The other day I was watching this show on the recent cruise ship sinking over in Italy. It was very interesting. They had ship experts looking over the data from the cruise ship, specifically GPS position and speed. One of the important factors that they said saved a lot of lives that night was the wind. After the ship had hit the underwater rocks and started taking on water, it continued out to deeper water. When the engines became submerged and stopped working, the ships momentum kept it moving out further from the island for a while. But, slowly the ship lost speed, and because there was an onshore wind of 25 miles per hour that night, it was first twisted/yawed to be perpendicular to the wind, then pushed back to shore.
The ship experts credit this as saving hundreds of lives that night, as people weren’t ordered to abandon ship for a long period of time. So long a time in fact that half the life boats couldn’t be used because the ship was leaning at too great an angle by the time people were told to abandon the ship. Many people ended up swimming for shore, which would not have been possible had it not been for the wind.
This made me wonder how much force it would take to move such a giant ship? I mean, this ship weighed 114,500 gross tons (256,480,000 lbs), and the wind was able to blow it a large distance in about an hour or 2? I needed to see what amount of force the air could exert on a ship this big.
I went on to grabcad.com, and found a cruise ship model. It was more for a desktop type model, but I easily scaled up the CAD to the dimensions of the real ship. Then, in FloEFD I simply defined the 25 mile/hour wind, set some goals to track the force on the ship, and the roll torque. Then I started to solve. It really was a very easy model to setup.
Here is what I saw. In this first image you get an idea of how large the wake is for a ship this big. I also displayed the mesh so you could see how it automatically adapted to the ship, and also the wake, which is very important to getting accurate results.
Zooming in, we get more detail on the mesh local to the ship. You will also notice an area of light blue higher speed air coming out of the middle portion of the ship.
What is happening here, is since I didn’t make this CAD myself, I missed the fact that there were no windows in this ship CAD, just holes through the ship. You can see in this image here the air moving through the ship where cabins should be. Obviously this isn’t correct, so a corrected model was run with the holes sealed.
I also plotted velocity vectors with the velocity contours to see how the air was moving as it came toward the ship.
- Cruise Ship Wind Wake with Velocity Vectors
- From the top view, we can see the width of the wake downstream. Also, the mesh was plotted so we could see how the mesh adapts to adding cells in the areas of velocity changes to accurately capture the wake structure. The adaptive mesh is one of my favorite FloEFD features, as it saves so much time getting an accurate mesh.
We can get a better feel for the airflow using streamline animations, like the 3 below showing different views of the same streamlines.
Another good way of getting a feel for the extent of the low speed air is an isosurface, which is a 3D plot that connects points with the same value to make a surface. Here I’m looking at anywhere that the airspeed is 15 miles/hr, a good amount slower than the 25 miles/hr wind. You can see the extent of the air being slowed down, which gives an indication to the forces affecting the ship.
Similarly a surface pressure plot will show where the high pressure regions, which when added up over the entire surface provide the force on that object.
But of course what we really want is the actual values for the force and the torques. FloEFD can output this into excel spreadsheet tables. Below are the results from the second analysis with the cabin holes “sealed”.
My main surprise is how low the actual force was compared to the weight of the ship. The ship weighs 256,480,000 lbs! 264,175 lbs of force is about 0.1% of the ship weight, yet it was enough to move the ship back to shore. Granted, we didn’t include any ocean current/wave effects in this analysis, which may have played a part, but as there was no data for that, I couldn’t include this into the simulation.
The more I think about it though, the more I can see that the force shouldn’t be massive. If it was, imagine how many docks would be crushed when a stiff wind blew in on a ship that was docked. Also, unlike most things that we can reference when thinking about forces, a ship isn’t on solid ground. There are no ground forces to resist this wind force. Similar to those “World’s Strongest Man” competitions where men pull airplanes or 18 wheel transport trucks, the pulling force is small compared to the size of the object being pulled, which is possible because the tires on the objects. Or how little tugboats can move ships much larger then they are.
In the end, what I take out of this analysis was that I was really surprised at the size of the wind wake behind the ship at about 1450 ft. But more then that, I was amazed at the tiny amount of wind force that moved this massive boat to shore and saved so many lives that day.