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Of
all the complicated subjects one needs to know as a boat owner,
this is probably the most difficult, and least understood.
This essay is intended to give you a fundamental
understanding of the causes and effects of corrosion, as well as
how to identify problems and correct them before they become severely
damaging.
There are many types of corrosion that boat owners
have to deal with. Well, actually, there are only two, but there
are many different causes with different names. The two basic
types are erosion and electro-chemical. The following descriptions
have been simplified for clarity since most corrosion mechanisms
are very complex. This is intended for laymen so if you are a scientist,
kindly cut me some slack here.
Erosion is a strictly mechanical
form of corrosion that is caused by friction. This can be mechanical
corrosion, such as that of sandy water flowing around a bend in
a pipe, which acts just like sand paper. Then there is another type
of erosion, which is caused by high speed water flow. The pitting
one sees on rudder blades behind propellers is an example of non-abrasive
erosion. In fact, many people mistake this condition for electrolysis,
a subject that I'll get to in a moment. This is caused by the stream
of bubbles from the propeller hitting the rudder. High speed
flow corrosion is rarely found in boats, other than this instance.
The most frequent occurrence is within the cooling systems of engines.
Electro-chemical corrosion
is the primary type of corrosion that boat owners have to deal with.
First we need to understand that all corrosion except mechanical
erosion is electro-chemical in nature. This is just as true of a
drop of water on a piece of raw steel, as it is of a stray current
leak going through a bronze propeller. There is no need to understand
this phenomenon completely, but a brief description will help.
All particles of basic elements or compounds have
electrical charges, be they positive or negative. If two different
materials have the very same electrical charge, nothing will happen.
These materials or substances are, we say, "compatible"
as in joining certain types of stainless steel and bronze together.
If two materials have a sufficient different charge, then a flow
of current (electrons) will occur. This is the principle that makes
a dry cell battery work. Dry cells use carbon and another metal
to generate an electrical current flow between the negatively charged
carbon, and a highly charged metal.
Electrolysis People generally
do not understand this term, using it as a catch-all to describe
any kind of corrosion below the waterline. Electrolysis is simply
the result of stray current, and nothing else. Galvanism and electrolysis
produce similar results, only they have different causes. We would
be better off using the term "stray current corrosion"
because this identifies the cause.
Galvanism This is the term
applied to the flow of electrons when two dissimilar metals are
mated together, as was described above.. Basically, there will be
very little flow when two metals are mated together dry. But add
water to the join and suddenly corrosion blossoms. That's because
water is a conductor and becomes the facilitator of the current
flow. This is why mating dissimilar metals is much less of a problem
inside your house than it is on your boat. All forms of galvanism
involve metals, but all metals don't look like metals. Carbon is
a metal that is used in making rubber, and so carbon rubber when
mated to stainless steel can produce quite a reaction.
Galvanism is a very complex issue. Boats, of course,
have a lot of different metals in them, including those below the
water line. This is complicated by the fact that all bronzes, brasses
and all stainless steels are not the same. There is a very wide
range of alloys -- meaning the mixing of different metals to achieve
specific metallurgical properties -- between what we usually think
of as basic metals. This accounts for why there is such a wide range
of performance of these metals, and sometimes why they corrode when
they shouldn't. If the right alloys aren't used, we have a problem.
We attach pieces of zinc to the underwater metals
of boats to protect those metals. What actually happens is that
the zinc reverses the normal flow of current between dissimilar
metals. The zinc will emit current that raises and equalizes the
electrical potential of all the metals in the system. It does this
by releasing electrons, which are positively charged ions of the
metal itself. This causes the zinc to erode and disappear. These
ions will attach themselves to the other metals, which explains
why your props and other metals may end up with a rough, scaly surface;
they've become covered with zinc oxide.
Scale of Nobility Metals
are rated on what is called a Scale of Nobility, which simply means
the materials ability to resist this kind of corrosion. There is
also a chart called the "galvanic series"
which shows the electrical potential of metals in seawater.
