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DocLightning
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The Subject Of Ship Design

Sun Sep 09, 2012 7:48 am

Living on the SF Bay, I've become increasingly interested in large ships. We have a lot of container carriers coming into Oakland and tankers going to points further inland, like Richmond.

A few things I wonder:

1) Passenger ships tend to place the bridge up front. Why do merchantmen tend to have rear-mounted bridges?

2) Passenger ships tend to have multiple screws. Merchantmen, even the biggest, newest. and fastest, tend to be single-screwed. Why?

3) Why does it seem like direct diesel drive gets used more for merchantmen than for passenger ships?
-Doc Lightning-

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Mir
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The Subject Of Ship Design

Sun Sep 09, 2012 8:42 am

Quoting DocLightning (Thread starter):
1) Passenger ships tend to place the bridge up front. Why do merchantmen tend to have rear-mounted bridges?

So that the crew can keep an eye on the cargo deck from the bridge, I'd imagine. It also puts the bridge closer to the engine room.

-Mir
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BMI727
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The Subject Of Ship Design

Sun Sep 09, 2012 8:47 am

Quoting DocLightning (Thread starter):
3) Why does it seem like direct diesel drive gets used more for merchantmen than for passenger ships?

Noise and vibration maybe? Or the much higher electrical loads demanded by passenger ships.
Why do Aerospace Engineering students have to turn things in on time?
 
smittyone
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The Subject Of Ship Design

Sun Sep 09, 2012 11:11 am

Step into to my office, Doc.

Quoting DocLightning (Thread starter):


1) Passenger ships tend to place the bridge up front. Why do merchantmen tend to have rear-mounted bridges?


Generally, placing the pilothouse somewhere between amidships and the pivot point (about 1/3 of the way aft from the bow) is ideal from a shiphandling perspective. You'll notice that a lot of your older, smaller cargo ships and tankers had this arrangement but they're just about all gone now, for a few reasons.

- from a regulatory standpoint you can't put the pilothouse over a flammable cargo.
- the midship superstructure interferes with long, uninterrupted cargo / container spaces, which are most efficient.
- It is cheaper and more compact to construct all of your accommodations/machinery spaces together - minimizes space not devoted to payload (as well as hogging forces caused by hanging weights at either end of a shell). Putting all of that stuff all the way forward would not be practical - so far from the propeller - and would also leave the bridge vulnerable to getting nailed by large waves. So you put it all aft.

Great Lakes freighters used to have their pilothouse on the bow (for visibility) and their machinery at the stern, but that has been superseded by the more efficient "everything aft" design. Of course in the Great Lakes you don't see the same wave heights that you see on the ocean (though it can be just as 'unpleasant' overall due to the short period).

Passenger ships really have nowhere else to put their pilothouse but well forward...if located aft it would be really tall and goofy to see over the passenger accommodations. Cruise ships are so tall that they can get away with putting it far forward from a seakeeping perspective.

Quoting DocLightning (Thread starter):


2) Passenger ships tend to have multiple screws. Merchantmen, even the biggest, newest. and fastest, tend to be single-screwed. Why?

3) Why does it seem like direct diesel drive gets used more for merchantmen than for passenger ships?

A large diesel engine turning one screw is the cheapest solution to install and is efficient. It is not very responsive (especially in a direct drive configuration) or maneuverable but those things are not as important as cost for merchantmen.

Multiple shafts turning controllable-pitch propellers offer a lot of benefits to maneuverability and redundancy that are important to cruise lines. If a passenger ship had a problem with its only engine/shaft/screw in heavy weather, that could be really bad for the human cargo. Likewise, controllable pitch propellers can be 'reversed' almost instantly, which increases safety and the ability to maneuver in tight spots.

Noise/vibration is a consideration but passenger electrical load is not an issue because the ship has auxiliary diesel generators specifically installed for that purpose.
 
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kngkyle
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The Subject Of Ship Design

Sun Sep 09, 2012 11:35 am

I'm also fairly interested in ships, particularly in lake freighters. They are just so long and skinny, looks like they would just split in half in rough seas, which I know the great lakes get their fair share of.

http://4.bp.blogspot.com/-8eNNUO64hM4/UCsuPyzddCI/AAAAAAAAk1I/ujbYgkLMGq8/s1600/jbarker15.jpg

Cool video showing arrivals and departures from the Port of Duluth, Minnesota. (well cool to someone who finds ships interesting)
http://www.youtube.com/watch?v=YUyRfltyAwU
 
smittyone
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The Subject Of Ship Design

Sun Sep 09, 2012 12:08 pm

Quoting kngkyle (Reply 4):

Awesome Kyle - I had the chance one time to sail GEORGE A. STINSON for an overnight trip...the captain took us down to the tunnel leading under the cargo holds running the length of the ship...he said that you could definitely see the hull flex in a storm.

