and other undesirable creatures and plants
thrive in this environment. The problem is exacerbated by taking on
water from different sources. Various water supplies contain
different pollutants which can “gang up” to create problems they
would not normally cause by themselves.

We are usually our greatest enemy in the fight for clean
water. We fill the tanks at the beginning of the season and use the
water sparingly during our time aboard. Weekend to weekend the water
sits in the tank and “grows” things. We don’t want to waste our water
supply and dump it every week, and we can’t clean the tank every
week, so, what to do?

The first step in assuring a clean, sweet water supply is to
find a supply that is sweet to begin with. We are going to use a lot
of water to clean up our onboard supply. Next, we add 2/3 cup of
bleach (sodium hypochlorite) diluted in one gallon of sweet water for
every 10 gallons of tank capacity. Fill the tank to the brim with the
mixture and let it sit for 24 hours. Dump all of the water and start
over, this time add one quart of white vinegar for every 5 gallons of
tank capacity, fill to the brim again, and let it sit for 48 hours.
Then dump it – all of it.

Next, fill the tank with sweet water with no additives, let
it sit for another 24 hours and dump it. Refill the tank adding one
teaspoon (1/6 oz.) of sodium hypochlorite for every 10 gallons of
water. The water in your tanks should now be sweet and clean. This
procedure is time-consuming but not hard to accomplish. The hardest
part will be assuring you have completely drained the tank at each
stage of the cleaning process. (Editors’ note: If you have any
physical reaction to water with bleach in it – a sore throat, for
example – keep flushing until you don’t, and then don’t add more
bleach. We have experienced problems with bleach, even in minute
quantities, in our drinking water.)

The bleach should have killed off any mold, mildew, or other
bacteria in the tank. It will not kill cysts and parasites, but we
will handle them with our onboard filters. The vinegar will
neutralize the bleach taste and fiberglass smell. The series of
rinses will remove any particulate matter leaving a clean water

Be sure, when you are accomplishing the above, that you flush
the supply lines from the tank to the fixtures, as these sometimes
enable things to grow. Pulling water through the system for all
treatments will accomplish this for you easily. You need not pull all
the water through the system. Pull till it runs clear, then dump the
rest. As the water is not hydrocarbon contaminated, it can be pumped
overboard through the bilge pump system.

Our water supply is now back in business, and all we need is
a final treatment to remove those things we cannot see but will hurt
us. We need a high-quality water filter between the tank and the
outlets. Water filters range in design from UV (ultra violet) water
sterilizers such as Water Fixer to in-line carbon filters similar to
units we would use in our homes. There are sediment filters, taste
filters, softeners, odor filters, and Structured Matrix technology
that combine the capabilities of several types of filters. The
Seagull IV System has an ultra fine submicron filter layer to remove
all visible particles combined with a molecular sieving and broad
spectrum absorption layer which removes chlorine, organic chemicals,
specific pesticides, herbicides, solvents, taste, smell, and color.
The final layer of the filter works by electrokinetic attraction
removing small positively charged particles of the larger
contaminants by attracting them to the negatively charged surface of
the filter to remove colloids and other even smaller particles than
those removed by the microfine filtration layer. By the time the
water has passed through a filter of this type, it is probably purer
than the tap water we have at home.

The cost of these filters runs from $30 to $500. You
definitely get what you pay for, but for most of us a good annual
tank cleaning and a $30 carbon filter will meet all of our
requirements. Water purifiers are available if the quality of water
you receive in a foreign port is in doubt. Purifiers can be used in
conjunction with other filters to clean up most potable water
problems. Remember, the water you take on must be potable. The
world’s best filter cannot make contaminated water safe to drink.

The filter you have on board will need to be serviced once a
year, preferably in the spring when you flush the tank after the
winter layup. This usually means pulling the old cartridge and
installing a new one at a modest cost.

The best way to assure a clean water supply is to exercise
diligence in selecting the source of supply in the first place.
Protecting your own and your family’s health are worth all the effort
in cleaning your water supply.

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