A more noble metal is one that has a neutral or negative electrical
potential. It will not generate a flow of positive ions, and is
called "noble." The reverse of this is the least noble
metal, which has a high positive charge, and which will generate
an electrical current. These include such metals as zinc, unalloyed
aluminum and copper, iron and steel. Graphite and carbon bottom
out the list, being the most highly charged metals.
Crevice Corrosion This is
the most common form of corrosion found on fiberglass boats, and
is the least understood. Electrical currents are generated anytime
there is a change in chemical composition. That's why powerful explosives
can be made of such ordinary things like plastic. As its name implies,
crevice corrosion involves water, metals and crevices. For our purpose,
a crevice is any cavity that will trap and hold water, while at
the same time reducing or eliminating air exposure to the water/metal
interface. Crevice corrosion is the same thing as galvanism, only
it occurs under different circumstances.
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| Crevice corrosion on screws. The
one on the right shows the typical wasting of the shank
right under the screw head. The one on left was exposed
to water both under the head, and on the inside of the
hull where it has thinned at both locations. This is probably
what the bolt in the photo below (top) would look like
when removed. |
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This is also called "closed cell" corrosion
by virtue of the fact that little or no air is allowed to get to
it. The water/metal interface results in oxidation of the metal
which concentrates the hydrogen content of water, and turns the
water into an acid. This changes the electrical make up of the affected
materials, generating an electrical current that "dissolves"
the metal involved. These crevices or closed cells can become dynamic,
meaning that the process can perpetuate itself for a long time --
either until the acidic water is exhausted or an oxygen source is
created that lowers the acidity of the water and stops the corrosion.
If no oxygen source is introduced, the corrosion process continues
until the metal is completely gone.
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This is is one of the
telltales of crevice corrosion in through hull bolts. The
fact that this is Taiwan stainless only makes the problem
worse. |
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This is what copper-based
paint looks like when stray current is involved with these
through hull bolts. The copper-based paint has reacted. You
can't get a better indicator of a problem than this. |
To illustrate this phenomenon, consider that you
could hang a stainless steel bolt over the side on a string. It
would hang there forever and nothing would happen to it. But put
that faster into the bottom of a hull and watch what happens (see
photos below). Water gets into the screw or bolt hole where there
is no free-flow of water, so that the small amount of water in the
screw crevice turns acidic and creates a galvanic cell. This usually
occurs right under the screw or bolt head, eroding the shank of
the screw or bolt until it becomes loose. Once it does become loose,
then a better flow of Ph balanced water is introduced, and the corrosion
stops because the water is no longer acidic. Virtually the same
thing will occur with stainless fasteners into an aluminum mast.
But in this case, the corrosion stops as soon as the water evaporates
from the crevice. In the case of an aluminum fuel tank, installed
in such a way as that water gets trapped against the tank, like
a foamed in place tank, or a tank sitting on a plywood deck, the
very same thing happens. Which tends to leave us mystified why you
could throw your aluminum parts over the side and they'd sit there
forever without corroding, while the seeming protected parts on
your boat corrode badly. Crevice corrosion always occurs
in places you can't see, though it usually leaves telltale evidence.
Stress Corrosion is yet
another form of corrosion, as it's name implies, occurs when a metal
is under heavy stress. This is a combination of crevice corrosion
cells combined with heavy loading. It most often occurs on sailboat
rigging and power boat propeller shafts. Old style swage fittings
on sail boat rigging combines both stress and corrosion cells from
entrapped water within the swaged cable. It also occurs at mast
rigging attachments where water is entrapped between the mast and
bolt-on parts, or even getting under welded parts. See photo below.