But my favorite lakers are the old school:

http://www.boatnerd.com/pictures/images/ryerson-icerockcut-jb.jpg
 
Kiwirob
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RE: The Subject Of Ship Design

Sun Sep 09, 2012 1:48 pm

Quoting DocLightning (Thread starter):


1) Passenger ships tend to place the bridge up front. Why do merchantmen tend to have rear-mounted bridges?

Ease of control, plus is cheaper to build them when the majority of electrical and mechanical systems are in a single location.

That said the new Maersk Tripple E class will have the bridge amidships, and will have twin engines. You won't see these in the US as they are too big for your ports.



CMA's Christopher Colomb class also have the bridge further forward. These vessels also have cabins for 10 passengers in very luxurious berths.

Quoting DocLightning (Thread starter):
2) Passenger ships tend to have multiple screws. Merchantmen, even the biggest, newest. and fastest, tend to be single-screwed. Why?

They also tend to be single engined, single engines are more efficient. There are exceptions, tankers which operate in Arctic regions have twin engines, this was one of the outcomes of the Exxon Valdise grounding along with double hulls.

Quoting DocLightning (Thread starter):

3) Why does it seem like direct diesel drive gets used more for merchantmen than for passenger ships?

Not so, most cruise ships burn bunker oil, it's cheap and helps keep costs down, converting to lighter grades of oil or gas will increase the price of your cruise holiday. Until the industry is forced to change ships will continue to be built burning bunker oil, the industry won't do it by themselves.
 
comorin
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RE: The Subject Of Ship Design

Sun Sep 09, 2012 2:23 pm

Excellent thread with great, informative posts!

Q: In sailboats (non-planing), max speed is proportional to the square root of hull length. Is this true of large vessels also? Thanks.
 
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DocLightning
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RE: The Subject Of Ship Design

Sun Sep 09, 2012 5:15 pm

Quoting SmittyOne (Reply 3):
A large diesel engine turning one screw is the cheapest solution to install and is efficient. It is not very responsive (especially in a direct drive configuration) or maneuverable but those things are not as important as cost for merchantmen.

Why not? They still have to go into port and dock, right?

Quoting SmittyOne (Reply 3):
Noise/vibration is a consideration but passenger electrical load is not an issue because the ship has auxiliary diesel generators specifically installed for that purpose.

So I was reading about this. On the newest ships, like QM2, there are a number of diesel engines that drive a single electric bus. The hotel and propulsion loads are then automatically separated from this single electric output. To me, this seems like the simplest systems architecture for such a design.

On merchantmen, there are dedicated propulsion engines and dedicated engines for ship's power. The reasoning I read is that this is easier to install and cheaper to design, but offers less flexibility and minute-to-minute response to changes in demand.

Quoting SmittyOne (Reply 5):
Awesome Kyle - I had the chance one time to sail GEORGE A. STINSON for an overnight trip...the captain took us down to the tunnel leading under the cargo holds running the length of the ship...he said that you could definitely see the hull flex in a storm.

I've been aboard Stinson myself as a boy. She shipped a lot of my father's company's steel.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Kiwirob
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RE: The Subject Of Ship Design

Sun Sep 09, 2012 6:15 pm

Quoting SmittyOne (Reply 3):
the midship superstructure interferes with long, uninterrupted cargo / container spaces, which are most efficient.

Not true anymore the Maersk Tripple E class carries some 2500 TEU more than the previous E class, the ship is only 3 m longer and 4 m wider. With the twin engine design they can use smaller engines which in turn means less space taken up by machinery and more cargo capacity

Quoting SmittyOne (Reply 3):
Great Lakes freighters used to have their pilothouse on the bow (for visibility) and their machinery at the stern, but that has been superseded by the more efficient "everything aft" design.

A yard in Russia which I supply built a design called RSD44, this had a forward pilot house, the design was by all accounts a disaster, the ship had significant control problems over similar sized conventional vessels, it's much easier to see what's going on in front of you than what's happening behind.

Quoting SmittyOne (Reply 3):
Noise/vibration is a consideration but passenger electrical load is not an issue because the ship has auxiliary diesel generators specifically installed for that purpose.

