Can a ship be truly top heavy and still be sea worthy. I just recently started looking a little closer at ships thanks to my grandsons interest in model ships. Some of the very large vessels appear to be massively top heavy. Is this possible, are they pusing the limits on safety?
A ship’s center of mass must be below its center of buoyancy, or it won’t be stable. But it’s really hard to tell where either of those points is at a glance.
Would the water line be considered the center of bouancy? Center of mass I can easily grasp, having a hard time with center of bouancy.
The center of bouancy is the center of gravity of the water the hull displaces, and so will be below the water line.
Actually, I take that back: A ship can have its center of mass higher than its center of buoyancy and still be stable. For instance, a uniform sphere with density less than that of water will always have its center of mass higher than its center of buoyancy, and yet will be neutrally stable. Put even a very small mass concentration at one point on the surface, and it’ll be absolutely stable while still having COM above COB.
Which means that I’m not entirely sure what the stability criterion is, or if there even is one that can be expressed succinctly.
For what it’s worth cruise ships look top heavy because, well, they kinda are. They fill and empty water tanks at the bottom of their hulls for ballast to manage buoyancy. Obviously they can’t be too top heavy or it becomes unstable and unsafe, but they also can’t be too bottom heavy either or a large percentage of the passenger cabins will rock significantly (more and more the higher up they are) and make passengers very uncomfortable.
This has become an issue of contention between cruise lines and maritime regulatory agencies, safety vs. comfort…
Wouldn’t a ship rotate until its center of mass is as low as possible? Which means that the center of mass must be below the water line.
I’ve seen pictures of some really tall cruise ships and it looks like they wouldn’t be too stable. I assume that in good weather they’ll stay upright, but they must catch an awful lot of wind, add waves and you could have trouble. Although I guess cruise ships have ballast tanks that are / can be filled with water to make them more stable.
Top heavy is fine. The important criterion is that the center of buoyancy always “shifts” more than the center of mass does, during a roll condition. This causes a restoring moment that rights the ship.
Diagram here:
Note that the center of buoyancy can move laterally as the ship rolls. Stability happens when a roll causes the center of buoyancy and/or the center of mass to move in such a way as to create a restorative torque. The easy way to do this is to put the center of mass near the bottom of the hull, e.g. a sailboat keel full of lead: ship rolls starboard, CoM pivots over to port, tends to right the vessel. But this adds weight, which makes the ship sit lower in the water (more drag, less speed) and reduces cargo capacity. The more elegant solution is to design the shape of the hull so that the center of buoyancy moves toward the direction of the roll. You can have a center of mass well above the water line if you want, just as long as a roll causes the center of buoyancy to move farther out to the side than the center of mass does.
The easy example of the latter case is a catamaran. Its CoM is well above the water line, but when it heels over and pushes one hull down into the water, the CoB moves way over toward that hull and tends to keep the ship right side up. Now you don’t need a ton of ballast in the hull to keep things stable: you can carry more cargo, or more speed, or both.
A uniform sphere will not be stable. The CoM will always be directly above the CoB, so roll displacements can never create a righting moment; if you define the current top point on the sphere as “up” and then tip the sphere over, there will be no righting moment to restore your “up” point to the top again.
This demonstrates the elegant solution I described earlier. Rather than having a ton of ballast to force the CoM to be below the CoB, stability is achieved by assuring that the relative lateral movements of the CoM and CoB during a roll create a righting moment.
Ninja’d by YamatoTwinkie…
Right… like when you put something heavy on top of a flat piece of styrofoam. The center of mass will be above the water line but the structure won’t flip because that requires pushing a large part of the styrofoam under water, but the buoyancy pushing upward is larger than the weight pushing downward.
I said “neutrally stable”. That is to say, if it gets turned, then it won’t experience a restoring force, but neither will it experience a force that increases the amount of turn. It’s the same as a sphere sitting on a flat solid surface.
Keep these things in mind:
If you could see the part of the hull that’s underwater, a ship wouldn’t look so top-heavy.
The stacks of shipping containers that make a ship so tall are relatively light for their size, and the dockside crane operators put the heavier ones on the bottom.
Cargo ships pump seawater into the bilges for ballast. They’re supposed to take on and flush out ballast water outside the harbor. At sea, they ride lower than in the harbor.
I am building a container ship model, I plan to try and ballance it out just as if it were full size. I guess I would have to test it in salt water for an accurate evaluation.
The knowledge on here never ceases to amaze me, maybe it is the small world I live in.
A related interesting story.
In the early days of crabbing in Alaskan waters the crabbers figured out what they were doing and the market exploded. So, a now well healed capitiain/company had several custom built crabbing boats built. Probably some of the first and most probably the new pride of the fleet.
On the sides of the boats they painted wide blue? line. That line was to tell the operators when they had loaded enough/too many crap traps on the deck.
The boats went out, some bad weather came up, and in short order all the boats disappeared off the radar.
Turns out there was some confusion about the line. It was a couple feet wide. You were supposed to stop when you reached the bottom of the line, not the top. That or the line was painted with the bottom being where the top would have been.
Marine engineering is a fascinating science/art.
Also, keep in mind that just because a ship “looks” topheavy, doesn’t mean it is. The Oasis of the Seashere looks very top heavy. But in the top are passenger cabins, open spaces, restaurants and whatnot that are mostly full of air. Below the waterline you have ballast tanks, fuel and other stores that take up less volume, but are extremely dense.
Compare that to this supertanker full of fuel.
For a second I was wondering why tankers sit lower in the water when they are full of fuel (which is lighter than water). Then I realized that it’s because the fuel is still heavier than the air that an empty tanker is full of.
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I’ve seen pictures of some really tall cruise ships and it looks like they wouldn’t be too stable. I assume that in good weather they’ll stay upright, but they must catch an awful lot of wind, add waves and you could have trouble. Although I guess cruise ships have ballast tanks that are / can be filled with water to make them more stable.
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I was on a cruise a few years ago and took a “behind the scenes” tour. I asked about how the thing stays upright with a 26 foot draft and 11+ passenger decks. The answer is that the above-water section is essentially hollow and filled with air. Everything below is by comparison, massively dense - engines, generators, fuel bunkers, bilge, etc.
While on a cruise I did watch part of the “making of the ship” video. As mentioned, the heavy parts (engines, generators, ice rink, etc.) are down low, and the relatively light cabins are up high. The top few floors are made of aluminum (instead of steel) to further lighten the top.
According to the video the ship could tilt 49.5 degrees and still not tip over
Brian
That would make for some interesting Yelp reviews. :eek:
And now I’m wondering if there’s any way to contrive a situation where a floating object has its center of mass below its center of buoyancy, while still being unstable.