Propeller shaft breakage has reach almost epidemic
proportions these days. That's because builders are opting for low
grade stainless shafts made of lesser alloys. All it takes is for
a tiny pit to form on a shaft to initiate the crevice/stress corrosion
cycle that will ultimately result in fatigue failure. This usually
occurs at the stuffing box or keyway cuts, the natural weak points.
Good propeller shafts don't break because they don't corrode. If
you have this problem, it's ultimately a question of how many new
shafts do you want to buy before you replace them with better quality.
Highly polished stainless steel is more corrosion
resistant than those without a mirror finish. The reason is that
unpolished metal has machine marks on it that serve as crevices
for corrosion to start. Polishing smoothes these crevices over.
However, high polishing won't help much for lesser grades of stainless.
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| Micro photograph of stress corrosion
cracking. Initially invisible to the eye, water gets into
these fissures and hastens the destruction of the part. |
It's All the Same All forms
of corrosion are the same electro-chemical process caused by different
circumstances. That there are so many different circumstances is
why we have so much trouble understanding the nature of our corrosion
problems. Every combination of metals in different locations on
our boats corrodes for different reasons. And if we don't understand
why, then there's little hope of preventing it, either by the boat
builder, or the boat owner.
Stray Current The electrical
systems on boats have improved sufficiently over the last 20 years
that stray current corrosion is much less of a problem. It begins
to show up in older boats because of all the jury rigged wiring
and systems that get added on over the years. In newer boats, it
usually occurs due to ground wiring faults on the dock.
The basic mechanism of stray current corrosion
is the same as all other types, only this time the introduction
of an outside source of electricity screws up the normal electrical
balance of the boats metals, plus adding one other pernicious problem.
Energizing all the underwater metals in the boat, that raised electrical
potential will seek a path to ground. And rest assured that it will
find one. That path will be the piece or pieces of metal below the
water line that are the weakest like. It will start with the zincs
and once those are destroyed, it will move onto the next lest noble
metal. That's usually a poor quality alloy of prop shaft, propeller
or through hull fitting. In other words, it's likely to attack one
specific piece of metal. As the current leaves the piece of metal
on its way to ground, it is carrying bits of the metal with it,
as well as the erosion that may occur from part to part.
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| The owner of this stern drive has a
big problem, though he doesn't know it. A dry stored boat,
the white stuff all over this drive is zinc oxide from the
zinc collars around the hydraulic rams. This is the
result of stray current and it won't be long before he's looking
at a $3500 bill. This one is less than a year old. |
Classic crevice corrosion.
In this case caused by a carbon rubber exhaust flapper clamped
onto a stainless exhaust flange plate. Notice that the worst
corrosion is where the clamp pressed the rubber the tightest. |
Stray current is the most difficult corrosion problem
of all to identify and correct because the source could anywhere
amongst that mass of wiring in your boat. The first thing that must
be done is to meter out the dock circuit to make sure that it isn't
feed-back from the dock grounding system (i.e. the grounding system
is positively charged, which can feed back into your boat), or that
the grounding wires are not reversed.
The conditions created by AC and DC current is
not the same, with DC current being the most damaging. The reason
for this is that AC current is pulse current that moves in two directions,
greatly reducing the corrosion potential for reasons I won't get
into here. Technically, AC current requires some kind of diode that
converts it into DC current before it causes metallic corrosion.
There are lots of naturally occurring diodes in crystalline forms
of metallic oxides. Aluminum oxide is VERY good at converting AC
to DC current. The aluminum oxides that form inside aluminum
boats!
Yet AC current may be just as damaging because
the voltage is so much higher. If you ever wondered why there is
so much misinformation concerning AC current corrosion, this is
why. Unless the right conditions exist to covert the AC to DC current,
the corrosion does not occur. Thus, only some boats on a dock with
faulty AC wiring suffer damage, while others don't. These are almost
always older boats where corrosion is already present, as it is
the crystalline nature of metallic oxides that exacerbates the rate
of corrosion.