On QM2 the 4 diesels also provide electrical power for the ship not just for propulsion. When higher speeds are required the 2 gas turbines are turned on to provide the extra go. From what I understand the QM2 drive train was considered by B.E.A. for the CVF project.
 
smittyone
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RE: The Subject Of Ship Design

Sun Sep 09, 2012 11:01 pm

Quoting comorin (Reply 7):
Excellent thread with great, informative posts!

Q: In sailboats (non-planing), max speed is proportional to the square root of hull length. Is this true of large vessels also? Thanks.

Yes - pretty much all displacement hulls.

Quoting DocLightning (Reply 8):
Why not? They still have to go into port and dock, right?

True, but ships of this size generally to use tugs alongside anyway (except the Jedi Master shiphandlers on the Great Lakes who generally don't unless they're being dragged backwards somewhere). I'm not up to speed on the docking procedures and use of tugs by cruise ships, so I'll have to punt on the question of why more maneuverability would appeal to them. The redundancy of multiple screws/azipods would have to be attractive, though. There is also the issue of hull design etc. that might make it hard to fit a big enough single screw under the transom of some ships.

Quoting DocLightning (Reply 8):
So I was reading about this. On the newest ships, like QM2, there are a number of diesel engines that drive a single electric bus. The hotel and propulsion loads are then automatically separated from this single electric output. To me, this seems like the simplest systems architecture for such a design.

My experience has been on diesel-electric buoy tenders and icebreakers, where two large diesels generated 440vDC power for the propulsion motor, and two much smaller diesels provided the ship's AC 'hotel' power. These were relatively small ships though (140' and 180'), so generating auxiliary power with a smaller diesel made much more sense when you weren't running the main engines. You don't want a diesel running with too little of a load - wasteful and not great for the engine long-term.A practical consideration since we often anchored (engines not running) or tied up somewhere that didn't have a shore tie plug for power. Either way, the electrical system was mechanical and incredibly simple. My guess is that QM2's system is not simple and is run by computers  
Quoting DocLightning (Reply 8):
On merchantmen, there are dedicated propulsion engines and dedicated engines for ship's power. The reasoning I read is that this is easier to install and cheaper to design, but offers less flexibility and minute-to-minute response to changes in demand.

I don't think ship's auxiliary power plays much into the decision on whether or not to use electric propulsion for merchant ships - the technology has advanced to the point where if you are going to opt for electric propulsion in the first place you might as well power the rest of the ship via an integrated system too. The lower cost and efficiency of the big diesel direct drive systems is probably still too compelling in a cargo application.

Quoting DocLightning (Reply 8):

I've been aboard Stinson myself as a boy. She shipped a lot of my father's company's steel.

Awesome - she's AMERICAN SPIRIT now!
 
Ken777
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RE: The Subject Of Ship Design

Sun Sep 09, 2012 11:43 pm

Quoting SmittyOne (Reply 5):
he said that you could definitely see the hull flex in a storm.

I was on the USS LONG BEACH (a cruiser) where there were flex points built in. I don't know how long a ship needs to be before benefitting from these points - the LONG BEACH was a long, thin design.
 
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DarkSnowyNight
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RE: The Subject Of Ship Design

Mon Sep 10, 2012 12:04 am

Quoting SmittyOne (Reply 10):
Awesome - she's AMERICAN SPIRIT now!

Did her lay-up last long? She looks like the same class of ship as Mesabi Miner, which is one I remember seeing in the locks way back when. Though I could be wrong about that classification...
Be A Perfectionst, You're Nothing If You're Just Another; Something Material, This Isn't Personal...
 
smittyone
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RE: The Subject Of Ship Design

Mon Sep 10, 2012 7:47 pm

Quoting DarkSnowyNight (Reply 12):
Did her lay-up last long? She looks like the same class of ship as Mesabi Miner, which is one I remember seeing in the locks way back when. Though I could be wrong about that classification...

I think they did the name change as part of her normal winter layup. (Btw I used to live in Sturgeon Bay, WI...so cool to watch the big ships come in for the winter.)

Mesabi Miner and Stinson were both built at American Shipbuilding in Lorain, OH (hulls 906 and 907).
 
rwessel
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 2:05 am

Quoting SmittyOne (Reply 10):
I don't think ship's auxiliary power plays much into the decision on whether or not to use electric propulsion for merchant ships - the technology has advanced to the point where if you are going to opt for electric propulsion in the first place you might as well power the rest of the ship via an integrated system too.

