 Is a Retracable Better Than a Tunnel
Thruster?
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> Is a Drop-Down Better Than a Tunnel
Thruster?
By Joe R. Bekker
As oil and gas exploration moves into deeper waters, we see
an increased demand for bow thrusters. OSVs and crew boats
are getting larger and are becoming more difficult to dock.
Deep-water drillers and producers don't allow support vessels
to dock at their platforms and demand that vessels hold station
in open sea while offloading supplies. More and more vessel
operators specify retractable steerable thrusters, also known
as drop-down thrusters, instead of the old and proven tunnel
thrusters. Are drop-down thrusters better? It depends.
Performance
Figure 1 - Tunnel Thruster
Figure 2 - Drop-down Thruster
Let's compare performance.
A tunnel thruster typically generates about 23 pounds of thrust
for every horsepower you put in. That is with a short tunnel,
chamfered tunnel ends and no grating. Thrust is less when
using a tunnel longer than twice its diameter, when using no
chamfer or radius at the tunnel inlets, and when using an inlet
grating at the tunnel ends. Since most installations require
relatively long tunnels and most yards do not chamfer tunnel
ends but do install grating, the actual thrust ends up somewhere
around 18 pounds per horsepower on a typical installation.
That is at zero vessel speed. It drops off pretty quickly
when the vessel is making forward speed.
A drop-down thruster with nozzle typically generates 25 pounds
of thrust per horsepower. No corrections apply. That's
what you can expect on a typical installation. The drop-down
is an azimuthing thruster. You can direct the thrust at
any angle, 360 degrees of the compass. Conversely, a tunnel
thruster is limited to directing lateral thrust straight to port
and straight to starboard only.
Ease of Installation
Either type of thruster can only be installed while the vessel
is out of the water. A tunnel thruster does not take up
much space and is relatively easy to install. The
major restriction is that the top of the tunnel must be at least
one tunnel diameter below the waterline to avoid vortex action
and air entrainment into the tunnel. Use the waterline
in unloaded condition, since that is when you need the thruster
the most, as that is when the vessel has most of its hull area
exposed to the wind.
Installation of a drop-down thruster is more complicated.
A well or trunk must be fabricated in the bottom of the bow.
The thruster is then dropped into the trunk from the top or sometimes
it is pulled up into the trunk from below. Drop-downs are
rather high and require a good bit of hull depth. For instance,
a 150 HP unit requires at least 8 feet of hull depth, a 250 HP
unit needs 10 feet, a 500 HP unit needs 14 feet, and a 1000 HP
drop-down requires not less than 17 feet of hull depth.
Cost
A tunnel thruster is the least expensive type of thruster
available. A drop-down thruster is at the opposite end
of the spectrum: it is the most expensive type. Depending
on the size, a drop-down may cost anywhere from twice to six
times as much as a tunnel thruster.
Power Source
Many tunnel thrusters are driven directly by an engine through
a reversing marine gear. This is often the most simple
and least expensive arrangement. It is also the least responsive
arrangement. Reversal of thrust requires slowing the engine
down to low idle, disengaging the clutch, shifting in reverse,
reengaging the clutch, and speeding up the engine. When
operating the thruster at lower engine speeds, the engine torque
exceeds the propeller torque by only a small margin, so there
is not much excess torque available for acceleration. This
results in slow response when increasing the speed setting.
And unless there is a shaft brake, response to reducing the speed
setting is even slower. See Figure 3.
 Figure 3 -
Blue lines indicate available torque for acceleration.
There is no torque available for deceleration: The thruster can
coast down only, unless it is provided with a shaft brake
Electric drives and hydraulic drives are much more responsive.
They can be reversed instantly and full torque is available for
acceleration and for deceleration at any propeller speed.
See Figure 4.
 Figure
4 - Torque-speed diagram for electric and hydraulic variable
speed drive. Red lines indicate available torque for acceleration.
Blue lines indicate available torque for deceleration (dynamic
braking). A drop-down thruster with direct engine drive requires disconnection
of the driveline every time the thruster is retracted.
This allows the engine to remain on its foundation while the
thruster, with its horizontal input shaft, is raised to its stowed
position. Thruster manufacturers have developed remotely
operated disconnects to facilitate this. These devices
are usually internally splined sleeves that slide over the splined
shaft ends. They are actuated by air or hydraulic cylinders.
