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  #1 (permalink)  
Old July 20th, 2002, 10:26 AM
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Default Lose the Azipods ...

I'd like to at least have a HINT that I'm on a ship !!!

"Give me that old-fashioned Rock and Roll"

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Old July 20th, 2002, 04:29 PM
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Default Re: Lose the Azipods ...

Right, lose the Azipods and the terrible toupees ! ! ! There is little that is more entertaining than a toupee acting like a kite on a Speedo clad beer gut bald guy. <<g>>

Actually, the Azipods are more efficient (but you already knew that) however I wonder if they are more reliable? Wasn't it the Summit who limped back to port on one Azipod a couple of months ago and then cancelled the next 2-3 cruises for repairs? And wasn't her sister ship having the same problems?

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Thomas
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Old July 20th, 2002, 09:10 PM
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Default Re: Lose the Azipods ...

Well... I'm not sure what the problem is with Azipods or the Mermaid systems... they do away with the need for stern thruster mechanisms and the rudder... and all the mechanical systems involved with the rudder... and its loss of efficiency as speed decreases (see Titanic)... I think these new propulsion systems are the way of the future, actually. I do not believe, as an example, a QM2 could be built or efficiently operated with deissel engines and long shafts leading to propellers, etc.... at least not a liner of that size able to efficiently operate at high speeds.

Post Edited (07-20-02 22:04)
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Old July 20th, 2002, 09:12 PM
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Default Re: Re: Lose the Azipods ...

Thomas,

"There is little that is more entertaining than a toupee acting like a kite on a Speedo clad beer gut bald guy. <<g>>"

Reminds me of a song by Alan Sherman that played regularly on the radio in the early 1960's, then suddenly disappeared. I don't remember all the lyrics, but one line has always stuck in my mind: "Did you ever see a whale in a bikini?"

"Actually, the Azipods are more efficient (but you already knew that) however I wonder if they are more reliable? Wasn't it the Summit who limped back to port on one Azipod a couple of months ago and then cancelled the next 2-3 cruises for repairs? And wasn't her sister ship having the same problems?"

It seems that every azipod ship that has been in service for any length of time has had serious propulsion problems that have caused the ship to limp to a yard somewhere for repairs about once per year or so. Fundamentally, the design puts an incredible amount of stress on the bearings that keep the pod shaft aligned vertically, which must carry the full thrust of the propulsion in addition to the torque induced by the fact that the propulsion units are at the very end of the shaft. The result is crushed thrust bearings that prevent the shafts from rotating as designed.

The azipod design also has the problem that the motors that drive the propellers are in the pod, where the crew can't get at them to service them if they malfunction. In fact, one has to put the ship in drydock in order to get to the motors. Most electric motors do have brushes that generate electrically conductive dust as they wear (induction motors being the most significant exception). If the dust builds up in the motor, it shunts power across the brushes and reduces efficiency. On a conventional ship with electric drive, the crew can stop a shaft and clean out the dust at sea if necessary because the motor is inside the hull. With an azipod, the ship has to go into drydock because the motor is in the pod.

Overall, I'll take the reliability of conventonal propulsion over the slight gain in efficiency that an azipod might provide. Prospective passengers tend to be most upset when their cruises get cancelled because the ship has to go to drydock on an emergency basis.

Norm.
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Old July 20th, 2002, 09:17 PM
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Default Re: Re: Lose the Azipods ...

Ernie,

"Well... I'm not sure what the problem is with Azipods or the Mermaid systems... they do away with the need for stern thruster mechanisms and the rudder... and all the mechanical systems involved with the rudder..."

Actually, the "azipod" system also replaces the rudders and the stern thruster.

Actually, a more accurate description is that it mounts the motor and the propeller on the rudder -- which can turn a full circle in aziumth. Unfortunately, this design puts incredible stresses on the bearings that align the rudder sharts. It also puts the motor where it's inaccessible to the ship's personnel -- thus requiring the ship to go into drydock if the motor needs service.

Overall, the azipod is not this engineer's idea of good design.

Norm.
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Old July 20th, 2002, 10:07 PM
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Default Re: Re: Re: Lose the Azipods ...

