Use of rudder systems vs. azimuth thrusters

[jokerabc]

Is there a reason why military ships continue to use prop and rudder systems instead of azimuth thrusters? Azimuth thruster ships like huge cruise ships don't require tug boats and have better handling.

Is it because traditional prop and rudder systems have more speed? more reliable and easier to repair at other ports?

 

[Belesari]

Many new military vessels are still not all electric an yea i think alot of it is the relability and ease of replacment.

Is there a reason why military ships continue to use prop and rudder systems instead of azimuth thrusters? Azimuth thruster ships like huge cruise ships don't require tug boats and have better handling.

Is it because traditional prop and rudder systems have more speed? more reliable and easier to repair at other ports?

 

[My2Cents]

Is there a reason why military ships continue to use prop and rudder systems instead of azimuth thrusters? Azimuth thruster ships like huge cruise ships don't require tug boats and have better handling.

Is it because traditional prop and rudder systems have more speed? more reliable and easier to repair at other ports?

See http://www05.abb.com/global/scot/scot293.nsf/veritydisplay/3355105c6026658dc12575c4002fcfbf/$file/azipod_xo2100_product%20introduction.pdf
There are a number of reason azipods may not be optimal for warships.

• All the thrust from the azipod in transmitted to the hull through a fairly small strut and slip ring/bearing with a considerable bending moment, these forces can also be applied in any direction because of the steerable nature of the unit. This will require major stiffening of the area where it is attached, which will in turn effect weight distribution and locations of additional equipment (weapons) to compensate. The bending moment will probably increase in proportion to the 3/2 power of the azipod output.
• Current designs have severely limited access and maintenance, the unit has to be removed and brought topsides for repairs. It is therefore recommended that the units be located “as far astern and as close to the ship’s sides as possible”. This may interfere with other equipment not common on cruise vessels, like a towed sonar array.
• Fixed propeller limits capability for sudden maneuvers, such as a quick stop or turn at high speeds.
• Generates more difficult to control flow noise than traditional arrangements.
• Military ships have much higher power to weight ratios than commercial ships. The Carnival Elation, for example, is 70,000 tons and uses 2 x 14MW azipods, for a total of 28MW of propulsive power. A Burke class destroyer is only 10,000 tons but has 81MW of propulsion power.

A few warships have used small retractable azipods to improve docking and low speed maneuverability, but this does not seem a trend.

Fixed propulsion pods are also being considered for future designs, but that is a completely different concept.

 

[Belesari]

Seeing a Burke at full throttle is a bit strange. Ships that large shouldn't function that way LOL.

If i remember there were designes showing future cruisers with fixed pods up from for better handling but havn't heard much from them.

[nomedia="http://www.youtube.com/watch?v=mzveUz-WRGQ&feature=related"]Navy ship taking "evasive action" - YouTube[/nomedia]

See http://www05.abb.com/global/scot/scot293.nsf/veritydisplay/3355105c6026658dc12575c4002fcfbf/$file/azipod_xo2100_product%20introduction.pdf
There are a number of reason azipods may not be optimal for warships.

• All the thrust from the azipod in transmitted to the hull through a fairly small strut and slip ring/bearing with a considerable bending moment, these forces can also be applied in any direction because of the steerable nature of the unit. This will require major stiffening of the area where it is attached, which will in turn effect weight distribution and locations of additional equipment (weapons) to compensate. The bending moment will probably increase in proportion to the 3/2 power of the azipod output.
• Current designs have severely limited access and maintenance, the unit has to be removed and brought topsides for repairs. It is therefore recommended that the units be located “as far astern and as close to the ship’s sides as possible”. This may interfere with other equipment not common on cruise vessels, like a towed sonar array.
• Fixed propeller limits capability for sudden maneuvers, such as a quick stop or turn at high speeds.
• Generates more difficult to control flow noise than traditional arrangements.
• Military ships have much higher power to weight ratios than commercial ships. The Carnival Elation, for example, is 70,000 tons and uses 2 x 14MW azipods, for a total of 28MW of propulsive power. A Burke class destroyer is only 10,000 tons but has 81MW of propulsion power.

