Ballast: How a Ship Keeps on Sailing Upright
From a log book of the Atlantis, a research ship for the Woods Hole Oceanographic Institute.
“As I write this, the engine room is carefully adjusting the ship’s ballast. This is an interesting physics problem for all you scholars out there. As we burn fuel, we transform the heavy liquid into a gas exhaust that leaves the confines of the ship. Therefore, we’re getting lighter.
As we get lighter, we sit higher in the water, losing some stability. How do you correct for that? Can you think of a way to do it? Well, this is actually a topic I overheard Captain George Silva discussing with Chief Engineer Jeff Little and First Engineer Jim Schubert.
If you guessed that the best way to offset the weight of the fuel lost is to replace it with salt water, you’re right! Underneath the lowest decks of the ship are the fuel containers and ballast containers. They run all the way from the bow to the stern and all the way from port to starboard. As you may imagine, it pays to have many tanks of different sizes in many locations. But how do you know when to fill certain tanks?
There are many things to consider, including how far the ship could tip before it would roll over, bow vs. stern trim, port vs. starboard list, and the comfort of passengers (to prevent seasickness).
In addition to all those, this program is designed specifically for Atlantis. It factors in the location and weight of critical equipment such as the cranes, A-frame, and Alvin.
As we’ve been traveling and burning fuel, we’ve been filling the ballast tanks. So now, we have a fair amount of water and less fuel.”
When they come into port and add fuel, they will release exactly the same amount of ballast water. Wow, the things you didn’t know