Monday, January 21, 2008

A Technical Interlude (The Cruising Life V4)

I have a question about cruise ship stability.

Many people--my wife and father-in-law and sister-in-law included--are prone to motion sickness, and cruising can be difficult or impossible for them. Susan acclimates after a day or two, but we long ago concluded that her father simply could not withstand the experience. The fact is, no matter how big the ship there is always some motion involved, and this can be a problem. Before this cruise I wondered if this present ship of almost a thousand feet in length would be so large as to preclude much motion while underway, but in fact this ship seems to roll even more than the somewhat smaller ones we were on previously. There is very little up-and-down movement or fore-and-aft rocking, but the ship rolls side-to-side noticeably, even on calm seas. I happen to love this motion--it lulls me to sleep, and it's a gentle reminder that though the ship feels for all the world like a huge building on land we are in fact on a moving vehicle. It's one of those little places where the routine touches the extraordinary. But there is something vaguely unsettling in there being so much easily perceptible movement in something so large.

I got to thinking about all this over the last couple days. Of the ship's 14 elevators, four of the midship cars are all glass and rise and fall in housings in the side of the ship (visible in photos as smoked glass towers) with a view of the outside--like something in a Disney hotel. Our cabin is down on Deck 1, which is probably 15 or 20 feet above the waterline. From the elevator right around the corner from our room, it's eight stories up to the Lido deck for mealtimes, with another two levels yet above that. Riding up 10 stories in the glass elevator of a moving cruise ship is surreal: it defies some branch of common sense that you can ascend so far and not have the ship require so much balancing mass below the water that you're constantly running aground. Turns out, the ship draws 26', but the top of the ship must be at least a hundred feet above the waterline. Given that the center of gravity MUST be below the waterline--or the ship would simply roll over to make it so--there must either be an unbelievable amount of mass down in that 26' below the water, or other means must be employed. (Even then, however heavy things are below the waterline, there's not much arm to work with--something less than 26' to counter all that weight above.) I know that much of what is above the waterline is air; the decks are mostly open spaces filled with relatively light things like furniture. But the structure that supports all this is really heavy gauge steel.


(This photo is the thinner hull plating, about 1/2". Down below thicknesses reach 3/4", and the structural elements are at least that thick. Even a quite small piece of this would be too heavy for a person to lift.)

Life boat davits, for example, are massive welded structures that rise from deep within the ship and hold huge, twin-engine boats of 35 or 40 feet--12 or 14 of them!--up some 50 or 60 off the water. There are two swimming pools up on Deck 9, plus a bunch of hot tubs and spas on the same level. When you look at all this metal and the stone decorative details and countertops and numerous grand pianos and industrial soda and coffee machines and on and on, you get a sense of where that 82,000 tons comes from. But again, the stuff stuffed into the 26' below the waterline must solidly exceed the weight of all that's above.

And my sense, from how the ship rolls, is that it might not exceed it by much. The five massive diesel engines and all the huge fluid reservoirs--fuel and fresh water to the tune of nearly a million gallons EACH--are surely kept well below the waterline, but I suspect there are tricks as well; water ballasting and directed thrust are employed to keep things mostly steady. Water ballasting, I think, has been in use for decades, where sea water is pumped into chambers to put mass where otherwise there would be air. But this directed thrust thing seems new to me--and indeed, I'm just speculating out of whole cloth here.



Modern ships like this do not have the usual propellers hanging off the back, but are rather propelled by what are called Azipods. The azipod takes the place of both the standard propeller and rudder, with the whole pod being able to rotate. The pod contains an immense electric motor which is fed by generators driven by the diesel engines. One positive of this solution is that the pods can be steered rapidly, presumably to help counter the ship's tendency to roll. Looking out the stern of the ship on my daily walk, our thrust wake often snakes back and forth in pronounced fashion, not in long lazy arcs, but almost whipping back and forth (look at the wake behind the opening photograph of the Oosterdam in the first cruise post below). The overall track of the ship is straight, but the thrust seems clearly to be steered quickly from side to side, back and forth continuously as we go; I can only surmise that an attempt to preempt a rolling motion is what's going on.