AC current corrosion occurs much less frequently,
mainly due to the fact that the high voltage is dangerous, and it
is treated with more respect. Moreover, if there is a 125 volt leak,
the chances are that it's going to be found rather quickly as the
boat owner doesn't appreciate the shocks he gets occasionally. Amongst
the thousands of boats I have surveyed, the number that I have found
with AC ground faults is amazingly few.
DC Current leaks are the
most common form of stray current problem. The fact is that any
boat that utilizes high quality underwater metals and has a good
bonding system can tolerate a fairly high amount of stray current
leakage. This is because the low voltage current rather quickly
dissipates throughout the system. Like trying to wash your car with
a cup of water, there isn't enough to go around to do the job. The
small amount that finds its way to underwater metals is usually
taken care of by the zincs, or dissipated by a large amount of metal.
Substandard bilge pump wiring is the most common
source of stray current. So are batteries mounted in a wet environment,
such as sitting on a wet deck. Take a fully charged battery
and sit it on the floor of your garage. Come back a month later
and measure the voltage. Surprise! It's dead! Where did the
power go? Well, right out through the casing and into the damp concrete,
that's where. Put that battery up on dry wood blocks and this won't
happen. The same thing happens when you have wiring laying in a
wet bilge. Plastic, you see, is not an absolute isolator. Just as
your fiberglass hull will absorb water, battery casings and wiring
insulation will absorb some water too, just enough to leak out a
small amount of current. And cause you a lot of problems. Having
said that, I needn't say any more about batteries and wiring. You
now know what to look for.
Detecting Corrosion Forty years
ago, the paint companies tried using pure copper in bottom paint.
It stopped marine growth cold, but it turned the entire bottom of
the boat into a bonding system. Well, the EPA has us back to copper
again, only this time cupric oxides are the toxic agent. Fortunately,
it works pretty good. It works even better as a telltale for stray
current, as the photo above shows. These oxides are still highly
conductive, and still contain not completely reacted copper, so
that the paints will corrode. Don't worry about that because
there is no material in this world that does not corrode, including
you and me.
If you are getting white and/or green halos around
your zincs or underwater metals, you have a stray current leak.
Even a very small amount of current will cause a paint reaction,
so we have a built-in litmus tester here in our bottom paint. You
can judge the severity by these photos, as this degree of paint
reaction is associated with very rapid wastage of the zinc. Also
make note of the fact that differing paint formulations react differently.
I've seen some that are so reactive that you'd think the whole boat
was being dissolved, and yet only a small current (1/2 volt) created
this result with no damage to underwater metals. Therefore, judge
the severiy by the condition of the zincs, as well as the paint.
The evidence of corrosion appears in one of three
forms. The one that we are all familiar with is the appearance of
oxides, the byproduct of a metal that has chemically changed. Copper-based
metals like brass and bronze leave green oxides, white for aluminum,
and reddish-brown for stainless steels. These are the telltales
of ordinary oxidation corrosion.
However, we have these other forms of corrosion
to deal with, and these have different affects. Galvanism and stray
current are abnormal, the result of something that shouldn't
be happening. When the current generated by galvanic action is weak,
it will generate the usual corrosion byproduct, the oxide of the
metal. Stronger galvanic and stray current will more often result
in rapid erosion of the metal, usually to the point where there
are no oxides present, but will leave an appearance of bright metal.
If you see any part of any underwater metal that is showing bright,
regard this as a red flag. It's the indicator of very rapid erosion.
Examples of this would be eroded, but shiny surfaces on zincs, or
propeller blade tips that are bright yellow and showing a crystalline
texture or pattern. It will look like frost on the inside of a window,
only it is bright yellow.
The effects of galvanism most often occur very
slowly with bronze or brass. Here, a condition known as dezincification
occurs. Since copper is alloyed with zinc to make bronze, zinc is
the weak link and will leach out of the alloy to leave raw copper.
