QM2 Propulsion Pod Questions

[Emu]

As the "pods" contain large electric motors............ I am assuming any repair that involves breaching the skin would have to be done in a dry dock............. as electric motors and water don't really mix.

 

Is this correct??? or is it possible to work from the exterior underwater??

 

There also would be internal access to the mechanicals available from inside the ship???

 

Are the pods double skinned (ie outer and inner walls)?????

 

The most likely candidate for damage must be the propeller blades............ can they be changed underwater???

 

What does an entire pod weigh?????

 

Any information would be welcome.

 

Thanks all

[Bollinge]

Pod Info: http://search.abb.com/library/ABBLibrary.asp?DocumentID=3BFV000246R01&LanguageCode=en&DocumentPartID=&Action=Launch

[Emu]

Although the ABB site gives some info........... still doesn't really answer my questions.

 

It seems to me that anything major requires a dry dock..........after all if you hit something hard enough to damage blades......... more than likely the shaft would be bent too. (unless the blades are designed to fail before reaching the stress limit of the shaft......... which is possible) As the electric motor is mounted on that shaft........... well it could possibly be damaged as well.

 

This will be an interesting process........how & when it all takes place.

 

I will be asking all my questions when I'm on board for the return LA - NY voyage................. (If we are still going!!!!!!)

[mufi]

There is a long history of problems in new, untried technology in engineering. This seems to have happened because of the pods, and would be unlikely with convential propellers. And if the pods are not servicable outside dry dock, this could be a big problem for the future of this ship when she leaves the Atlantic crossings.

 

David.

[Bollinge]

This ship hit the bottom. The pod was damaged.

 

The QE2 hit the bottom a few years ago off Martha's Vineyard. She had no pods to be damaged, but she suffered a mighty gash in her hull.

 

On both occasions, a local pilot was embarked and officially in charge of the ship's course.

 

I seem to remember in the case of the QE2, the U.S. Hydrographic Survey charts were blamed for the unknown rock sticking up out of the sea bed.

[mufi]

Well yes, any ship will be damaged after hitting a rock. QM2 hit a sandbank ?? with presumably no damage to the hull, but to the pod. We must be thankful that only one pod was damaged considering what could have happened.

 

The question is why. There are only two possible reasons - faulty charts or human error. It will be interesting to learn the outcome.

 

David.

[gatour]

Hmm, baby the hurricane moved some sand around so the channel is not as deep in all the places that it was thought to be. Just speculation on my part

[Jedidad]

There is a long history of problems in new, untried technology in engineering. ....

Actually, pods aren't new and untried. They've been around at least since early '90s, if I recall correctly.

[gatour]

And they have been having problems ever since :) , although I think they have gotten better. I don't this problem is attributable to the pods. It will probably come down to what others have said some type of human error or as I said that the channel may not have been in perfect shape.

 

When the hurricanes hit Florida in 2004, they actually had to dredge out Port Canaveral's channel to allow the cruise ships back in.

[Capt_BJ]

Cunard links provide some answers

 

 

As the "pods" contain large electric motors............ I am assuming any repair that involves breaching the skin would have to be done in a dry dock............. as electric motors and water don't really mix.

 

As noted here: http://www.cunard.co.uk/QM2/default.asp?Active=about&sub=shipfacts There are 4 PODS, two fixed and two that azimuth (turn), each is a 21.5 megawatt electric motor. It has been noted that the pod that was damaged is forward and one of the fixed ones. One concludes that the forward PODS are outboard. Elsewhere I've read that the primary PODs are the aft azimuthing ones and the forward are used for speed. This is logical as the steering also comes from the azimuthing pods. Note the arangement in this profile: http://www.cunard.co.uk/QM2/photoGallery.asp?path=QM2+Ship+Images%2FQM2%5Fprofile+%2Ejpg&page=1

 

 

Is this correct??? or is it possible to work from the exterior underwater??

 

There also would be internal access to the mechanicals available from inside the ship??? While possible, all POD arrangements I've seen have not been built to afford access from inside the ship.

 

Are the pods double skinned (ie outer and inner walls)????? Not double on any that I am familiar with, but if the POD had been breached they would not have been cleared to sail if for no other reason than pollution risk; altho the safety risk would be there too.

 

The most likely candidate for damage must be the propeller blades

I agree What is unclear from the profile is if the screw projects beyond the side of the ship altho it does not appear to project below the keel. In any case it appears they found the bottom and or side of the dredged channel (and not a sea wall)...possibly new shoaling following the hurricanes this year? To be determined I'm sure. I'd note that with a draft of 32 feet and 4 PODs, QM2 puts the props closer to the banks than say a tanker also drawing 32 feet, but with only 1 prop on the centerline. But from the various reports I've seen I'd guess that as they began to increase speed after clearing the channel, sensors indicated vibration in the pod. Tests determined this not to be a false reading so they suspected blade damage, returned to port and had that verified by divers. Question then becomes what to do....