A few warships have used small retractable azipods to improve docking and low speed maneuverability, but this does not seem a trend.

Fixed propulsion pods are also being considered for future designs, but that is a completely different concept.

 

[PCShogun]

A few warships have used small retractable azipods to improve docking and low speed maneuverability, but this does not seem a trend.

Fixed propulsion pods are also being considered for future designs, but that is a completely different concept.

I believe the Independence class littoral combat ship uses the retractable pods, not sure about the Freedom class. It is interesting that they do use water jet propulsion though.

 

[Belesari]

They have to they are supposed to get up to 50kts fully loaded. Dont think they have but it was a requirement.

I believe the Independence class littoral combat ship uses the retractable pods, not sure about the Freedom class. It is interesting that they do use water jet propulsion though.

 

[My2Cents]

They have to they are supposed to get up to 50kts fully loaded. Dont think they have but it was a requirement.

I think that waterjet propulsion was one of the requirements. It eliminates the prop and rudder which are the most vulnerable elements if they hit an underwater obstruction.

 

[Anixtu]

[*]Fixed propeller limits capability for sudden maneuvers, such as a quick stop or turn at high speeds.

If you mean "fixed pitch propeller", not an issue IME of azimuthing thrusters. To stop quickly you apply "full ahead" revolutions with the thruster reversed. Better than CPP!

There is however a severe trade off between rate of turn and forward speed with the azimuthing thruster systems I've used. You can turn the ship very quickly at any speed, but you lose headway and have to build it up again on coming out of the turn.

 

[My2Cents]

If you mean "fixed pitch propeller", not an issue IME of azimuthing thrusters. To stop quickly you apply "full ahead" revolutions with the thruster reversed. Better than CPP!

Correct. I meant a fixed pitch propeller

Questions:

1. How big (approximate MW or HP) were the units you had experience with?
2. How long would it take to turn a large azimuthing thruster 180 degrees?
3. Is it usually done with the thruster running, or is the thruster slowed or stopped, rotated, and then brought back up to speed?
4. Assuming that the ship is equipped with 2 azimuthing thrusters, when you want to crash stop do the thruster pods pivot in opposite directions when they reverse? If so, how much control would you have in the event you had to maneuver before they completed the rotation?

 

[ngatimozart]

Methinks that the rudder and prop is preferred because it is less noiser than Hamilton jets and thrusters. Ok travelling at speed through water is noisy but slow quiet is another story. Secondly the energy out put on a thruster to change the direction of a ship at speed in water. Is it efficient? A rudder is mechanical. Also the one thing that limits a ships speed is, IIRC the ratio of its length at the water line to its beam at the waterline. I'll have to go and look it up but IIRC (and I may be wrong so correct me if I am) the wave length of its wake is critical to its max speed and no matter how much power you put on once you reach that wave length you won't go any faster. Now I apologise that's a real confused explanation but I know minds far better than mine will give a far better explanation.

 

[Anixtu]

Correct. I meant a fixed pitch propeller

Questions:

1. How big (approximate MW or HP) were the units you had experience with?
2. How long would it take to turn a large azimuthing thruster 180 degrees?
3. Is it usually done with the thruster running, or is the thruster slowed or stopped, rotated, and then brought back up to speed?
4. Assuming that the ship is equipped with 2 azimuthing thrusters, when you want to crash stop do the thruster pods pivot in opposite directions when they reverse? If so, how much control would you have in the event you had to maneuver before they completed the rotation?

