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?

 

[ktm5]

In order to answer aircraftengr’s original three queries:
“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?”
1. I believe it depends on the degree of maneuverability required under the specific seakeeping circumstances occurring during the docking/undocking procedure.
2. Some of the criteria have been discussed in this thread, being existing propulsion design limitations, sea state experienced, air draft of vessel vs wetted surface area, size, etc… Each vessel has it’s unique turning and maneuvering characteristics. Other criteria not discussed yet here are CAPEX vs ROI, will the thrusters preclude use of harbor tugs for docking and undocking, pilotage requirements for the specific ports to be visited, etc…
3. I don’t believe there has ever been a sternthruster installation without a bow thruster also.
Rgds,
ktm5

 

[AngelAlvarez]

IRstuff: You are right saying that if you applied the same amount of thrust at the extremes outboard location of a ship, you can generate more torque. We mainly are talking about physical considerations. From this point of view, in fact we can look at two problems:
1 To steer the bow
2 To steer the stern

1. To steer the bow:
On one engine installations, at slow speed is almost impossible to steer the bow: You need a bowthruster or a tug in close quarters because as soon as you use your engine the ship will move astern or forward.
On twin engine installations, you can use the moment that develops with different setting of the propellers thrust and the rudders to move the bow. The dimensions that determine how easy you will be able to control the bow are the separation between the shafts and the total thrust you can use on each shaft.
When the L/B relationship is relatively low, and the shaft are relatively far from each other, you can very easy turn the ship and "walk" sideways using the main engines and rudders
If you look at any sportfisherman, tug, etc, you will see how "easy" they turn and dock relying only on the engines and rudders.
On very long ships, with high L/B relation, the maximum moment you can get is relatively low so it makes a lot of sense to use a bowthruster.

2. To steer the stern:
Even having only one engine, you can help the ship a lot using the propeller and ruder. Having a bow thruster, the ship can be steered without a stern thruster or tug using the main engine. One main consideration will be the ability of the propulsion plant to switch back and forward.
If you have a twin engine installation, the need for the stern thruster is dubious except in the case when you need huge amounts of torque to steer the ship stern or you can't or do not want to use the main engines to steer the stern. The distance between shafts is not so critical as is the amount of thrust you can apply on each direction. One engine holds the ship in place while the other move it to one side or the other.
If you can use your main engines and they are flexible enough - controlable pitch props, electric motors, etc- the stern thrusters do not represent a big advantage: For the stern thruster and the propellers, the lever is almost the same relatively to the center of the submerged area of the ship but the main engines carry hundreds and hundreds of time more power and the propellers can develop a lot more thrust. You can move the stern sideways without any headway if you choose to.
Of course, when you have both bow and stern thrusters, the handling of the ship is a lot easier and safer, you do not need so much to relay on somebody's skill to dock the ship and the probability of an accident is lower. Also you do not stress the propulsion system.
There are a lot of considerations when you are trying to make a decision to install or not certain type of thrusters. I agree 100 % with KTM5's considerations .
I never found a ship with stern thruster without a bow one.
I have seen hundreds with bow thrusters and no stern thrusters.
If somebody is interested in a deeper knowledge about this topic and ship maneuverability in general let me know.I might write a little bit deeper about this and send to the e-mail of the interested person.
Thanks

 

[mbridger]

In 30 years I have never seen a vessel with a stern thruster and no bow thruster presumably because most boat handlers can coax some rear lateral movement out of the craft by use of the props residual side thrust and flow across the rudder. In improving the manouverability of several vessels we have always retrofitted a bow thruster before stern thrusters.
In a typical offshore support vessel required to Dynamically position you could find three azimuthing thrusters at say 2Mw each and three bow tunnel thrusters at about 1.2 Mw each this may well be an indication of their relative effectiveness?? We need the Naval Architechts for this one.

 

[cyclops]

I have operated and been a passenger in many Hacker mahogany speedboats. They are 23 to 38 feet long and powered by engines of at least 350 to 800 hp. Old style - shaft and prop / very small rudder ( just like the originals) the area of them is your palm. The boats are a joke to dock. Rudder area and how deep you can get them is the only safe and fail safe solution to docking under all bad conditions. WW II ELCO PTB had 2 small rudders. HIGGINS PTB had 3 large area and very deep rudders. Guess who rarely was hit by any enemy weapon during emergency evasive action? A well designed and powered stern area should only need a bow thruster if tugs are not available. If a stern thruster is needed --- it is a barge piloted by a untrained crew.