The result is metal that is pinkish in color, is granular in texture
and tends to crumble when probed. It is soft and very weak. Any
pinkish looking copper-based metal is waving a red (pink) flag.
Problems with stainless is typified by the appearance
of pits or very coarse tunneling, creating cavities that are very
rough, and which have very sharp edges. Stainless can be very quickly
destroyed by stray current, so if you see any sign of pitting on
things like shafts, rudders or trim tabs, you need to seek out the
cause and eliminate it quickly. This is one of the reasons why we
don't recommend the use of stainless for any sea water plumbing
systems. Screws and bolts underwater will usually end up with rusty
looking oxides around them when crevice corrosion is involved, but
no telltale oxides when stray current is the culprit.
About Zincs You can have
serious stray current problems and yet your zincs seem unaffected.
Once again, understanding this is not easy. First, there are many
alloys of zinc -- some are more durable than others. You may have
gotten the wrong kind of zinc. Secondly, like aluminum, zincs tend
toward self protecting. That means that as the oxide layers build
up, the metal becomes insulated from the water, so that the corrosion
rate diminishes or stops altogether. This layer can get as much
as 1/4" thick. At this point, the zincs have lost their effectiveness.
If there is still a large amount of zinc left, you can just scrape
off the oxide and the effectiveness will be restored. This condition
means that you don't have a stray current problem.
Very rapid zinc loss that results in bright, shiny
metal being exposed is a clear indication of electrical activity,
be it galvanic or stray current, usually the later, since galvanism
rarely creates enough current to destroy zincs quickly . Bright
zinc in association with heavily corroded bottom paint means you
have a problem that needs to be addressed immediately(see photo
above). The brightness of the zinc is telling you that there is
too much current for the zincs to handle. Adding more zinc is NOT
the solution.
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This is how you can tell if your going
to have an aluminum fuel tank problem. Sitting on a flat deck
that gets wet, here you can see the corrosion stains coming
out from under the tank. This one's days are numbered. |
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Crevice corrosion of through
hull bolts. In this case, the water got at the bolts from
the inside. The use of nuts with nylon inserts (shakeproof
type) accelerated the process. Notice that the acid etching
leaves the metal very bright. This is a total lack of
oxygen to the cavity, whereas the exposed threads look rusty
and had more oxygen available. That's because the nut completely
wasted away. |
The vast majority of inboard powered and sail boats
don't even need zincs. We put a few on as a precautionary measure,
and to serve as a telltale should a problem develop. Why? Because
on most boats everything is in balance without a lot of dissimilar
metals creating electrical current. Do you need to put a zinc on
every piece of underwater metal? No, you don't if your bonding system
is in good condition. That means that the wire connections are still
making a good electrical connection. If your boat is 10 years old
and you have never serviced the wire connections, then rest assured
that your bonding system is now useless. The wire ends to components
like sea cocks, rudders, struts, etceteras, should be serviced every
few years. If you have wire splicing tools, it will take you about
an hour to redo your entire system.
Stainless trim tabs are the exception to this.
Most often tabs are NOT tied onto the bonding system unless they
are bolted, rather than screwed to the hull. That's because there's
nothing to attach a bonding wire to. The screws are all on the outside.
Over zincing, putting too much zinc, on causes
the opposite problem. It will reverse the flow of current in the
other direction and actually cause corrosion. Over zincing is discernable
when you find your props all covered with a coarse layer of zinc
oxide. It will feel like sandpaper. This condition will reduce your
boat's speed significantly and increase fuel consumption. I'll bet
that statement got your attention!!!
Which is better: using one big zinc on a central
bonding system, or putting zincs on rudders, shafts, etceteras?
A single large zinc tied to the internal bonding system is unreliable
because of the internal corrosion problems to wiring. And
you have no way of knowing, short of doing a conductivity test,
to know if the system is functional. Surface area of zinc is the
most important criteria, and you'll get more surface are with more
small ones. Multiple zincs are far more reliable.