On many modern large cruise ships* the power needed for the hotel load exceeds that needed for propulsion. Rather oversimplified, that makes the motivation to simply upsize the electrical plant a bit, and then do electric propulsion. That makes azipods particularly attractive (since you can do away with stern thrusters too).


*Excluding the liners like the QM2, which have rather higher speed requirements (consider the 30kt, 150,000ton, 1130ft, 117MW QM2, vs. the 22kt, 225,000ton, 1180ft, 96MW Oasis of the Seas - the Oasis normally sails with twice the passengers and crew of the QM2, and would have a substantially higher hotel load).

Quoting comorin (Reply 7):
Q: In sailboats (non-planing), max speed is proportional to the square root of hull length. Is this true of large vessels also? Thanks.

Approximately - you can always go faster with more power, but it's an exponential curve. So there's a practical speed limit for most of us, and a slightly higher one for the money-is-no-object folks like Navies and folks running luxury liners.
 
Stealthz
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 2:57 am

Quoting SmittyOne (Reply 10):
so I'll have to punt on the question of why more maneuverability would appeal to them

Ships like the Emma Maersk and Christopher Columb only travel between major industrial scale ports with deep water, tugs and facilities geared for their operation.

Cruise liners often visit small ports or "hover" off shore at even smaller locations so maneuverability becomes much more important to them.
If your camera sends text messages, that could explain why your photos are rubbish!
 
rwessel
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 3:11 am

Quoting stealthz (Reply 15):
Ships like the Emma Maersk and Christopher Columb only travel between major industrial scale ports with deep water, tugs and facilities geared for their operation.

Cruise liners often visit small ports or "hover" off shore at even smaller locations so maneuverability becomes much more important to them.

Not to mention that cruise ships typically dock at a different port approximately every day, getting in and out quick would be a big plus (and makes for a better passenger experience).
 
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zkojq
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 5:24 am

Just curious (because this seems like a good place to ask), is there is a maritime inequivalent of ETOPS, regulating how reliable a passenger-carrying ship's engines must be?

Quoting SmittyOne (Reply 5):
he said that you could definitely see the hull flex in a storm.

Just like in a Boeing 757-300 during turbulence! =D

Quoting rwessel (Reply 14):
That makes azipods particularly attractive (since you can do away with stern thrusters too).

Does anyone know if the Azipod manufacturers fixed the reliability issues that many operators were experiencing a few years back? I remember reading a piece in Ships Monthly magazine a few years ago that indicated a substantial number of cruise ships weren't happy with them due to reliability issues.

Quoting rwessel (Reply 16):
Not to mention that cruise ships typically dock at a different port approximately every day, getting in and out quick would be a big plus (and makes for a better passenger experience).

And no-doubt save the Cruise Line on tug fees.
First to fly the 787-9 (ZK-NZE, NZ103, 2014-10-09)
 
comorin
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 6:18 am

Quoting SmittyOne (Reply 10):
Quoting comorin (Reply 7):
Excellent thread with great, informative posts!

Q: In sailboats (non-planing), max speed is proportional to the square root of hull length. Is this true of large vessels also? Thanks.

Yes - pretty much all displacement hulls.
Quoting rwessel (Reply 14):
Quoting comorin (Reply 7):
Q: In sailboats (non-planing), max speed is proportional to the square root of hull length. Is this true of large vessels also? Thanks.

Approximately - you can always go faster with more power, but it's an exponential curve. So there's a practical speed limit for most of us, and a slightly higher one for the money-is-no-object folks like Navies and folks running luxury liners.

Appreciate the replies, thank you. Now to try and understand why! Time for Cliffs notes on Hydrodynamics...
 
Kiwirob
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 6:33 am

Quoting zkojq (Reply 17):
Just curious (because this seems like a good place to ask), is there is a maritime inequivalent of ETOPS, regulating how reliable a passenger-carrying ship's engines must be?

The closest thing would be the SOLAS regulations, I'm not sure if propulsion is covered.
 
Stealthz
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 6:39 am

Quoting zkojq (Reply 17):
And no-doubt save the Cruise Line on tug fees.


I was at Sydney's Circular Quay one evening a few years back awaiting the departure of what was then the largest passenger ship seen in Sydney. As departure time approached I commented to a couple of crusty old shipwatchers about whether she would leave on time, they responded along the lines of " won't be going for a while yet, the tugs are not here yet to assist her out of her berth".
I should have taken photos of the expression on their faces as she blew her whistle, then quietly and serenely moved sideways from the quayside and backed out into the main channel, did a 360deg + pirouette giving all her passengers their last look at the Opera House and Harbour bridge and slipped silently away into the night... all totally unassisted.
If your camera sends text messages, that could explain why your photos are rubbish!
 