 

[cyclops]

One other point about ships that do have and use them all the time. IF, and the IF will happen, you lose the thrusters in a condition where they must not fail to perform, close your eyes, yell " TUG assistance " and kiss you reputation goodbye, so a owner can spend it on something more important.

 

[aircraftengr]

I can see big ships yelling "TUG assistance" but not yachts.

 

[colinp]

When a ship is nearly stopped bow thrusters are somewhat more effective than stern thrusters. When the ship is moving faster stern thrusters are far more effective for maneuvering.

When a ship maneuvers, the rudder changes the angle of attack of the hull. The hull then generates a hydrodynamic force (lift) in the horizontal plane which is responsible for causing the maneuvering acceleration. The center of lift is typically 1/4 to 1/3 LWL aft of the forward perpendicular. A stern thruster is farther away from the center of lift of the hull, so it's effect on maneuvering is greater than that of a bow thruster.

A bow thruster's effect on maneuvering decreases very quickly with increasing speed.

Colin Pitts

 

[mshimko]

"When Are They Necessary" should be a question asked of both bow and stern thrusters, and the answer would be "when the ship owner thinks they are worth the expense and other trade-offs".

At this time, VERY FEW Navy and Coast Guard vessels have thrusters of any sort, with buoy tenders being an obvious exception (and the only one I can think of off the top of my head).

 

[IRstuff]

Again, as mentioned above, many cruise and cargo ships that visit ports that do not have full or full-time facilities tend to be equipped with side thrusters.

TTFN

 

[aircraftengr]

Personally, I don't really find a need to use stern thrusters when the boat is moving at a reasonably high speed. That is because one is usually not within reach of obstacles when high speed is engaged.

That means the maneuverability is less critical at this point (unless you're doing a stunt act in a movie).

But when you are constricted for space and yet need power to move, it appears that the bow thruster comes in handy. Guess that's why bow thrusters are used more often.

 

[ssreese1500]

The application of stern and bow thrusters depend almost exclusively on the service of the vessel. Cruise ships have them for the obvious reasons, they simple cant crash a dock with a deck full of passengers watching. most new vessels that really need that kind of maneuverability are being equipped with Z drives, which is a propeller housing that rotates 360 degrees, or "egg-beaters" which is a downward pointing controllable pitch propelling system.

I personally have never dealt with a stern thruster but i have worked on several bow thrusters. The point that was made about large vessels not having them is not true. As a matter of fact, the largest oil barge in the world has two seperate ones, each with its own engine. And let me tell you this, I would NEVER design a vessel with one of them, the maintenance is a nightmare and very expensive. For a true cargo vessel, forget the thruster and get a tug. Most cargo ships use two tugs when docking, even when they DO have a bow thruster. Also, dont forget that the tunnels where these things are installed take a beating from the water flow forward and make it hard to keep the coatings on them. In these areas, erosion is as big a factor as corrosion.

Lastly, dont forget the golden rule of Cargo Ships. Every square inch of space, and every pound of weight used for something installed on the ship, is that amount of cargo that cant be carried. Its all about money people.

 

[oxilume]

Dear aircraftengr,

First and foremost a good ship driver with a twin screw ship does not need a bow thruster or tugs. Both are relied upon for safety. But then of course, the most valuable assest during mooring is the ship's pilot. He comes aboard prior to entering the harbor.

However, mooring a large vessel is just the opposite of parallel parking. You make an approach 30 degrees relative. Come to all stop about 400 yds. Dead in water about 10 yards. Throw over #1 mooring line.

Take tension on #1 line.

Walk stern over. Throw over #6 line. Take tension on 6 line.

Getting underway, you twist the ship. Similar to a bulldozer or skid steer. One track goes forward and the other reverse. If you are starboard side to, starboard engine ahead 1/3, port engine back 1/3.

If ship has gas turbines (LM2500), then shaft always rotates in same direction. CRP screws rotate to full pitch ahead (over 20+ feet) and back to about (over 15+ feet).

One thing everyone has forgotten is sail area. Large Naval vessels project a tremendous amount of sail area. Think very large main sail for a sail boat.

In a 30 knot wind at 90 or 270 relative, if the ship's pilot orders you to deadstick it in, then you will need four very large tugs to push you pierside.

With the new electric pods, many thruster applications are becoming obsolete.

