Bow
and stern thrusters can make
a BIG difference in
your boating
How
They Work
Bow
thrusters and stern thrusters
consist of either one or two
propellers, powered by either
hydraulics or an electric motor.
These reversible propellers
are typically housed in a tunnel-shaped
compartment -- and in the case
of bow thrusters, are connected
to a gearbox that is mounted
inside the boat.
With
the push of a button or the
flip of a joystick at the helm,
the propellers start turning,
and change direction to provide
thrust in one direction or
the other.
With
bow thrusters, the result is
an extra set of props to move
the boat’s bow right or left.
With stern thrusters, the back
of the boat moves right or
left. If you have both types
of thrusters on your boat,
get ready to practice that "Candid
Camera" parking stunt
-- and just watch those jaws
drop at your marina.
Thrusters
by Bruce McDugle
Over
the last 30 years I’ve
watched and been involved in
various devices designed to
create better maneuverability.
These have ranged from rudder
design, keels, skegs, twin
screws, drogues and thrusters.
One
of the more memorable ones
was the gentleman who had a
single screw 30’ powerboat.
He had two old trailer tires
on eight-foot lines cinched
to amidships cleats, one on
the Port side and one on Starboard.
From the aft helm station he
could launch the proper tire
overboard to enhance last minute
control. His success was often
regulated by the amount of
vodka needed to build up his
courage.
I
was the one who had to patch
the odd scratches and gouges
from mis-timed tire launches.
Since then, my experience in
augmented control has notably
increased. A giant step forward
was the fireboat we built which
used high-powered water jets.
A
diesel engine powered a large
pump that fed four 3-inch diameter
hoses. They terminated in a
nozzle at each corner of the
boat. After much trial and
error the system worked well.
This system did the required
job but was heavy, expensive,
and noisy taking up too much
space.
Several
years later, we installed another
water jet on a yacht. It used
an electric motor to drive
the main water pump and I ran
tow 3-inch hoses forward to
nozzles on the Port and Starboard
Sides. The pump, motor and
hoses, when charged, were between
500-600 ft. pounds. It was
awkward finding room to fit
very stiff 3 inch hoses behind
cabinetry and through storage
areas to the forward nozzles.
The electric motor was a 24-volt
but still drew a tremendous
amount of amperage and again
it was too heavy, expensive
and bulky.
Both
of these water jet thrusters
were installed on boats under
36 feet and left me convinced
that in their present state
of technology, they didn’t
belong on yachts.
In
between the water jets I installed
two hydraulic bow thrusters
on commercial fishing boats.
By today’s standards,
they were a bit heavy and awkward
to install. The mounting surfaces
didn’t match up well
with the supplied fiberglass
tunnels. It took two people
to actually bolt the units
in place and then there was
the cost of hydraulic pumps,
oil reservoirs, controls, hoses
and fittings.
Even
with their shortcomings, all
four of the above units are
still in use today. I do believe
that hydraulic thrusters are
the best you can get, but for
vessels less than 45 feet are
just (usually) too expensive.
If you already have a hydraulic
pump setup, like some sailboats
with hydraulic drives, then
you are the exception and the
installation could be relatively
inexpensive.
The
alternative is DC electric.
Two varieties are available.
Direct gear or belt drive.
I remember an early drive that
required the boat to be hauled
out to replace the belt when
it broke. The lower unit had
to be disassembled and the
prop shaft pulled out to fit
a new belt. I am sure the owner
could have found other things
to spend his money on. There
are belt drives that can be
repaired in the water but eventually
belts wear out or have to be
readjusted. A good gear driven
unit only needs the oil checked
and zincs changed periodically.
The simple facts are that direct
drive electric thrusters outsell
all of the others by a wide
margin. As an example, of the
last 30 bow thruster we installed
4 were hydraulic and the other
26 were electric gear drives.
There
is another choice for powerboats
but it won’t work on
sailboats. Dickson Thrusters
makes a nice bolt-on hydraulic
stern thruster. The unit is
mounted on a stainless steel
transom bracket and it doesn’t
use a tunnel. It is less expensive
than a good bow thruster and
would almost have to be engineered
retractable. Currently, it
is not a viable option for
sail boaters.