Ken777
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 3:31 pm

The Queen Victoria has had a bit of a problem and will be heading to dry dock in Germany. Would really like to know what specifically is the problem with the propulsion unit as the ship is pretty new - delivered in 2007.

Quote:

Hundreds of people have had their holiday plans ruined after a Southampton, England-based cruise ship suffered a mechanical failure.

Cunard's Queen Victoria has suffered a problem with one of its propulsion units with hundreds of passengers on board and it cannot travel at its top speed.
http://travel.usatoday.com/cruises/s...fers-mechanical-failure/57743392/1
 
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DocLightning
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 3:44 pm

Quoting SmittyOne (Reply 3):
A large diesel engine turning one screw is the cheapest solution to install and is efficient. It is not very responsive (especially in a direct drive configuration) or maneuverable but those things are not as important as cost for merchantmen.

Why not? An azipod-driven ship with bow thrusters could dock without tugs, which would have to save some fees, no?

Quoting SmittyOne (Reply 3):
Multiple shafts turning controllable-pitch propellers offer a lot of benefits to maneuverability and redundancy that are important to cruise lines. If a passenger ship had a problem with its only engine/shaft/screw in heavy weather, that could be really bad for the human cargo. Likewise, controllable pitch propellers can be 'reversed' almost instantly, which increases safety and the ability to maneuver in tight spots.

As I understand it, relatively few cruise ships have variable-pitch props. Azipods have become more popular because they allow tugless docking procedures, but particularly with the electric drivetrains in use on most cruise ships, reversing the propeller is trivial.

Quoting stealthz (Reply 15):
Ships like the Emma Maersk and Christopher Columb only travel between major industrial scale ports with deep water, tugs and facilities geared for their operation.

Cruise liners often visit small ports or "hover" off shore at even smaller locations so maneuverability becomes much more important to them.

That does make sense. Still doesn't explain to me why even large and fast ships like Emma Maersk have single screw propulsion.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Kiwirob
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 7:43 pm

Quoting DocLightning (Reply 22):
Still doesn't explain to me why even large and fast ships like Emma Maersk have single screw propulsion.

Simple cost, one large engine uses less fuel than two smaller ones, it also costs less to install and to maintain, it's all a matter of economics.
 
smittyone
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RE: The Subject Of Ship Design

Tue Sep 11, 2012 11:19 pm

Quoting KiwiRob (Reply 9):
A yard in Russia which I supply built a design called RSD44, this had a forward pilot house, the design was by all accounts a disaster, the ship had significant control problems over similar sized conventional vessels, it's much easier to see what's going on in front of you than what's happening behind.

I don't know, my guess is that the Great Lakes forward pilothouse design must have been superior 100+ years ago when it became the rage. Of course these were much smaller ships without radar or VHF radios, operating in congested waters in the days before we had Vessel Traffic Services etc. I could see how putting the pilothouse forward would be useful in that situation. Especially in limited visibility, which happens a lot due to fog (or sea smoke during the ice season).

Quoting DocLightning (Reply 22):
Why not? An azipod-driven ship with bow thrusters could dock without tugs, which would have to save some fees, no?

I suppose that they crunched the numbers and found it cheaper overall in that application to just rely on tugs...which could very well be compulsory in a port for a large ship regardless of how maneuverable it is.

Quoting KiwiRob (Reply 9):
Not true anymore the Maersk Tripple E class carries some 2500 TEU more than the previous E class, the ship is only 3 m longer and 4 m wider. With the twin engine design they can use smaller engines which in turn means less space taken up by machinery and more cargo capacity

I was thinking more about the large self-unloading bulk carriers that they have on the Great Lakes - full-depth cargo holds feeding the long, continuous unloading belt are the most efficient way to handle coal/wheat/taconite etc.

But the container ship example you cited does show how each succeeding technological improvement makes previous design 'rules' less relevant than before!
 
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DocLightning
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 5:58 am

Quoting Ken777 (Reply 21):
The Queen Victoria has had a bit of a problem and will be heading to dry dock in Germany. Would really like to know what specifically is the problem with the propulsion unit as the ship is pretty new - delivered in 2007.