Likewise, a ducted propeller doesn't need 3 to 5 kts for rudder effectiveness. Knort nozzles have been around for quite some time.

Designing a ship is one thing. Driving it is another!

Todd
Former Navy Officer Ship Driver
Naval Engineer



Todd

http://www.oxilume.com

 

[kander]

Stern thrusters are used in vessels involved with the oil & gas industry where a very high accuracy in positioning is required. For this a dynamic positioning system (DP) is utilised. With this bow and stern thruters or azimuth thrusters are used, taking information from satalite to determine position and computer control over thruster to hold position or move position. An example would be landing out a oil wellhead in all sorts of water depths. Typically the DP system will hold position to within 1-2m (3-6 ft) and accuracy decreasing in relation to wind, waves, current, etc.
Another example of where position holding is paramount is in ROV (Remotely Operated Vehicle) operations. These are remote control underwater robots typically capable of upto 3000m (10,000 ft) seawater depth.

Such a vessel is the world class offshore construction vessel the BOA Deep C, operating out of Norway. A 13,000 ton vessel, equipped with two work class ROVs and capable of deploying equipment upto 250 ton in all sorts of water depth. This has two bow, two stern thrusters and one retractable azimuth thruster. An incredibly sophisticed vessel! See:

http://www.akerkvaerner.com/NR/rdon...6-EAB184F739B1/10663/2004BOADeepCbrochure.pdf

For your info, there is also another type of propulsion system, completely independent from conventional prop/thruster systems. This utilises rotating 5 blades extending below the hull of the ship. As the blades rotate the pitch angle is altered and the blades in effect 'swim' through the water. As this pitch angle can be directed towards any azimuth angle, the vessel can move in any direction, or rotate, whatever you want. This system patented and built by Voith Schneider has been about for about 80 years and is typically used for pilot boats, short distance passenger vessels, etc. See:

http://www.voithturbo.de/vt_en_pua_marine_vspropeller.htm

Hope this is of help to you.

 

[AngelAlvarez]

Contrary to what some poeple might think putting a relative small ship in a pier with no tugs in complex meteo conditions a lot of times is just impossible no matter how skilled you or your pilot are.
One thing is a Naval ship, with mainly hughe power to displacement ratio -compared to merchant ships- more than one shaft, very good inherent maneuverability and a moderated relative sail area and another is an empty cargo, or tanker ship.

To maneuver a cargo ship or a tanker in copmplex meteo conditions is a nightmare and the cause of some ports just do not allowing maneuvers of certain ships even with the help of powerfull tugs and or bowthrusters during certain meteo conditions.
The inertia, and the reaction time since you apply your own power and/or from the tugs to the moment you get some movement are really hughe. The wind and current forces are really hughe.
I have seen a lot of accidents during maneuvers even with tugs present, destroying ships and piers and costing lives. The human error is the main cause of the accidents, but of course, the meteo conditions at the time you are maneuvering and the characteristic of the ship you are running and the layout of the piers sometimes induce such a high stress on pilots and captains that errors occur and accident happens.
Even if you have twin screws, on a big container or passenger ships the distance between the shafts is not enough to control the bow -even at relatively high power settings- unless there is no wind or a very moderate one. Sometimes the thruster is a necesity of the ports the ship is visiting. If a ship will go to ports that do no have tugs and the pier layout is difficult, and the meteoconditions are mainly rough, when you are designing the ship probably you will put thrusters of some kind to make the ship safer.
As oxilume pointed, the electric pods made the stern thrustes necessity dubious at best, but even in this case, sometimes a bow thruster remains a necesity.

 

[aircraftengr]

Thanks for your comments, folks. It's really very compelling reading. I'm back in the aviation /aerospace sector and reading this again brings back memories.

R.A.
Structural Analysis Engineer
Excelnet

 

[jabonet]

I can only think of three cases where using a stern thruster could be a must, if you have twin rudders and one propeller, therefore no flow over the rudder, in which case you need some speed to steer.

in case you want to keep a very precise position over the seafloor, and your main engine is too big to react as fast as you want for this

or in the case that you do extensive backward maneuvers at low speeds, like channel barges.

I particularly don´t like thrusters, they are made, and installed for faster and easier maneuvers, but if you can´t dock you ship without them due to meteo conditions, or other factors, depending 100% on them will get you into problems sooner or later.

I am only talking about yachts and small ships, I now very little about tankers and the likes.

Jabonet