Unfortunately, these disconnects are rather maintenance-intensive
due to problems resulting from improper alignment, lack of lubrication,
or control related issues.
A better solution is to use a drive that does not require
disconnection of the driveline. This can be achieved by
using hydraulic or electric drive in an L-drive configuration
with a vertical motor installed on top of the thruster.
The drive motor goes up and down with the thruster, as it raises
and lowers. This also simplifies the thruster arrangement
as it eliminates the need for an upper right-angle bevel-gear
transmission. As an alternate, a podded drive may be used,
whereby the electric or hydraulic motor is in the lower pod of
the thruster leg and the motor shaft is directly connected to
the propeller shaft. This configuration eliminates bevel
gearing completely.
Hydraulic drive thrusters, tunnel or drop-down, are available
up to 1000 HP. Electric drive is available to 3000 HP and
beyond. Hydraulic drive is more compact and requires less
space than electric drive. Hydraulic drive is also less expensive
than electric unless the vessel already has ample electric power
available.
Docking in Port
For docking in port, a tunnel thruster in the bow can be very
useful, especially when the wind is blowing at the beam.
A drop-down will do at least as well except in shallow ports.
The drop-down extends quite a bit below the bottom of the vessel
and it could get damaged when hitting bottom. It also may
get fouled when picking up debris from the bottom. A tunnel
thruster can never hit bottom and is much better protected against
floating logs, car tires and other debris found in ports.
The grating at the tunnel inlets keeps the larger debris from
being sucked into the thruster.
Holding Station in Open Sea
When the thruster is used for station keeping in open waters,
a number of factors need to be considered. First,
the thruster should be sized for the worst environmental conditions
(wind and current) that the vessel must be able to operate in.
How is the thruster going to behave under those conditions?
Second, most of the time, wind and current will be much lower
than during those worst conditions. How will the vessel
hold station in mild weather conditions?
 Figure 5 - Tunnel thrusters come
up out of the water when station-keeping during rough weather
conditions. With a tunnel thruster operating in rough weather conditions,
the vessel is normally headed into the wind and the main engines
are used for forward thrust while the tunnel thruster is used
to maintain heading. The vessel rolls, pitches, and heaves
due to the weather. With the vessel moving like that and
with seas going up and down around the bow, the tunnel openings
may come up out of the water on a regular basis. This allows
large slugs of air to get sucked into the tunnel resulting in
thruster propeller ventilation, causing instant loss of thrust
just when you need it the most. This propeller ventilation
also causes severe pounding and shock loading on the thruster
drive train. It may result in premature failure of gears
or couplings unless a podded hydraulic drive is used. A
podded hydraulic drive acts as a shock absorber and torque limiter
capable of withstanding this kind of abuse, and it does not use
gears or couplings.
During mild weather conditions, the tunnel thruster should
do fine, provided that it has good speed regulation even at low
propeller speeds. Here again, the electric or hydraulic
drive will perform much better than a direct engine drive.
The main engines may have a hard time trying to control forward
thrust during low wind conditions. You may put one screw
in forward and one in reverse, but it still requires compensation
of unequal thrust, and the bow thruster will have to compensate
for the turning moment generated by the opposing forces of the
main propellers. Extended operation at idle is bad for
the main engines, and the frequent use of the clutches puts a
lot of wear on them.
How does a drop-down cope with bad and with mild weather?
Even during rough weather conditions, there is no likelihood
of propeller ventilation. The propeller and nozzle are
right at centerline and well below baseline. Maximum thrust is
available when you need it the most, and the azimuthing capability
allows it to help the mains in the forward and reverse direction.
During mild weather, the drop-down can do it all. You
can disengage the clutches of the mains. The bow thruster
will be able to keep the vessel on station with its nose in the
wind without any help from rudders or main screws.
Conclusion
For docking and undocking maneuvers, tunnel thrusters are
preferable. But if you need to hold station in open seas
on a regular basis, a drop-down is a much better solution.
If you have to use a tunnel thruster for station-keeping, at
least make sure it has an electric or hydraulic variable speed
drive.
If you want the best of both worlds, use a combination thruster.
This is a drop-down that, when retracted, operates as a tunnel
thruster. These Acombi-thrusters@, with hydraulic or electric
variable speed drive have become very popular with offshore service
and supply vessel operators working in deep waters.
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