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  #31 (permalink)  
Old August 13th, 2006, 09:05 PM
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Dave,

Important to note: There is a difference between displacement when talking about air craft carriers and tankers versus the interior volume of cruise ships. For example, the Voyager class have an interior volume of 138,000 tons. Who knows what their actual displacement is??? I think is is far less than 138,000. Roland? Norm? I guess I can try to find it...

I read "Supertankers" awhile back and it was written in the 70's about how dangerous those ships were. They had drafts close to 90 feet, some of them.


Unfortunately, that seems to be cruise writers proving their ignorance again -- this time, in the form of half truths. Let's go back to an ancient Greek named Archimedes, who demonstrated that the weight of a floating object is equal to the weight of liquid that it displaces. Thus, the weight of a ship is equal to the weight of sea water that would occupy volume of the portion of the hull that's below the water line (except in the Panama Canal and most of the great lakes, where it's the weight of fresh water that would occupy the volume of the hull). Theoretically, one can compute this by multiplying the density of sea water (64 pounds per cubic foot) by the volume of the underwater portion of the hull.

Now, the catch is that the displacement of a ship is not static. Rather, it changes constantly because it depends upon the ship's load, so the ship's engineering officers have to monitor the ship's load continuously (and it's actually more complicated than that because they also have to monitor the distribution of the weight in order to maintain trim). Historically, the draft markings on the side of the hull, forward and aft, were precisely for this purpose but modern ships also have instruments inside the hull that provide similar information.

Anyway, the U. S. Navy actually publishes three displacements for each ship -- "Navy light" (minimum in port, no load-out), "standard" (normal leaving port for routine operations, with partial food and minimal ordinance) and "full load" or "combat load" (loaded to the gills for a sustained deployment). Merchant ships are registered based upon their maximum load. Thus, the published displacement is stated in "gross registered tons" (GRT). If you want to relate this figure to volume, it's the volume of the underwater portion of the hull at maximum load -- that is, with a full complement of passengers and their luggage aboard, a full load-out of stores, and full water and fuel tanks.

Many cruise ships do not operate under "full load" conditions -- which probably would include enough provisions for 90 to 180 days at sea and a goodly amount of fuel -- because they don't operate that far from resupply. By operating at less than full load, they cut drag and draft, and thus fuel consumption, considerably. You may have noticed that some ships have two-tone hulls with a horizontal break a few feet above the actual waterline. The break in color is actually at the design waterline, which probably is the maximum safe displacement.

Here is the answer to my own question. The QM2, for example, has gross tonnage of 151,000 but displacement of only 76,000 tons. It is alo longer and with a deepr draft than the Freedom so one could assume the Freedom has less displacement unless the hull bottom is more flat than QM2.

Do you know what happens when you ass-u-me???

These ships have completely different hulls that you simply cannot compare in this way. The Freedom of the Seas class has a standard cruise ship hull. If you look at it from the side, it's perfectly flat for most of the length of the ship. A few feet at the front curve into a bow and there's a more or less square stern at the back. If you were to look at the underwater portion, it would go straight down and then curve into the keel, which tapers up at the stern, forming a wide "U" for most of the length of the ship. By contrast, MV (RMS?) Queen Mary 2 has an ocean liner hull. It's curved for most of its length, with a very long bow and some taper inward toward the stern. Going down into the water, it forms a fairly sharp "v" at the that broadens somewhat as you move toward amidships, then moreso when the keel turns upward to accommodate the propellers.

There's a parameter called the "coefficient of form" that's defined to be the ratio of the underwater volume of the hull to the product of the length at the waterline, the maximum beam at the waterline, and the maximum draft. The standard cruise ship hull has a coefficient of form that's very close to one whereas the ocean liner hull has a coefficient of form that's much smaller. Unfortunately, you can't calculate the coefficient of form from the numbers provided because they are extreme length and extreme beam rather than the respective values at the design waterline.

Norm.
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  #32 (permalink)  
Old August 14th, 2006, 03:57 AM
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Norm,

My goodness.... You're even worse than I am when it comes to technical specifications........

But you are correct -- a given ship's "weight" if you will, is the amount of sea water it DISPLACES as it bobs about at anchor.

And you're right -- various hull designs contribute to various displacements.

It would be nice if ship displacements were standardized. Heaven forbid there be a common standard.

But I guess, Gross Registered Tonnes (GRT) is a recognized standard. Perhaps if the cruise lines were to stick with that, life would be easier..........

