Is a Retractable Better Than a Tunnel Thruster?

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.


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.


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.


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.



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

tunnelThrusterOutOfWaterWhen 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?

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.


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.