Norm... if you reread what I wrote, that is exactly what I said... that the Azipod system replaces the rudder, the stern thruster and the throbbing noisy vibration laden shafts from the engines to the propellers... at least that is what I wrote! ()

Post Edited (07-20-02 22:08)
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Old July 20th, 2002, 10:24 PM
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Default Re: Lose the Azipods ...

Norm - is there anything you don't know about ships and cruises and ports of call! You are a font of information and I'm so glad you post here. Thanks mucho.

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Old July 21st, 2002, 10:52 AM
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Default Re: Re: Re: Lose the Azipods ...

If thrust bearings are not "beefy" enough then wouldn't a set of bearings alleviate the problem if stronger ones were not available. I've not studied the design of the azipods but wouldn't a motor coupled with a paraflex to a gear reducer help? Since a 3-stage gear reducer would transmit the torque to three sets of bearings, shafts, and gears. The reducer could even be a 1:1 ratio. Maybe a reducer would be too bulky to fit into the design.

It's amazing to me to design something that is unmaintainable for Christ's sake. Doesn't make any sense to me to forfeit the good will and revenue of several thousand passengers to save money on construction costs. I'm currently in the middle of a construction project now and maintainence costs are at the top of the pecking order.

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Thomas
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Old July 23rd, 2002, 09:23 PM
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Default Re: Re: Re: Re: Lose the Azipods ...

Thomas,

Marine thrust bearings are a radically different design. They actually go between the end of the shaft and a verical plate inside a casing that's welded to the hull with incredible reinforcements. Both surfaces are machined perfectly flat.

The bearing design itself involves a set of links configured in a circle and designed so that they tilt away from the rotation of the shaft. In other words, if the shaft is turning clockwise, the links against the shaft tilt such that the counterclockwise side lifts slightly off the shaft and the clockwise side presses against the shaft, while the links against the casing's faceplate do the reverse, creating wedge-shaped gaps. Since the casing is full of oil, the rotation forces the oil into both gaps so that the oil actually absorbs the thrust. There's also a flange on the end of the shaft with a smaller set of thrust bearings on the other side that absorb thrust when the ship operates astern propulsion.

Now, consider the azipod design. I understannd that the propellers are on the forward end of the pod, which means that the shaft must have a flange with thrust bearings to transmit forward propulsion to the pod itself. That is not a technological problem, though -- a standard thrust bearing will do the job (again, pity that you have to send the ship into drydock to service it!). Rather, the real problem is where the vertical shaft of the pod unit enters the hull. In this location, a standard thrust bearing won't work -- partly because a standard thrust bearing is designed for a longitudinal shaft (thrust on the end of the shaft) rather than a shaft that's either verical or transverse (thrust on the side of the shaft). A further complication is that the propulsion, located at the bottom of the shaft, is exerting a substantial torque as well as the ship's entire thrust.

I don't have the dimensions of the azipod unit, but we can do a simple "back of the envelope" calculation using some rough engineering estimates. If we guess that the vertical shaft probably enters the hull about midway along its length and that it has horizontal support from bearings at that point and at the top, the fulcrom of countertorque from the hull will be 3/4 of the way up the shaft. The propeller is three times as far from this fulcrom as the bearings. Since torque is the product of force and distance, the force required to generate the necessary countertorque is three times the force generated by the propeller -- split euqually between the two bearings, so each bearing must carry 3/2 of the force generated by the propeller. This is added to the propulsive force at the lower bearing. Thus, we can expect that the bearings where the azipod unit enters the hull will have to absorb about 5/2 of the ship's thrust -- or 2 1/2 times the load on a standard set of thrust bearings. That's a pretty hefty load on a rotational bearing that generally is designed not to be load-bearing.

The azipod problems do not surprise me.

Norm.
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Old July 23rd, 2002, 09:32 PM
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Default Re: Re: Lose the Azipods ...

Jane,

"Norm - is there anything you don't know about ships and cruises and ports of call! You are a font of information and I'm so glad you post here. Thanks mucho."