 

............ can they be changed underwater??? Sometimes yes, and that appears to be the case here. One report I've read says they removed the prop from this POD. Why do that? Well a prop that is not turning is a big drag... Usually if a ship wants to run on less than all available shafts, not unusual, thay will free wheel the unused shaft which produces less drag. So, why did they remove the prop and not put on the spare? Well, another thing you need to worry about in this situation is whether any other damage was done to the POD - specifically bearings. Spinning the shaft under no load, but with a damaged bearing could do more damage. They could have put the spare screw on and conducted tests but that takes time. So my guess is they decided to take the blade off and get on their way and let the engineers use the time 'til their next port to determine what tests they can do... And yes if/when they get to the point of opening the POD....I'm 95% sure that involves Mr Dry Dock ....

[travel-to-go]

Cunard links provide some answers

 

Wow! Thanks for some pretty comprehensive explanations! You're obviously knowledgeable of some of the engineering principles. Just an interest? Just curious as to your background. (none of my business- just curious!)

 

Thanks for the explanation!

 

Time will tell what actually did happen, and what has to be done to fix it!

 

Karie

[c-legs]

:cool: Hi,

 

Excellent speculation !!! You might be the closest to the truth here.....

 

Cheers

CG

;)

 

Hmm, baby the hurricane moved some sand around so the channel is not as deep in all the places that it was thought to be. Just speculation on my part

[Capt_BJ]

Wow! Thanks for some pretty comprehensive explanations! You're obviously knowledgeable of some of the engineering principles. Just an interest? Just curious as to your background. (none of my business- just curious!)

 

 

A) I'm "in the industry" ... I drive ships and have overseen several dry dock periods, both planned and unplanned.

 

B) Had a somewhat similar experience:

 

a number of years ago while sailing in the Old Bahama channel the ship that I was on hit something we didn't see. We heard a loud racket in the engine room and quickly stopped everything to check things out. Part of the standard drill in this situation was for a crew member to go aft and look over the stern - I always thought that was dumb.

 

We were sitting stopped and tring to determine waz' up when the guy back aft reported that there was a big tree trunk sticking out from under the ship back aft. :eek: We'd hit a 'dead head' in the middle of the ocean and unfortunately as luck would have it when the shaft stopped the tree - with a big root-ball - wedged itself in the blades of the prop. When we tried to turn the shaft the tree would hit the rudder. Amazing but true - I have pictures! A 1 in 1,000,000 shot. Thank goodness for twin screw ships and multiple rudders.

 

We limped to port and needed divers with an underwater chain saw to get the thing out...fortunately there was no significant damage to the prop or shaft - just a few scuffs. But if you hit something and it bends a blade just a little bit, you can get serious vibration in the shaft and that can cause big time bearing damage ... Then you get to experience an unscheduled drydock. Very expensive....very....

 

Another ship, another time, we started experiencing bad vibration from one of our 2 shafts. Got so bad we finally had to 'lock the shaft' which seriously slowed us and increased fuel consumption. Divers determined the bearings in a shaft support strut had failed. The only way to replace was to dry dock and pull the shaft out of the ship. Very expensive task...

 

c) I like to track the NTSB, Coast Guard and industry reports on things like this....good professional reading in my business.

[Emu]

Thank you for providing some answers.............

I find it really strange that "pods" of the size in question would not have access from inside the ship............. It must make inspections of the electric motor and the bearings very difficult.

 

So it seems increasingly likely to me that the entire pod will have to be replaced............. I just hope that Cunard or ABB have (1) a replacement of the same size available............ (2) a rather large plane to shift it in & (3) a vacant dry dock.

 

I will also be interested to see what the actual damage was!!

 

Thanks again for your professional advice.

[Capt_BJ]

Thank you for providing some answers.............

I find it really strange that "pods" of the size in question would not have access from inside the ship............. It must make inspections of the electric motor and the bearings very difficult.

 

So it seems increasingly likely to me that the entire pod will have to be replaced............. I just hope that Cunard or ABB have (1) a replacement of the same size available............ (2) a rather large plane to shift it in & (3) a vacant dry dock.

 

I will also be interested to see what the actual damage was!!

 

Thanks again for your professional advice.