I know that QM2 had problem with its RR-powered units when the thrust bearings kept wearing out in the pounding of the North Atlantic. I believe Cunard took legal action against RR.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Stealthz
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 12:19 pm

Quoting DocLightning (Reply 22):
Still doesn't explain to me why even large and fast ships like Emma Maersk have single screw propulsion.

It is so much more efficient, Emma makes 16 port visits in a 3 month cycle between Gdansk, the far East and return, most of that 3 months is spent with her Wärtsilä RT-flex96C operating at close to 100RPM. The trade off between paying for tugs etc and their services seems pretty clear cut to Maersk.
Emma also has a systen that generates flashs steam from her exhaust that not only provides "hotel" heat for the crew but drives a steam turbine gen set that contributes something like 10,000 hp to the propellor shaft.
If your camera sends text messages, that could explain why your photos are rubbish!
 
Klaus
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 12:26 pm

Quoting stealthz (Reply 26):
Emma also has a systen that generates flashs steam from her exhaust that not only provides "hotel" heat for the crew but drives a steam turbine gen set that contributes something like 10,000 hp to the propellor shaft.

How is that power coupled into the shaft?
 
Stealthz
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 12:34 pm

Quoting Klaus (Reply 27):
How is that power coupled into the shaft?

Steam turbine alternators and electric motors geared onto the shaft
If your camera sends text messages, that could explain why your photos are rubbish!
 
Klaus
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 12:47 pm

Quoting stealthz (Reply 28):
Steam turbine alternators and electric motors geared onto the shaft

Okay... are these electric motors capable of driving the shaft alone as well in an emergency (such as keeping the ship maneuverable in bad weather when the main engine should be inoperable)?
 
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DocLightning
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 7:32 pm

Quoting stealthz (Reply 26):
It is so much more efficient, Emma makes 16 port visits in a 3 month cycle between Gdansk, the far East and return, most of that 3 months is spent with her Wärtsilä RT-flex96C operating at close to 100RPM. The trade off between paying for tugs etc and their services seems pretty clear cut to Maersk.
Emma also has a systen that generates flashs steam from her exhaust that not only provides "hotel" heat for the crew but drives a steam turbine gen set that contributes something like 10,000 hp to the propellor shaft.

So if single-screw propulsion is more efficient (and I believe her screw has six blades), why do cruise ships seem to have a minimum of two?
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DocLightning
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 7:35 pm

Quoting Klaus (Reply 29):
Okay... are these electric motors capable of driving the shaft alone as well in an emergency (such as keeping the ship maneuverable in bad weather when the main engine should be inoperable)?

Since the steam for those turbines comes from the main engine exhaust, I'd say that a total main engine failure would lead to loss of that electricity, too.

It says that she also has diesel generators for shipboard power. Whether those generators can power the propeller, even at a reduced speed, is not clear from my reading.
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Klaus
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 7:40 pm

Quoting DocLightning (Reply 30):
So if single-screw propulsion is more efficient (and I believe her screw has six blades), why do cruise ships seem to have a minimum of two?

I'd guess that it has to do with the cost of delays or interruptions of the voyage, among other things.

A passenger cruise is much more time-critical with a much higher financial risk should something go wrong than a cargo shipment, so for a cargo vessel the risk might not justify a less efficient / more expensive engine configuration but for a cruise ship or passenger liner it might.

A giant single diesel engine and a single propeller also probably generate more noise and vibrations than multiple electrical engines driven by multiple diesel or gas turbine generators which can impact passenger comfort and achievable sales and passenger satisfaction (return business).
 
Klaus
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 7:42 pm

Quoting DocLightning (Reply 31):
Since the steam for those turbines comes from the main engine exhaust, I'd say that a total main engine failure would lead to loss of that electricity, too.

It says that she also has diesel generators for shipboard power. Whether those generators can power the propeller, even at a reduced speed, is not clear from my reading.

Being without any propulsion in heavy weather can be dangerous as far as I know, so I would expect there to be at least some kind of backup for emergencies...
 
N766UA
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 8:02 pm

Quoting kngkyle (Reply 4):
They are just so long and skinny

They may appear so, but many Lakers are actually as wide or wider than their salty counterparts, excluding the big wide oil tankers and container ships, of course.

Many of the "newer" lakers, especially the 1000 footers, are over 100 feet wide, and with their big bluff bows they're actually bulkier looking than the built-for-speed salties.

I guess I'll use this opportunity to say that it's a real shame that the new generation of Canadian lakers have to be built in China. How ridiculous that we can't even build our own lake freighters anymore.
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zckls04
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 8:02 pm

Quoting DocLightning (Thread starter):
Living on the SF Bay, I've become increasingly interested in large ships.