Sure, there are lots of things I don't know. For example, I don't know why anybody would ever choose to cruise with certain cruise lines. I also don't know why anybody would build a whole class of vessels with a new but unproven propulsion technology. I also don't know what clueless engineers come up with these hair-brained schemes. That's just for starters....

The truth is that most of what I know about this comes from experience in one form or another -- but I won't bore you with the details here as they tend to emerge from time to time on the boards.

Norm.
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Old July 23rd, 2002, 10:39 PM
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Default Re: Lose the Azipods ...

Norm - does Princess have any ships with this Azipod system ... specifically the new Coral Princess? Thanks.

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Old July 24th, 2002, 11:08 AM
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Default Re: Re: Re: Re: Re: Lose the Azipods ...

Yes, I can agree with the torques and moments imposed upon the shafts and bearings but I have a hard time understanding why a redundant system was not installed which would allow each pod to be removed for routine maintenance while aboard.

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Thomas
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Old July 24th, 2002, 10:02 PM
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Default Re: Re: Re: Re: Re: Re: Lose the Azipods ...

Thomas,

Yes, I can agree with the torques and moments imposed upon the shafts and bearings but I have a hard time understanding why a redundant system was not installed which would allow each pod to be removed for routine maintenance while aboard.

That would be a rather difficult engineering challenge. The vertical shafts enter the hull from the bottom of the vessel, below the waterline. The only way to remove the entire Azipod unit would be to undo the ring bearings and drop it down out of the hull since the pod containing the motors at the end of the vertical shaft is longer than the shaft itself. Unless the drydock is exceptionally deep, it probably also is necessary to remove the propeller from the pod before removing the pod from the vessel in order to provide clearance to drop the pod.

You know, the more I think about the Azipod concept, the worse it seems!

Norm.
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Old July 24th, 2002, 10:12 PM
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Default Re: Re: Lose the Azipods ...

Janie,

Norm - does Princess have any ships with this Azipod system ... specifically the new Coral Princess? Thanks.

Thankfully, I have not seen a single mention of azipods or similar units in any of Princess's press releases or brochures.

MV Coral Princess and MV Island Princess do have the line's first gas turbine generators in addition to diesel generators, though. You can see the turbine pods on each side of the "stack" above the stern of the vessel. Princess's previous MV's had only diesel generators.

The gas turbines that power the generators are basically the same engines that power large airplanes. They are proven technology, extremely reliable, durable, economical, and enviromentally clean. The U. S. Navy has been using them to power cruisers, destroyers, and frigates nearly three decades.

Norm.
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Old August 4th, 2002, 01:10 PM
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Default Re: Re: Re: Re: Lose the Azipods ...

Ah, give me the Royal Princess. It looks like and feels like a real ship. We had the least expensive non view blocked cabin on the ship last September. At night the waves were crashing against our windows. I slept like a baby. It was great.

Carole
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Old August 17th, 2002, 08:49 PM
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Default Re: Lose the Azipods ...

The problems with Azipods seem to have been worked out. The Paradise was the first cruise ship to have problems. She was drydocked for two weeks for repairs, and as a preventive measure, the Elation was brought in for repairs, too. The Voyager class ships for Royal Caribbean all have Azipods, and I haven't heard of any problems with them. Carnival's Spirit class is also equipped with Azipods and I havet heard of any engine problems with them either, although the Carnival Spirit did have an air conditioning problem earlier this year resulting in the cancellation of one cruise and the trunkation of another. Since the Elation came into service, the Celebration, Tropacale, and Destiny all have had engine room fires, and the Celebration had to have engine repairs eirlier this year. Also, the Ecstasy lost power to its stern thrusters when a laundry room fire spread to the mooring deck, and the fire there damaged the electrical lines to the thrusters.

Celebrity uses a slightly dirrerent design on its pods, and all three ships had to be drydocked within the last year.

Also, Azipod is a trade name for the podded propulsion system developed by ABB of Finland. Celebrity uses a different brand, named the Mermaid and manufactured in France. To be technically correct, we should refer to Azipods as podded propulsion systems unless we are specifically refering to the ABB product.

TomS
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