Dean
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  #33 (permalink)  
Old August 14th, 2006, 08:54 AM
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All cruise ships are meassured to GT "standard".
The term GRT is no longer used.






Quote:
Originally Posted by Mean Dean
Norm,
But I guess, Gross Registered Tonnes (GRT) is a recognized standard. Perhaps if the cruise lines were to stick with that, life would be easier..........
Dean
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  #34 (permalink)  
Old August 15th, 2006, 08:37 PM
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ISSUE 2 of Crown & Anchor arrived yesterday. Page 19 has a photo of Freedom at 160,00 gross tons--what a monster. It mentions two more of same size...Also lists Genesis with 5,400 pax That's bigger than some towns----really over the top-- Among many negative notions, any type of erergency,, illness etc. will not be very easily handled--Hope to get in as many cruises as we can before these monsters kill the fun of it!
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Old August 16th, 2006, 12:58 PM
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Dean,

My goodness.... You're even worse than I am when it comes to technical specifications........

What can I say? Imprecision creates confusion which causes misunderstanding that leads to conflict....

But I guess, Gross Registered Tonnes (GRT) is a recognized standard. Perhaps if the cruise lines were to stick with that, life would be easier..........

For all merchant ships, the published size ("displacement") is the gross registered value. It's commonplace to drop the words "gross" and "registered" because they are understood. There's a slight disparity (less than two percent) between English measure (always long tonnes) and metric measure (metric tonnes), but it's negligible for practical purposes.

Norm.
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Old August 16th, 2006, 01:02 PM
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Roland,

All cruise ships are meassured to GT "standard".
The term GRT is no longer used.


The standard of "gross registered tonnes" is so ubiquitous that "gross" and "registered" are often unstated. It's the same number. I still see listings with the abbreviation "GRT," though.

Norm.
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  #37 (permalink)  
Old August 16th, 2006, 01:09 PM
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hcat,

ISSUE 2 of Crown & Anchor... Also lists Genesis with 5,400 pax

Did Issue 2 the Crown & Anchor also mention the other number -- 220,000 tons?

(Just curious.)

That's bigger than some towns...

Ay-yup!

Among many negative notions, any type of erergency,, illness etc. will not be very easily handled

Thise issues should be no worse than on the present behomoths. The "Project Genesis" ship will have proportionately more medical personnel and facilities, etc., to deal with those problems.

Hope to get in as many cruises as we can before these monsters kill the fun of it!

You might find that these monsters are even more fun because they offer even more variety of activities. I'm not so sure about the boxing rings aboard the Freedom of the Seas class, though....

Norm.
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  #38 (permalink)  
Old August 17th, 2006, 11:29 AM
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Norm,

"International Convention on Tonnage Measurement of Ships, 1969

Adoption: 23 June 1969
Entry into force: 18 July 1982

Introduction
Gross tons and net tons

Introduction
The Convention, adopted by IMO in 1969, was the first successful attempt to introduce a universal tonnage measurement system.

Previously, various systems were used to calculate the tonnage of merchant ships. Although all went back to the method devised by George Moorsom of the British Board of Trade in 1854, there were considerable differences between them and it was recognized that there was a great need for one single international system.

The Convention provides for gross and net tonnages, both of which are calculated independently.

The rules apply to all ships built on or after 18 July 1982 - the date of entry into force - while ships built before that date were allowed to retain their existing tonnage for 12 years after entry into force, or until 18 July 1994.

This phase-in period was intended to ensure that ships were given reasonable economic safeguards, since port and other dues are charged according to ship tonnage. At the same time, and as far as possible, the Convention was drafted to ensure that gross and net tonnages calculated under the new system did not differ too greatly from those calculated under previous methods.

Gross tons and net tons
The Convention meant a transition from the traditionally used terms gross register tons (grt) and net register tons (nrt) to gross tons (GT) and net tons (NT).


Gross tonnage forms the basis for manning regulations, safety rules and registration fees. Both gross and net tonnages are used to calculate port dues.

The gross tonnage is a function of the moulded volume of all enclosed spaces of the ship. The net tonnage is produced by a formula which is a function of the moulded volume of all cargo spaces of the ship. The net tonnage shall not be taken as less than 30 per cent of the gross tonnage."

Roland










Quote:
Originally Posted by Rev22:17
Roland,

All cruise ships are meassured to GT "standard".
The term GRT is no longer used.


The standard of "gross registered tonnes" is so ubiquitous that "gross" and "registered" are often unstated. It's the same number. I still see listings with the abbreviation "GRT," though.