1) 4.4MW
2) Single digit seconds. The thrusters turn 180° in a shorter time than it takes the propellers to stop and reverse.
3) Propeller at whatever speed you like. For a crash stop, simultaneously turn the thrusters whilst pushing the speed lever to maximum "ahead".
4) Opposite directions, outwards, is the norm. Bad things may happen otherwise, including unintended turning moments and wash from one prop being directed at the other. You don't have to set the thrusters to exactly 180° for the crash stop, so you can manoeuvre as required whilst stopping in a shiplength or two.

 

[My2Cents]

1) 4.4MW
2) Single digit seconds. The thrusters turn 180° in a shorter time than it takes the propellers to stop and reverse.
3) Propeller at whatever speed you like. For a crash stop, simultaneously turn the thrusters whilst pushing the speed lever to maximum "ahead".
4) Opposite directions, outwards, is the norm. Bad things may happen otherwise, including unintended turning moments and wash from one prop being directed at the other. You don't have to set the thrusters to exactly 180° for the crash stop, so you can manoeuvre as required whilst stopping in a shiplength or two.

1. What is you maximum maneuver speed? Is there a speed beyond which some maneuvers should not be performed?
2. About what I would expect. Faster to reverse than a fixed propeller, slow than a variable pitch propeller, slower than reversing a water jet. More maneuverable than either, but still cannot go sideways unless you have 4+ thrusters.
3. The stopping distance for a 30+ knot warship would depend on the time to reverse and the available breaking thrust. Water jet is probably the faster yet. The variable pitch propeller would be the quicker to reverse, but I’ll bet the azimuthing thrusters have the advantage in thrust, with water jet propulsion somewhere in between, so it’s a real guess.

 

[My2Cents]

Also the one thing that limits a ships speed is, IIRC the ratio of its length at the water line to its beam at the waterline. I'll have to go and look it up but IIRC (and I may be wrong so correct me if I am) the wave length of its wake is critical to its max speed and no matter how much power you put on once you reach that wave length you won't go any faster. Now I apologise that's a real confused explanation but I know minds far better than mine will give a far better explanation.

What you are thinking of is ‘hull speed’, and it is not an ultimate limit. But the power requirement vs. speed once you cross it goes up MUCH faster.

Methinks that the rudder and prop is preferred because it is less noiser than Hamilton jets and thrusters. Ok travelling at speed through water is noisy but slow quiet is another story.

The noise for a prop and rudder is generated by cavitation, the blade tips, and the turbulent flow from the propeller past the rudder. There is also some noise from the power plant that passes down the drive shaft

Water jets eliminating the blade tip noise and remaining noise and cavitation (and addition acoustic insulation can be added) is dampened because they are enclosed in the hull, and there is no rudder, so they are quieter except possible from dead astern. The generated noise is also at a higher frequency, so it is more rapidly damped by water.

Thruster are just maneuverable propellers, so no rudder is needed. The ones being discussed use electric drive, so there is no noise from the power plant passed on by the shaft. They are probably quieter going ahead, but there may be some beat frequencies generated during maneuvers. There will also be the noise from the rotation of the thruster mounts.

Secondly the energy out put on a thruster to change the direction of a ship at speed in water. Is it efficient? A rudder is mechanical.

Thrusters are more efficient than a propeller and rudder, especially a low speeds. The problems are a substantial increase mechanical complexity and impact on the hull design.

 

[Anixtu]

1. What is you maximum maneuver speed? Is there a speed beyond which some maneuvers should not be performed?
2. About what I would expect. Faster to reverse than a fixed propeller, slow than a variable pitch propeller, slower than reversing a water jet. More maneuverable than either, but still cannot go sideways unless you have 4+ thrusters.
3. The stopping distance for a 30+ knot warship would depend on the time to reverse and the available breaking thrust. Water jet is probably the faster yet. The variable pitch propeller would be the quicker to reverse, but I’ll bet the azimuthing thrusters have the advantage in thrust, with water jet propulsion somewhere in between, so it’s a real guess.

1) About 18 knots. No limitations that we've found.
2) Sideways with three: 2 x azimuth thrusters + 1 x transverse bow thruster.