Bonding Systems The purpose
of a bonding system is to equalize the electric potential of dissimilar
underwater metals by tying them all together with wire or copper
straps. The benefits of a bonding system are wide ranging but little
perceived. One is that it serves to dissipate stray current leaks.
12 volts of current focused on a small piece of metal will result
in rapid destruction. But that same 12 volts spread over a much
larger surfaces, causes less damage in proportion to the size of
the water exposed surfaces of the metal. Bonding systems can reduce
the corrosion potential of metals inside and on the bottom of the
boat. Boat which have all the hardware bonded, such as the railings,
will suffer much less corrosion.
As mentioned above, bonding systems are not maintenance
free. The wire connections corrode too, and need to be reestablished
periodically. This is done by cutting off the old terminal or connection,
and then establishing a new one. It's as easy as standing on your
head in the bilge ;-)
Corrosion in Systems Okay, so
now well know all about corrosion and the underside of the boat,
but we haven't even touched on all those internal systems through
which water passes. Yep, I mean the engines, airconditioning,
and other expensive stuff like that. Anything that water touches
has a potential corrosion problem. Some of these things we can perform
preventative maintenance on; for others you wait until the part
fails and replace it.
Those metallic components through which water flows
also need some corrosion protection. That includes the engines,
pumps, A/C units and so on. The metal chassis or housings of things
like pumps and air conditioners also need to be tied into the bonding
system. On an A/C system, the sea cock, strainer, and compressor
chassis all need to be wired together. The same applies to all other
metal housing pumps, but not to plastic housing pumps. The primary
reason pumps fail so often is because (a) they're not bonded, and
(b) they are located in places where they get wet. Pumps tend to
have a lot of dissimilar metals in them, which is why bonding is
so important. I'd be willing to bet that the pump motors on your
heads aren't bonded, which is one reason why you've replaced so
many pumps.
The general rule is that anytime a piece of metal
plumbing or hardware is isolated in a system, as with a sea strainer
that is joined by two hoses is electrically isolated, needs to be
wired into the system. This can be done by daisy chaining items
together, but it's a good idea not to include too many items in
a chain. Obviously, at any point where a connection is broken, all
those items upstream will be unprotected.
More On Stainless Stainless
steel comes in a very wide range of alloys. Naturally the best are
the most expensive because they contain higher percentages of nickel,
which is very expensive. Ergo, builders don't like to pay for this
stuff, and that includes all you screws, nuts and bolts. Stainless
is quite vulnerable to crevice corrosion, and it's most often to
be found on low grade fasteners. The better grades -- this doesn't
mean the best and most expensive -- when used for deck hardware
do not rust. If you have rusting stainless hardware, it's because
it's a low grade. Most corrosion occurs via crevices, such as around
screw holes, stanchion bases and sockets, rub rails, etceteras --
any place that can trap water and create a closed cell. There isn't
anything you can do about this.
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| This stern drive cavitation plate was lightly
scraped by a fork lift truck. With the aluminum exposed, the
metal now begins to erode strictly as a result of stray current
which has left the metal bright. |
Previously banned, ABYC has recently approved stainless
for use as fuel tanks. This is unfortunate for stainless has a very
poor track record for use as tanks. Stress, corrosion cracking and
welding problems are among the reasons why. Place a stainless tank
flat on a deck and the same thing will happen to it as an aluminum
tank. Water tanks are even worse; metallurgically altered metallic
structure around the welds results in unacceptably high corrosion
failure rates. Weld failures in stainless water tanks are legion
and can't be stopped.
Stainless piping and other fittings for sea water
use is not recommended. Not so much because it isn't any good, but
because no one can tell what grade of material it is. If it's a
low grade, there always exists the danger of failure. Bronze is
easy to make and is not subject to the alloying problems like stainless,
so when buying plumbing hardware, it's best to opt for bronze. The
stainless that looks so pretty today, probably won't after a few
years anyway.
