An
outboard motor fairing to prevent propeller ventilation
Attaching
an outboard motor to almost any type of double ended hull generally
involves
using a bracket mounted on the side of the hull.
Some small catamarans also have their outboards
mounted to a bracket between the hulls. Without a transom or deep
fairing in
front of the leg of the motor, severe ventilation of the propeller can
occur
and reduce the thrust of the motor.
Ventilation (where air is sucked down into the
propeller) occurs because
of the turbulence behind the leg of the outboard motor.
The two
horsepower motors are the worst offenders, with an almost round leg
between the
power head and the anti-ventilation plate.
Larger engines have a more streamlined leg shape and
don’t have as much
of a problem as the two horsepower size.
Unfortunately for me, the outrigger canoes and proas
that I build don’t
need any more than two horsepower, so a solution was needed. The new Honda two
horsepower motor, that I
had recently purchased, ventilated horribly at any speed more than an
idle, and
threw large amounts of spray into the aft section of the canoe.
I decided
to fabricate a fairing around the leg of the motor to reduce the
turbulence and
hopefully increase the thrust. I
considered many possible methods and materials, and I didn’t
want something
that would be too difficult to remove if it didn’t work as I
expected. Flat
panels of fiberglass bent around the leg
in a foil shape seemed to be the easiest solution.
Sheet metal could also do the job, but my
workshop was better equipped to make it out of fiberglass.
I
fabricated a flat panel of fiberglass, using a sheet of window glass
for a
mold. I applied a
coat of mold release,
allowed it to dry, and brushed on some gray polyester gelcoat. After the gelcoat had set
up, I saturated a
scrap piece of 18 ounce biaxial fibreglass fabric with vinylester resin
on top
of the gelcoat. Three
or four layers of
6 ounce woven cloth could also be used.
The next day, I peeled the fibreglass panel off of
the sheet of window
glass.
I used a
jigsaw with a carbide grit blade to cut two identical rectangles from
the
panel. The two rectangles were taped together along the future leading
edge,
and opened in a vee shape held at a 70 degree angle by a couple of foam
wedges. A small
fillet of
vinylester resin
and glue powder was
applied along the inside of the joint, followed by two strips of six
ounce
fibreglass cloth.
The next
day I tested the fit by bending the vee panel around the leg of the
motor, with
the bottom edge of the panels resting on the top of the
anti-ventilation
plate. I was
pleased with the curved
shape and decided to go ahead with bonding it to the leg. I applied packaging tape
along one trailing
edge of the panel with the tape overhanging to allow it to hold the two
panels
together when bent around the leg.
I
applied a bead of silicone sealant down each side of the leg, and a
generous
bead of resin/glue powder mixture along the inside of one trailing edge. I folded the panel around
the leg, and
wrapped the tape over the trailing edge.
So far so good and it didn’t spring loose
and fly across the shop.
The next
day I couldn’t wait any longer to test it, so I clamped it to
my Ulua outrigger
canoe, loaded up some fishing tackle, and headed out across the harbor. It performed beyond my
expectations, with no
ventilation occurring at any speed.
Normally with side mounted installations, a hard
abrupt turn can also
cause some ventilation and over revving, but this little problem was
cured
too. At this stage
the foil had no top
panel, and a thin sheet of water was peeling off the top edge. An over-size top panel or
fence would deflect
the water outwards and also stiffen the whole foil installation. I laminated another flat
fibreglass panel at
twice the previous thickness, cut it about 5/8” larger all
around the foil, and
bonded it (in two pieces) to the top of the foil with a resin/glue
powder
fillet.
It’s
important to be able to rinse out any salt water from the inside of the
foil to
avoid any corrosion of the leg, so I drilled two holes in the top panel. The two bottom corners of
the foil already
had small openings for drainage where they overhang the
anti-ventilation plate.
I choose
the Honda engine because it’s a four stroke, it’s
air cooled, and it has a
centrifugal clutch, but the clutch can cause a small problem when
approaching
the shore or a dock. If
you use the
motor to steer (rather than a rudder), reducing the throttle to slow
stops the
propeller turning, and you lose steering control.
An unexpected benefit of the large foil
around the leg is that it functions as a rudder after the propeller has
stopped.
If I ever
have to remove the lower unit from the motor, I’ll have to
cut a small hole in
the fairing to access one of the bolts, but since that
shouldn’t happen very
often (no water pump!), a simple patch job will restore it to its
former shape.
Ideally
this fairing could be made of some high density foam in two halves, so
that it
could all be removed with a couple of screws.
There is exhaust flowing in the leg though, so maybe
having cool water
around it is best.
Since this was written I have installed the same type of foil on two Yamaha 2HP motors with great success.