But even with these things, the ship rolls quite noticeably, even on absolutely calm seas. Even at rest (or maybe especially at rest). The ship will tilt so far--two or three degrees to one side, I suppose; enough that we clearly notice it (if we're paying attention), but not quite enough to make things roll off furniture--and then it seems to catch itself, and it rolls back level and then slowly two or three degrees the other way. Back and forth. I was particularly interested to watch them pull up the tenders today after they ferried us to Cabo San Lucas and back. These tenders are some of the ship's lifeboats, and they're substantial pieces of machinery in themselves: they have space for 120 for tendering (150 for lifeboat duties), are twin-engine-powered and have substantial fuel and water on them. These are already much bigger boats than most of us will ever operate in life. And yet their being cranked up along the side of the ship has no impact on the ship whatsoever. At one point they were hoisting two of them at once, and I stood there watching and couldn't sense the slightest impact on the larger vessel. That's not so surprising, I guess, given the Oosterdam's 82,000 ton displacement; but it makes you realize that the forces at work making the ship roll and right itself are many times larger than the 10 or 15 ton weight of each of these boats.



And that makes me wonder how this ship would do in high seas. I'm sure they have tried it out and have good computer models, but I can't help thinking that if it does THIS in calm seas how would it deal with a rough sea? I spent a little time as we pulled out of Mazatlan on a deck above the bridge near the bow with a good view aft at the whole of the ship. With a good view of the horizon, I can easily see movement around all three axes. And the ship seems almost oblivious to sea conditions and even a pretty stiff breeze. The rolling I noticed didn't seem to correspond to anything I could identify on the surface of the sea or to gusts of wind. Maybe it's just an oscillation.

Ah well, it was built in Italy, and the ships regularly make round-the-world trips, so my questions apparently want for the light of a little hard information to settle.

3 comments:

Dzesika said...

I'm stupendously jealous of your cruise. And your hat. Despite the motion sickness (which I get, too, to a ridiculously bad degree).

Jeffy said...

Your speculation on the stability of your cruise ship got me thinking about it too. It does seem remarkable that its structure above the waterline can be can so much higher than the depth under the waterline. You'd think that there would have to be a huge amount of ballast down there to keep it upright.

Unlike you at the moment, I am not constrained by an internet connection that I have to buy by the minute. I can do some browsing and see what I can find.

One item of interest is a detailed description of ship stability on Wikipedia -

http://en.wikipedia.org/wiki/Metacentric_height

This article points out that while the center of gravity is generally below the waterline, it is still above the center of buoyancy, and this is what produces a positive stability. I was also able to learn that the amount of roll a ship exhibits is related to its stability, with a more stable ship tending to roll more (or more rapidly at least). Thus, for comfort, passenger ships are generally made so as to be only as stable as necessary. This makes them not snap back right when they are pushed to a list, but they tip back slowly. This slow roll is generally considered to be more comfortable than the more rapid roll caused by a very stable design.

I also found a great report on the construction progress of one of its sister ships, the Noordam, with several pictures of the inner workings. You can find the PDF here:

http://www.worldshipsocietyrotterdam.nl/news/Shipping_news/Special/Noordam.pdf

This report shows the active stabilization system employed on these ships to reduce the amount of roll. It doesn't appear that the azipods are used to stabilize the ship, I suspect that they would have to really swing in wide arcs to do very much. These ships have retractable fins that are gyroscopically controlled to counteract the ship's roll.

In a way, it is a bit surprising that such measures are even necessary on a ship this size - you'd think that something this big would be essentially unaffected by the seemingly minor movements of the water. Just goes to show that there is no stopping gravity.

wunelle said...

I'll have to look the articles over. I imagined you'd be especially intrigued by the physics aspects!

A Wikipedia article on ship stability... there seems no limit to the wonder of Wikipedia.