Norm.
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  #39 (permalink)  
Old August 17th, 2006, 01:21 PM
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Roland,

The gross tonnage is a function of the moulded volume of all enclosed spaces of the ship. The net tonnage is produced by a formula which is a function of the moulded volume of all cargo spaces of the ship.

The term "moulded volume" (love those British spellings...) clearly refers to the hull rather than supersturcture, and probably is further defined as the portion of the hull that's below the waterline at maximum safe load since that's where the molded form is necessary to minimize drag.

Norm.
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Old August 17th, 2006, 02:02 PM
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Here is a picture. Familiar profile....just BIGGER!

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Old August 17th, 2006, 04:45 PM
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Quote:
Originally Posted by hcat
ISSUE 2 of Crown & Anchor arrived yesterday. Page 19 has a photo of Freedom at 160,00 gross tons--what a monster.
hcat,
I was on the first week-long passenger sailing of Freedom. I can assure you, she is no monster. Truly expecting she would be too big for my taste, I went for the novelty of the inaugural cruise. What a fantastic surprise! So much so, that I am going back in December to make sure I wasn't making an alcohol-induced premature judgement.

Week after week, I witness people coming back with raves about the ship. People still keep asking "but what about the long tender lines, and crowds in the Windjammer?", and it seems like they need to experience Freedom for themselves before they will believe that we are all telling the truth that the "horrors" just simply do not exist. There is some code that Freedom has "cracked"...she is BIG, but not crowded and monstrous.

I plan to be on the first two sailings of Liberty as well (my first B2B), with a bunch of friends, and imagine I'll be giving Genesis a whirl too. Can't knock it until you've tried it. (even though I used to be in your camp )
Tracy
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Old August 17th, 2006, 06:50 PM
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Dave,

Here is a picture. Familiar profile....just BIGGER!

Interesting picture. I must say that I'm not fond of the lifeboats obsturcting the view from the promenade, if there is one....

Thanks for posting the picture!

Norm.
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Old August 18th, 2006, 01:23 AM
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Norm,

Got a couple of questions for you.

As you note, there is a difference between cruise ship hulls and ocean liner hulls. The cruise ship hull being more or less rectangular, and the ocean liner hull being curved from stem to stern and "V" shaped profile when viewed bow-on.

So first question: If ocean liners ply only the Atlantic (a fairly rough patch of water), why would you call for a less stable ocean liner hull? Fuel economy, perhaps -- but at the cost of seasick passengers? A cruise ship hull would seem more suitable.

Second question: Battleship hull design. Every battleship that I'm aware of had an ocean liner type hull. And yet, a battleship is basically a gun platform. And as such, you want the most stable hull design possible. If the ship rocks too much from the recoil of the main battery, precious time is lost until the main battery bears upon the target.

Interestingly, the German battleship Bismarck (and its sister Tirpitz) had ocean liner type hulls, but had a beam of 36 meters! The Bismarck was regarded as one of the most stable battleship gun platforms ever built. But wouldn't it have been even more stable with a cruise ship type hull? Of course it's range would have been reduced, but I suppose everything is a compromise.

Just thought I'd pick your brain a little!

Dean
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Old August 18th, 2006, 07:20 PM
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Dean,

This post may be long, but you asked for it....

So first question: If ocean liners ply only the Atlantic (a fairly rough patch of water), why would you call for a less stable ocean liner hull? Fuel economy, perhaps -- but at the cost of seasick passengers? A cruise ship hull would seem more suitable.

Okay, time for an experiment. Rinse out an empty tuna (or similar) can and float it in seveal inches of water in your bathtub. You'll see that it's nice and stable because it's much wider than its draft.

Now, plunge your forearm up and down in the other end of the tub to make some fairly big waves and watch what the tin can does. You'll see it bob up and down and tilt side to side (that is, roll) as waves pass by its location.

The problem with the tin can is not one of instability, but rather one of too much stability, which causes the tuna can to conform to every passing wave. Basically, the tuna can rolls pretty seerely because it reacts very quickly to changes in slope in the surface of the water.

If you want to carry this expriment one step further, you can put a few small weights in the bottom of a vegetable can, so about its radius is below the waterline, for comparison. The weighted soup can won't respond as quickly to the wave actoin, and thus will roll less than the tuna can.