Ever since I've lived in Alameda I've loved the port of Oakland, especially at night:

http://viveier.com/photos/444564554_Nc5W8-XL.jpg

I was sailing down the stretch of water a few weeks ago between Alameda and Oakland and saw a huge cargo ship with hundreds of containers, and perched right on the top amongst a sea of red boxes was a 1950s Silver Streak camper. It looked very forlorn up there; I was quite surprised that they just plonk it on the top; I assumed it would be covered up in some way for its long journey.

One of the worst missed photo ops of my life!
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LMP737
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 8:05 pm

Quoting SmittyOne (Reply 3):
Of course in the Great Lakes you don't see the same wave heights that you see on the ocean (though it can be just as 'unpleasant' overall due to the short period).

The Great Lakes get some preety nasty storms, especially in the late fall and winter. Just look at the case of the Edmund Fitzgerald.
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 8:07 pm

Quoting LMP737 (Reply 36):
The Great Lakes get some preety nasty storms, especially in the late fall and winter. Just look at the case of the Edmund Fitzgerald.

Also, the lakes see very different kinds of waves than the ocean. In the open ocean, you get long, rolling waves going in a single direction. On the lakes, you get far more frequent waves and they come from 2 or 3 different directions, exerting a "yawing" effect on the boats. They'll never be as tall as open-ocean waves, but they can be just as nasty.
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 8:31 pm

Quoting N766UA (Reply 34):
I guess I'll use this opportunity to say that it's a real shame that the new generation of Canadian lakers have to be built in China. How ridiculous that we can't even build our own lake freighters anymore.

How do they get to the Lakes? They aren't really designed to tolerate ocean swells and they're too big to fit throught he locks in the Welland canals. Are they shipped in in sections?
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N766UA
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RE: The Subject Of Ship Design

Wed Sep 12, 2012 8:41 pm

Quoting DocLightning (Reply 38):
How do they get to the Lakes? They aren't really designed to tolerate ocean swells and they're too big to fit throught he locks in the Welland canals. Are they shipped in in sections?

I believe they are "seawaymax" size, meaning they will be able to enter the locks along the welland canal, and though not designed for oceans long-term, they are certainly built for wind and waves and should be able to tolerate such a journey with no problem.

Assuming their Chinese welds hold.
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RE: The Subject Of Ship Design

Thu Sep 13, 2012 12:04 am

Quoting zckls04 (Reply 35):
I was sailing down the stretch of water a few weeks ago between Alameda and Oakland and saw a huge cargo ship with hundreds of containers, and perched right on the top amongst a sea of red boxes was a 1950s Silver Streak camper. It looked very forlorn up there; I was quite surprised that they just plonk it on the top; I assumed it would be covered up in some way for its long journey.

You, sir...

...need to take me sailing.  
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Kiwirob
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RE: The Subject Of Ship Design

Thu Sep 13, 2012 8:46 am

Quoting N766UA (Reply 39):
Assuming their Chinese welds hold.

That's an unfair comment, the Chinese build better ships, faster and cheaper than yards anywhere else in the world. The US can't hold a candle to the Chinese when it comes to building commercial vessels.
 
sprout5199
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RE: The Subject Of Ship Design

Thu Sep 13, 2012 3:43 pm

Quoting KiwiRob (Reply 41):
The US can't hold a candle to the Chinese when it comes to building commercial vessels.

You need to add "for the same cost".

Quoting DocLightning (Reply 22):
That does make sense. Still doesn't explain to me why even large and fast ships like Emma Maersk have single screw propulsion.

Its like having four wheel drive. If its not needed, then dont have it. A single screw has less hull penatrations, less things to break and so on. Like having 2 jet engines vs four. If 90% of its time is in open water why have thrusters, azipods, and so on.

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RE: The Subject Of Ship Design

Thu Sep 13, 2012 4:26 pm

Quoting N766UA (Reply 37):
Also, the lakes see very different kinds of waves than the ocean. In the open ocean, you get long, rolling waves going in a single direction. On the lakes, you get far more frequent waves and they come from 2 or 3 different directions, exerting a "yawing" effect on the boats. They'll never be as tall as open-ocean waves, but they can be just as nasty.