Historically,
the majority of our thruster
installations have been on
powerboats ranging in size
from 26 to 75 feet. What has
really caught me by surprise
is the number of sailboats
installing bow thrusters and
they seem to be the most pleased
with the results.
In
retrospect, it is very understandable.
What architect in his/her right
mind would sacrifice the joy
of an easily controlled vessel
under full sail for slow speed
handling in a congested harbor,
especially when a bow thruster
will allow him the best of
both worlds.
It
has been challenging to say
the least, fitting bow thrusters
into places they were never
designed to be. Let me share
with you a few of the hurdles
that have to be overcome and
some of the solutions. We will
concentrate on the DC electric
thrusters.
1.
The power source: Ideally,
this will be the engine starting
battery. The requirements
for a thruster are very similar
to an engine starter and
have the same problems. The
effectiveness of direct current
(DC) is limited by wire size
and length. Simply put, if
the thruster is too far from
the battery then we install
one at the closest point.
The necessary cold crank
amps regulate the battery
size for the thruster. The
new spiral cell batteries
work very well and can normally
be integrated into most ships
charging systems. (A word
of caution here. DO NOT use
existing windlass wiring.
The wire size will be wrong,
the breaker/fuse will be
wrong and the power to run
the thruster will probably
not be adequate, resulting
in the motor being underpowered
or motor failure.)<![endif]>
2.
Tunnel Location: The
most favorable position
is as far forward and as
low as possible. The problems
are chain lockers, water
tanks and shallow draft.
Let’s use an example
of an average 32’ sailboat.
If the tunnel can be placed
forward and low then a
good 4 horsepower unit
will work. But suppose
a built-in water tank is
in the most “favorable
position”. The tunnel
would go a couple feet
aft and we would probably
use a 6 horsepower unit.
That equates to an additional
$1000.00 but is still less
expensive that rebuilding
a water tank. (Lack of
leverage arm can be overcome
with horsepower.) Another
consideration is the depth.
A good rule of thumb is
the top of the tunnel should
be at least one diameter
below the waterline. The
deeper the better. Higher
water pressure equals higher
efficiency.
3.
Cavitation: If a
thruster cavitates it loses
thrust and makes noise. We’ve
worked hard to learn how
to prevent this. Here are
a few things to consider.
Water always follows the
easiest path and its flow
can be directed into the
tunnel via proper radiusing.
The path is created by plan
not chance. A normal installation
will cost over 50.00 per
each pound of thrust. Why
throw it away, when by proper
design of the tunnel installation
you wont lose any thrust.
4.
Vessel Trim: The
new electric thrusters are
surprisingly light for the
amount of power they develop.
A good example would be an
8 horsepower unit in a 40-foot
sailboat. The brand we use
weighs 68 pounds. With the
addition of a spiral cell
battery overall weight is
118 pounds. Miscellaneous
fiberglass and accessories
finishes this installation
at a weight gain of approximately
135 pounds. A 4 horsepower
unit in a smaller vessel
that did not require an additional
battery could be as low as
60 pounds overall. There
is a new 3 horsepower unit
that develops 77 foot pounds
of thrust, yet weighs in
at a mere 22 pounds and uses
a 5 inch tunnel. This thruster
works well in a 30 foot and
under vessel. Trim considerations
are always important, but
as you can see, these weights
are pretty easy to deal with.
As
I said, Sailboats have been
challenging, but the results
have been worth it. Thrusters
have become popular enough
to warrant some pretty good
competition among the manufacturers,
which has brought prices down
and quality up.
Let
me finish with a couple of
observations. Boats with thrusters
generally sustain less cosmetic
hull damage, charter companies
prefer them and boat brokers
report they are easier to sell.
Oh yes, which thruster brand
to we sell the most? Side-Power
by Imtra.
Bruce
McDugle
Cap Sante Marine, Ltd.
P.O. Box 607
Anacortes, Wash. 98221
Fax: (360) 293-2804
Contacts:
Cap
Sante Marine *(800) 422-5794; info@capsante.com
Side-Power/Imtra
Corp.*(508) 995-7000; www.imtra.com
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