Ships face a similar problem, not so much with the short waves that are most noticeable but rather with the longer (typically hundreds of feet crest to crest) swells that your eye can pick out by looking at the crests of the smaller waves. In order to make a liner ride smootnly without stabilizers, one must design the hull to be considerably less stable than the tuna can. The "deep V" hull meets this requirement. The diminished stability causes the vessel to respond very slowly to changes in slope, so the rise of the swell passes and the fall of the swell begins to counter it well before the vessel fully responds to the rise.

Note, BTW, that I did NOT say that an ocean liner should be unstable! An unstable vessel would not remain on even keel even in calm waters.

You can also see this by comparing a houseboat (shallow, very stable, hull) to a cabin cruiser (less stable deep "V" hull). You'll feel a lot less motion on the cabin cruiser if there are waves of any size -- which is why most people don't take houseboats out to open waters.

Second question: Battleship hull design. Every battleship that I'm aware of had an ocean liner type hull. And yet, a battleship is basically a gun platform. And as such, you want the most stable hull design possible. If the ship rocks too much from the recoil of the main battery, precious time is lost until the main battery bears upon the target.

There are three parts to the answer to this.

>> 1. Battleships do indeed have deep hulls, similar to ocean liners, to diminish their susceptibility to heavy seas.

>> 2. Battleships have a LOT of armor plate, and thus a LOT of mass and inertia, for their size, which redues the amount of motion imparted by a given force or torque.

>> 3. The guns on all World War II vintage and newer warships are dynamically stabilized. Their control systems actually sense the motion of the hull and compensate for it, so the guns stay locked on their targets. It's really impressive to see this in action on a ship that's rolling twenty or thirty degrees!

Interestingly, the German battleship Bismarck (and its sister Tirpitz) had ocean liner type hulls, but had a beam of 36 meters! The Bismarck was regarded as one of the most stable battleship gun platforms ever built. But wouldn't it have been even more stable with a cruise ship type hull? Of course it's range would have been reduced, but I suppose everything is a compromise.

Stable in what sense? A platform that's too stable, by the definition of system dynamics, will roll a lot more severely than one that isn't. Unfortunately, laymen tend to use the term "stable" to mean "not susceptible to roll"....

Just thought I'd pick your brain a little!

I hope that this answers your questions!

Norm.
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Old August 19th, 2006, 08:31 PM
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i didn't even explore the entire Explorer OTS How do you think Im going to Find everything on THAT BOHEMETH!!!!!
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Old August 19th, 2006, 08:57 PM
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Donna I believe you but the thought of 4,000 others & me hitting an island paradise just does not work well. Even Summit with half that number in Mauai was too much for them to handle at the Lahaina tender spot...took almost two hrs to re-board everyone... I cruise so I can watch the sea, enjoy the sunsets etc--do not need a skating show, rock climbing. cirque d-soleile or rap music---but each to their own--just hope my ships are not obscured by the biggies... As for the technical debate--I am lost but know big, bigger & biggest!
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Old August 20th, 2006, 09:31 AM
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Quote:
Originally Posted by hcat
I cruise so I can watch the sea, enjoy the sunsets etc--do not need a skating show, rock climbing. cirque d-soleile or rap music---but each to their own--just hope my ships are not obscured by the biggies...
I'm the same. Don't spend much time in the promenade, ice-rink, etc. The great thing, however, is that many people do. This is why the ship doesn't feel packed to the gils. Thought I would have hives from the amount of children the H2O Zone would attract, but you know what? It attracted them sooo well, that the main pools were very pleasurable and almost kid-free zones!

I understand your thoughts on crowded ports, and think that is why we won't soon be seeing many creative itineraries on these ships. They'll only go to ports that have the infrastructure to accomodate them. In fact, I am really looking forward to Liberty's eastern next year. Only three ports, and one of them is Labadee (St. Thomas and San Juan are the other two...perfectly capable of dealing with the big ships). LOVE the sea days, and since there is so much to do on Freedom class, it really never feels crowded. Never in a million years did I expect to come back and be a huge Freedom Fan, but here I am!

Tracy
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Old August 22nd, 2006, 01:44 PM
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gambitscuba,

i didn't even explore the entire Explorer OTS How do you think Im going to Find everything on THAT BOHEMETH!!!!!

You'll just have to take longer cruises with more days at sea....

Norm.
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Old August 23rd, 2006, 09:38 AM
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Default Promenade deck

I have only been on one RCCL ship (The Serenade of the Seas) and enjoyed the deck 5 promenade, particularly in the early morning as the ship was docking. It may just be the drawing, but I see no exterior promenade on the Geneses drawing. The lifeboats seem to be hanging over a straight drop. In the event they were needed, how are they accessed without a promenade deck?
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