 

a) As far as I've seen the POD is just big enuf to wrap around the electric motor it carries, and the mount just big enuf to carry the weight, be turnable and support power transimssion. How much larger would it need to be to provide space for humans to get about, have ventilation, access from inside etc etc? and the increased size means increased drag etc etc.

 

One of the factors they are trying to exploit is that is a dies' (or turbine) electric drive system, the electric motor is supposed to be the very reliable, low maint' part of the system.

 

b) Spare POD readily available... Nope, don't think so. Altho they are basicly big outboard motors, I have not gotten a sense of standardization (pls send me one of you 2.15 MW pods....) I haven't seen a case yet where a spare was kicking around. Scan all the pages here and see the # of POD ships operating at reduced power cuz of problems.....some day maybe, but I don't think they are there today.

 

c) Once parts are available, finding an empty dry dock? Well, curiously this isn't as hard as one might think. Cuz any dry dock that is empty isn't making money and the ship yard is probably ANXIOUS to fill it. There are three outside my office window and all are empty. When I needed to get a ship in FAST, it was always the contracting that took time cuz we could find an empty dock. BUT, I wasn't trying to dock QM2... There's many fewer places she'll fit (none of the 3 outside my window) Sometimes size ISN'T everything :o

 

 

I think this stuff is cool as heck and Happy to share .... ain't ships neat? I just rec'd the new Professional Mariner mag - cover article is about....PODs!

[travel-to-go]

Capt BJ,

In the aviation industry what you described is called a Prop Stop..(Hey, there's an NTSB report in the archives with my name on it! Last I checked it was still searchable) If you get a prop strike on an airplane, and it stopped the prop, even briefly, chances are you have a bent shaft. replaceing a bent up prop is expensive. Replacing a trashed engine with a shaft that looks like an old mangrove root is more expensive, and a lot of time off-line. Thanks for educating us today. I'm learning a lot from this forum!

Karie

(look up Karie Parker N3099X- Johnnycake Airport, Burlington CT I forget- around 1987-88)

[Emu]

Capt BJ,

Why are the pods arranged with the props at the leading end??

 

Are the motors AC or DC........... I understand with AC the motors are more powerful and smaller than the comparable DC types, but the power controls are much more complicated???

 

When the QM2 was recently in dry dock in Germany, would the pods have been swapped with new or overhauled ones, or just disassembled and checked, before being reassembled??? (I think they only had about 10 days)

 

(I ask this question because in heavy machinery to minimise down time, it is common to replace a motor quickly then rebuild the one you removed and use it next time)

 

The new RCL "Voyager of the Seas", does it also use the pod system, or is it standard????

 

Yes......... ships are very interesting ........... I just love big things that move :) ships, trains and planes.

 

Thanks again for your most enlightening comments.

[Bollinge]

"Voyager class" vessels (137,000 tons) have three azipods of 14 mW each, compared to QM2's (148,000 tons) four pods of 21.5 mW each.

 

The benefits of the pod system are such that it has revolutionised ship design. The system is very smooth with little or no vibration, fuel consumption is up to 20% better than conventional shafts and screws, and there is also greatly improved manouvreability.

 

There have been problems with reliability, but the pod system is here to stay.

[Capt_BJ]

My reference is Issue 94 of Professional Mariner. They usually post their main articles on their web page: professionalmariner.com, the Feb/Mar issue is not there as of this date, but I expect it will be soon….

PM’s Feb\Mar 06 issue includes an article about POD propulsion with VOYAGER OF THE SEAS {hereafter VOS} as the example.

VOS @ 142,000 tons WAS the largest the largest cruise ship afloat when it went into service. QM2 150,000 tons?

VOS has 3 Pods… each is 14MW (QM2 has 4 @ 21.5 megawatt) The three are arranged with one pivoting on each side, and one centerline - fixed.

Azipods are noted (by PM) as having several advantages: they move a bunch of machinery out of the basic hull carrying with it both consumed space (able to be filled with consumer revenue producing stuff) and noise & vibration. You loose ‘shaft alley’, & bunch of bearings. In answer to a question, the article states that the pods have their screws forward so as to pull vs push…because this is more efficient. On VOS that applies to the 2 outboard pods only, the centerline pod has the screw aft…

I’d further note that POD systems have become so the vogue that they are even the system of choice for modern ice breakers. Note the recent launching of the replacement for the Coast Guard’s Great Lakes ice breaker, MACKINAW…so far known as NEW MACKINAW. After extensive research of state of the art ice breaker technology, ‘new MAC’ was built with PODS….

On the down side, when they break, it appears there is little that can be done without a drydock because they lay outside the skin of the ship….