More effective - bow thrusters or stern thrusters? 9

[aircraftengr]

I have personally found that more boats use bow thrusters rather than stern thrusters, for the simple reason that the keel and rudder are located towards the back of the boat, thus making the stern thruster "unnecessary".

However, by observation, the back portion of the boat is usually heavier, due to physical build and equipment stored, such as trim tabs (exterior), generator, batteries, inverters and engines (in engine room). It would be logical to require extra 'help' in the form of thrusters.

At which point is it necessary to use bow/stern thrusters? What are the criteria involved in making the decision? Have there been instances where only stern thrusters are used without the bow thrusters being present?

 

[djw2k3]

Hi aircraftengr,

I'm probably not terribly qualified to discuss this but I do know a bit about large vessels.

AFAIK the bow thruster tends to be used only during mooring operations. If the ship is self sufficient - ie reasonably powerful bow thrusters it can get alongside okay, high winds and swell cause havoc for an unassisted vessel with under-powered bow thrusters.

Large vessels such as tankers and container vessels usualy need the assistance of a tug to control at least the bow of the vessel. I am not sure if they would even bother having bow thrusters.

Vessels with a high turnaround - ferries, Ro/Ro's tend to be more self sufficient and are usually equipped with powerful bow thrusters to get alongside.

As an example a ferry/rail vessel that weighs 15000 Tonne, might have 10000 kW main propulsion and 1200 kW bow thrusters.

Also the rear of a ship is very manouverable - especially twin screw and becker rudders - can pertty much position a vessel anyway you please with those rudders.

I hope u found this rambling post helpful

 

[aircraftengr]

Yes djw2k3, you're right when you say that it is usually used for mooring operations. Especially at unprotected areas, bow thrusters can be a desperate necessity for maneuvering to get in and out as both the wind and waves/current can make it difficult. It's used mostly in yachts.

Tankers and container vessels would not be able to find a bow thruster powerful enough to even move their bows. The biggest bow thruster I've seen takes 15,000W and that is no easy electrical system to spec out. Thrust is only about 290kg, and compared to the displacement weight of the boat, it's quite insignificant.

But the question still remains, if rudders are quite sufficient, then why the need for stern thrusters?

 

[IRstuff]

For the same reason bow thrusters are used. In order for the rudder to do anything, you need to have forward motion, which means that if you need to essentially do a parallel park with a ship, it's absurdly difficult.

With the thrusters, you could stop the ship and essentially slide it into place.

TTFN

 

[aircraftengr]

That means the rudder is not enough to maneuver the boat. Stern thrusters still need to be utilized. When you say stop the ship, I assume you mean neutralize the throttle and use only bow/stern thrusters?

So have there been instances where only stern thrusters are used without the bow thrusters being present?

 

[IRstuff]

I'm not positive I've actually seen them in use, but I vaguely recall seeing a ferry moving sideways and it looked like both bow and stern had side thrusters.

TTFN

 

[djw2k3]

A vessel with bow thrusters but without stern thrusters can move easily sideways with no headway - just using rudders and twin screws at the rear + bow thruster if and when necessary.

But no aircraftengr I couldn't say if there have are instances of stern thrusters w/o bow thrusters.

 

[aircraftengr]

I thought as much. Thanks guys, for your input. I appreciate it!

 

[AngelAlvarez]

There are many reasons why some ships and boats use bow thrusters.
. A twin engine boat or ship will need only a bow thruster and move sideways with no major difficult. .
. A one screw boat will not necessarily need an stern thruster. With some experience you can make the vessel to move sideways without a stern thruster. Of course if you have both, it will be easier.
. The bow thruster is relatively easy to install in the bow because the witdth of the hull -in the region where you need to install the tunnel for the bow thruster- is not a big one and you do not have other machinery, shafts or equipment in that area.
. In the stern, the installation is mainly more complex because you have shahts, machiney, engines pipes, etc and usualy the stern is wider with some exceptions so you need longer tunnels, etc. Mainly the stern is thin only in the places where you have the main shaft in one screw vessels.
. No matter what is said before, there are ships with both stern and bow thrusters.
. Contrary to popular believe, a lot of big ships use bowthrusters with pretty big HP
Regards

 

[ktm5]

aircraftengr,
As Fleet Manager for a previous employer, one of our vessels had a rotating nozzle in the stern (no rudder) and a bowthruster. The rotating nozzle acted as a sternthruster. I don't believe I've ever heard of any ship with sternthruster only installed, although this particular vessel operated fine and maneuvered well without utilizing the bow thruster at times.
Rgds, ktm5

 

[aircraftengr]

Well, it seems to unanimous then. No one has exactly seen any vessel with only stern thrusters installed. I think stern thrusters and rudders seem to be somewhat interchangeable judging from what ktm5 says.

Any other comments?

Cheers!

 

[IRstuff]

Interchangeable only in a gross sense, though. There are clear occasions where stern thrusters are used where rudders are useless.

Likewise, most stern thrusters are probably inappropriate for underway steering.

TTFN

 

[aircraftengr]

If both the bow and stern thrusters were installed, would the bow thruster be more powerful or the stern thruster? Or does it depend on the function of the vessel?

How much more powerful anyway? The stern is usually heavier for the common bow-up position. I figure it requires more power. However, the stern already has the assistance of the rudder. How do I balance the necessity of stern maneuverability?

 

[AngelAlvarez]

It all depends on the specifics of your ship -boat. With a twin engine configuration, forget about the stern thruster, you just do not need it. As a rule when you have both thrusters, the bow thruster is the more powerfull one because you can use your engine and rudder to assist the control of the stern. The bow up position just mean that you need more power to control the bow in windy situations.
Regards
Angel

 

[tobbe]

AngelAlvarez> A twin engine configuration does not outrule the use of the stern-thruster. Large cruise-ship often are have that configuration because it improves their manouverbility?? partly due to their large superstructure creating large wind-forces. A must? Maybe not, but they make life simpler in one way. Example:

http://www.masa-yards.fi/publications/pdf/Imta2001.pdf

Today more and more ships are equiped with podded propulsion where several solutions allows you to rotate the propeller and redirect your thrust in any direction and really removing the need for a stern thruster (and rudder):

http://www.masa-yards.fi/publications/pdf/Motorship2000.pdf

 

[Cyclefrog]

http://www.dickson-thruster.com

used to sell stern thrusters for stern-only installation

 

[IRstuff]

Coincidently, I was just on the Carnival Ecstasy. Both bow and stern thrusters are used to separate the ship from the dock, as well as manuevering the ship to turn seaward in a small harbor.



TTFN

 

[AngelAlvarez]

Tobbe is right: Some cruise ships, Ro-Ro and Ferries use stern thrusters even they might have twin engine instalations.
On some ships with twin engine installation, the distance between the shafts is so relatively short with respect to the length of the ship that is hard to move the ship sideways using the engines and rudders alone. This dictate the use of stern thrusters or tugs during docking and undocking, etc.

 

[IRstuff]

Even if the shafts are reasonably separated, the efficiency is still relatively poor as far as generating the torque necessary to turn the vessel.

The same amount of thrust applied at the extreme outboard location would generate a much larger torque.

TTFN

 

[aircraftengr]

I certainly agree that the same amount of thrust applied at the extreme outboard location would generate a much larger torque.