On Lake Superior you'll get waves in rapid succession, referred to as "three sister waves". What's ends up happening is tons of water being dumped on the deck and not enough time to drain it away. End result for some ships, a trip to the bottom.
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Kiwirob
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RE: The Subject Of Ship Design

Thu Sep 13, 2012 5:03 pm

Quoting DocLightning (Reply 30):

So if single-screw propulsion is more efficient (and I believe her screw has six blades), why do cruise ships seem to have a minimum of two?

Cruise lines wouldn't want to take the risk, especially in tort happy America.

Quoting sprout5199 (Reply 42):
You need to add "for the same cost".

Not even for the same cost, simple fact is a Chinese shipyard is ligthyears ahead in technology than any yard in the US. Just look at the mess the Project America cruise ships turned into, the first hull was so badly build by Litton Ingalls Lloyd Werft in German had to completely rebuild her, the other ship was canned.
 
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RE: The Subject Of Ship Design

Thu Sep 13, 2012 7:11 pm

Quoting KiwiRob (Reply 44):
Cruise lines wouldn't want to take the risk, especially in tort happy America.

But they don't have redundant engine rooms. In the past several years, I've heard about several cruise ships losing all power due to engine room fires. I have yet to hear of one losing a prop until this one recent issue with the Queen Victoria (IIRC).

IMHO, losing all power is a lot worse than losing the screw. At least you have food and air conditioning if you lose the screw.

It stuns me that cruise ships don't have backup generators isolated from the engine room for just such an eventuality.
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Kiwirob
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RE: The Subject Of Ship Design

Fri Sep 14, 2012 6:25 am

Quoting DocLightning (Reply 45):
It stuns me that cruise ships don't have backup generators isolated from the engine room for just such an eventuality.

Some do.
 
kl671
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RE: The Subject Of Ship Design

Fri Sep 14, 2012 6:57 am

Quoting DocLightning (Reply 38):
They aren't really designed to tolerate ocean swells and they're too big to fit throught he locks in the Welland canals. Are they shipped in in sections?

Great lake freighters are certainly capable of operating in ocean conditions. A large number (133 according to Veterens Affairs Canada) were used in ocean convoy duty during WW2.

Quoting DocLightning (Reply 45):
It stuns me that cruise ships don't have backup generators isolated from the engine room for just such an eventuality.

All cruise ships, are built to the rules of one of the marine classifiication societies. (Lloyds, DNV & ABS for example). These rules require that all ships have an emergency generator located in a compartment outside of the main engine room, and connected to an independant emergency switchboard. However as an example the DNV rules require this generator provide power for essential services (Lighting, fire pumps, communications etc) for only 18 hours.

Quoting zkojq (Reply 17):
Just curious (because this seems like a good place to ask), is there is a maritime inequivalent of ETOPS, regulating how reliable a passenger-carrying ship's engines must be?

The marine classification societies do have some standards for reliability, but they are not as demanding as aircraft regulations. If something breaks in the engine room it can usually be fixed at sea, and usually without stopping the engines.

If you lose power in a ship, it is much preferable to have the failure in the middle of the ocean than close in to land, where the chances of hitting something are greatly increased. The exact opposite of the intentions of aircraft ETOPS requirements.
 
Stealthz
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RE: The Subject Of Ship Design

Fri Sep 14, 2012 9:14 am

Quoting DocLightning (Reply 30):
why do cruise ships seem to have a minimum of two?


Apart from reliability and maneuverability aspects that are likely important to cruise and passenger ship operators, there are significant physical and engineering factors that work against single engine/screw passenger ships.
Passenger ships tend to be of much shallower draft than large merchantmen.

This image from wikipedia illustrates the differences in draft of different types of ship



The propellor on the Emma Maersk is around 9.6 m diameter, It would be impossible to have such a propeller under the stern of a passenger ship there is just not enough room, indeed the follow on ships to the Emma Maersk, the so called Triple E class have 2 smaller propellors because it was just not possible to get a single propellor of the required size under the keel.

Quoting Klaus (Reply 29):
Okay... are these electric motors capable of driving the shaft alone as well in an emergency

I don't know the answer to that, the two electric motors total about 18Mw so they should be able to drive the ship at better than steerage way. There is a bit of work for those motors, the propellor weighs in at 131 tonne and the shaft is 605 tonne.
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Kiwirob
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RE: The Subject Of Ship Design

Fri Sep 14, 2012 10:14 am

Quoting stealthz (Reply 48):
the so called Triple E class have 2 smaller propellors because it was just not possible to get a single propellor of the required size under the keel.

They also have two smaller engines, this also enabled the height of the engine room to be reduced which in turn allowed for much great cargo capacity.

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