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Thursday, November 8, 2018

Why Don't Big Ships Capsize?

     Looking at the picture on the left, the ship looks extremely unsafe. Many ships seem to be out-of-proportionally tall, perched precariously on a hull which seems incapable of safely supporting a structure towering hundreds of feet into the air. So, what keeps these giant ships stable in a roll and reduces their potential to list to large angles in bad weather to the point that they just roll over? 
     At one time shipbuilders built a ship then put a hotel inside of it, but now they build a hotel and then wrap the ship around it. One expert claims that in the past there was a safe ratio between a vessel’s draft (below the waterline) and the portion that's above the waterline, but modern cruise ships are no longer built that way, making them dependent on stabilizers not only to combat rough weather but to stay upright with only slight to moderate winds and seas.
     Back in 2010, the Louis Majesty, a cruise ship hit by 26-foot waves off northeast Spain didn't capsize, but it resulted in two passengers getting killed and 14 people injured when water violently crashed through the windows of some of the ship's public areas. Experts said waves like those that struck the Louis Majesty are extremely rare and should be of little concern to the average cruise ship passenger. Still, the incident raised questions by potential passengers about the safety of cruising, especially, “How likely would it be for a cruise ship to encounter a wave that might tip it over?” 
     A professor and chairman of engineering at the Maritime College of the State University of New York, said it's not very likely and he would be as worried about it as an asteroid hitting the Earth. A captain at the California Maritime Academy said the only way that it could happen is if the ship were in extreme weather and positioned sideways to a 70- to 100-foot wave that would have the potential of rolling it over. He added that it would never happen because cruise ships avoid bad weather like the plague. They don't want the passengers in peril, they don't want to risk any injury or accidents. 
      How far can a cruise ship lean over to one side and still recover? It turns out, pretty far. In extreme cases, a ship can actually list 60 degrees and recover. When a ship lists 20 degrees, you almost can't walk on the deck. If a ship heels more than that, the problem is that you're going to get thrown off your feet and a lot of equipment and furniture is going to break loose and fly around. So, that possibility of injury is very high, but the ship should right itself without any problem. 

     Cruise ships seem to be getting bigger and taller and the big question is, does that affect their stability? It may appear that cruise ships are top heavy, but naval architects design them in such a way that all of the heavy liquids, machinery and the main engine are positioned very low, so the ship's center of gravity is also low even though it looks like the structure goes up high. 
     Modern cruise ships also have very intricate anti-heeling systems. They literally blast water from one side of the ship to the other side so that you can take a ship that might be rolling 20 degrees and when the anti-heeling system is turned on the roll will only be 5 degrees. Thus, because of the anti-heeling system on state of the art cruise ships, it's very difficult to feel any kind of movement on board. 
     If there is a tsunami warning, a cruise ship is only in danger if it is docked in port. A tsunami would go by entirely unnoticed on the open sea. But if the ship were still at port, an arriving tsunami would be devastating. 
     Whether a ship capsizes or not has a lot to do with its center of gravity. We think of gravity as a force that pulls things downward, but it doesn’t always work like that. Sometimes gravity can make things turn and topple over, especially if they are high up and unbalanced. Throw a ball in the air and gravity pulls it straight back down. Not everything moves like this when gravity acts on it. Ships are not nice, neat ball-shaped, so gravity acts on them in more complex ways. Even so, all objects behave as though their mass is concentrated at the center of gravity. 
     A small boat can swing from side to side pretty freely and because it’s light and it takes only a small force to make it move in the water. This makes a small boat relatively easy to capsize. By sitting down in a small boat, you lower the center of gravity and make it more stable; the lower the center of gravity the more stable boats are. 
     Large ships have heavy engines underneath the water level which gives them a low center of gravity and makes them more stable. They also have a big keel, the vertical board running underwater from back to front along the central spine. The keel helps stop boats rolling or capsizing because it means more force is required to push the boat sideways through the water or rotate it. 
     For a more technical explanation, see HERE.


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