Hose Connections Hose connections
to metal pipes and nipples, and threaded pipe fittings are all subject
to crevice corrosion. That's because of the tendency for water to
work its way between hose and the part it's clamped onto. One of
the reasons marine hose is so expensive is because it has to be
made of butyl rubber, and not carbon rubber which is cheap while
the former is expensive. The problem of leaking connections is particularly
acute on engines because of the effects of heating and cooling,
which changes the dimensions of the parts. When water gets under
the connection, crevice corrosion begins. This is the reason why
teflon tape is used on pipe connections, and why it's a good idea
to use silicon gasket cement on hose-to-metal connections. These
materials will greatly reduce the propensity for leakage, bearing
in mind that small leaks ALWAYS become large leaks. Always.
Stiffer reinforced plastic hose is very good for
plastic to plastic connections, but it should NOT be used for plastic
to metal connections. Plastic hose is too stiff to form up a good
seal with a metal pipe.
Hose Clamps No doubt you have
wondered why they can't make a hose clamp that doesn't corrode like
crazy. Well, they can't and the reason is that old bugaboo stress
corrosion. No mater how good the stainless, stress corrosion will
take its toll. For this reason, it is extremely important that all
hose connections be properly fitted. If you have to use a ton of
clamp pressure to make the seal, you will only cause the clamp to
fail that much quicker because of the increased stress on the clamp.
This is another reason why you shouldn't use plastic hose unless
the plastic is quite soft and will deform easily like a rubber hose
will. A good fit is one where little clamp pressure is needed.
Double clamping. For decades surveyors have
parroted the need to double clamp hoses. The truth is that you don't,
although it's a very good idea to have two clamps in place, the
other not tightened up. Two very tight clamps will fail just as
fast as one. Moreover, a properly fitted hose will fuse itself
to the pipe connection so hard that you'll have a hard time getting
it off. Tighten the second clamp just enough to hold it in place.
When it's time to replace the first clamp, just tighten up the new
one and remove the old one. If you're trying to clamp a hose with
a bad fit, THEN you need more than one clamp. All engine cooling
system and exhaust systems should be double clamped for obvious
reasons. You NEED more clamping surface area here.
Aluminum Ugh. This inexpensive
material has its uses on a boat, but it's used in too many unsuitable
applications just because it is cheap. It makes lousy hardware but
good sail boat masts and great fuel tanks, so long as they are installed
properly. It makes for lousy trim and moldings; it makes for lousy
machinery casings like winches and spotlights. Aluminum cannot be
cast in good marine grade alloys, thus all aluminum castings corrode
like a banshee. Aluminum is extremely vulnerable to crevice corrosion,
which is why so many aluminum tanks fail. Don't blame the material,
blame the installer for not doing it right. Done right, aluminum
tanks will last forever. Fuel tanks, that is, not water tanks. AL
should not be used for water tanks, period.
Aluminum is similar to Corten steel in that it
develops a layer of self-protecting oxide. This layer is so thin
you can't even see it, but it's there. Quality marine alloys don't
corrode and fail, except where crevices may exist, and when joined
to dissimilar metals. That's why it's nearly impossible to keep
paint on aluminum window frames when secured with stainless screws.
Keeping paint on aluminum is extremely difficult,
and requires very careful and proper preparation. All these boats
with painted but corroding window and door frames are the result
of just shooting the raw aluminum without proper preparation procedures,
many without even the proper primers that are indispensable to making
the paint stay on. Worse, many boats these days simply use residential
or recreational grade windows and doors that are not even a marine
grade aluminum. There's no hope for this stuff.
Needless to say, this is what makes maintaining
stern drives so difficult. Not only are they vulnerable to galvanism
because of all the different metals in the drive, but it only takes
a very small amount of stray current to cause serious damage, as
shown in the photos above.
Related Article: Corrosion
in Marinas
Posted January 4,
1999
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