The problem here is that all outboard motors have housings made from
aluminum alloys in order to save weight & cost. The outboard
manufacturers try various coatings such as hard anodizing, baked on
powder coatings, etc. But, in salt water, nothing works very well &
the aluminum alloys corrode. The outboards which are rated for use in
salt water all have a sacrificial zinc mounted somewhere on the lower
unit. You need to keep an eye on this zinc, don't coat it with
bottom paint, & replace it as often as necessary. It also helps to
raise the outboard up out of the water & do a fresh water flush on the
cooling system after every sailing session. This will extend the
life of your engine by 10-20X over an outboard that sits in salt
water all the time as well as prevent any banacles from getting
You don't need to do a corrosion test. Corrosion of the common
aluminum alloys in salt or brackish water is well documented in the
literature. Assume you have corrosion going on & take the above steps
to minimise it.
S/V Dynamic Equilibrium
15 Jan 2001
Electrolysis is caused by metals of differing valence or nobility
being immersed in a conductive solution,i.e. water. Salt water is
extremely corrosive because of its higher conductivity coupled with
high positive valence due to mineral content. Its called electrolysis
because the chemical reactions that occur generate electrical current.
Stray electrical currents from badly grounded boats can accelerate
the process, but the presence of an electrical source is not
necessary. An outboard motor typically has a lower housing of
aluminum alloy, but the gears and prop shaft are steel. Props may be
bronze, steel, aluminum, or plastic. The presence of these different
metals in close proximity in salt water, and in direct contact with
each other, is all that is required for electrolytic corrosion to
occur. Metals which have a higher number of electrons in their
moleculular structure, negative valence, will have the excess
electrons stripped away by metals or solutions which have fewer
electrons or positive valence. The reaction changes the molecular
structure of the metal to somthing else. Zinc anodes are used to
protect other under water metals on boats because they will corrode
first. That is why they are called "sacrificial anodes". Virtually
all outboards have a zinc anode bolted to the prop shaft or the
lower housing somewhere for this reason. They should be replaced when
they have lost about 1/3rd of their original mass.
15 JAN 2001
Why they need to clear:
- 1) The very first Rhodes22 used an outboard bracket or a well.
- 2) When Stan first came out with his system it would lift 15-inch
shafts clear, Long shafts tilt and clear.
- 3) Then 20 inch long shaft will clear (most Rhodes22), extra
Longs tilt and clear,
- 4) The last year or two extra Longs will clear, using the latest
- 1) Drag when sailing.
- 2) Growth on the lower, unless treated with tin based paint
(copper antifouling will destroy it).
- 3) Electrolysis of the lower portion depending on the motor
[the skid plate and prop can be in the water but not the zinc.]
A long shaft will hold in about 3-4 waves on a Rhodes 22, the extra
long will hold in about 4-5-foot waves. A sailboat makes a lousy
powerboat. You can play games quartering the wave to stretch this
number, and a trolling motor will not help you. A trolling motor
needs to draw 615 amps at 12 volts to equal a 9.9 hp motor (1 hp =
746 watts, (746 watts * 9.9 hp)/12volts= 615 amps, the motor are
rated at the shaft so the power head is about 20% higher), that why
you see the trolling motors rated in pounds of force in the 30-50
range (that is a peak number and can only be sustained for a few
minutes before burn out). My 9.9 Yamaha is will produce 240+ pounds
of force; the trolling motor might have a longer shaft but it is not
going to be of any help in those conditions, because they do not
have the power or the battery range.
Electrolysis is a fact of life: take a glass pan, put some water in
it, us a volt ohmmeter, hook a 1 oz gold coin to one lead and place
it in that water, take a 1 oz magnesium plate (they us them in place
of zinc's in fresh water), hook it to the other lead and place it in
the water, should read about 3 volts on the meter(add a little salt
if having trouble). The only metal in the water on a Rhodes22 is the
motor and bracket, on larger boats you read in the specs about the
bonding system, what this is doing is tying it everything to the
zinc's for protection. It also protects from lightning.
16 JAN 2001
[... the first part of this email concerns trailer maintenance and can
be seen at Trailer Maintenance...]
Besides the outboard motor, the other common place on a sailboat where
we encounter aluminum vs stainless steel corrosion is when we attach
things to the mast. The aluminum mast extrusion is less noble than
the stainless steel & thus gets to be the anode & go into solution.
It doesn't take very long for sufficient corrosion to occur to weaken
a threaded connection dangerously - like about one summer's worth of
exposure! Any place you have stainless steel in contact with
aluminum, look for pitting in the aluminum along with a granular
white corrosion product in the pits. Let's consider an example of
Assume we want to attach a 36 inch long section of black anodized
aluminum T-track vertically to the front of the mast with stainless
steel machine screws.
Why? This type of T-track is used to provide an adjustable spinnaker
pole car for supporting the in-board end of a spinnaker pole or
whisker pole. The T-track will be heavily loaded & failure of the
mast-to-track connection could ruin your whole day!
The T-track is attached to the mast with 1/4 inch diameter flat head
machine screws. The mast extrusion is only about 3/6 inch thick in
this area, so it's stronger to use the 1/4-28 UNF fine threaded
machine screws instead of the more common 1/4-20 UNC coarse threaded
machine screws. After you've drilled & tapped the mast for all the
machine screws, you need to decide if you will EVER want to remove
this T-track. On Dynamic Equilibrium, my answer was ,"No, I will
never take this thing apart." To isolate everything, make the
installation stronger, & preclude the machine screws coming loose; I
coated the backside of the T-track as well as the threads & the
countersunk holes in the T-track with epoxy during final assembly.
So far, this installation has shown no sign of corrosion (or
weakness) for 12 seasons & counting. If you wish to be able to take
an aluminum/stainless steel connection apart in the future without
having it corrode in the meantime, things get a little more
complicated. You should make some sort of isolator to go in between
the stainless steel & the aluminum. This could be a coat of epoxy,
or a thin sheet of an electrical insulator like an engineering
plastic, hard rubber, or ceramic. In some cases, you can make the
necessity of an isolator serve double duty. For instance, if you
wanted to mount some flat piece of harware on a curved surface, make
the curved-to-flat adaptor also be the isolator. Loc-Tite Corp &
many other companies make a thread anti-sieze compound which should
be coated on the machine screw threads during final assembly.
Coating the threads is probably the single most important step. The
stainless male threads have a lot of surface area & are forced into
intimate contact with the aluminum female threads. Without the
anti-sieze, corrosion in the threads is almost a foregone conclusion.
S/V Dynamic Equilibrium
17 Jan 2001