How do “Moon Pools” work? How can a ship have a big hole in it yet not sink?


How do “Moon Pools” work? How can a ship have a big hole in it yet not sink?

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Well imagine you take two boats next to each other. Both swim right?

Now you weld them together with two long sticks. Still swim right?

Then you place a platform on those sticks with a hole in the middle, and build a roof above them. Moonpool finished.

[Here is a drawing](

Objects float because they experience a buoyant force equivalent to the weight of the water that they displace. The form of the object is irrelevant, other than the effect that it may have on displacement.

Moon pools aren’t just open holes in the bottom of a ship’s hull. They have sidewalls that extend to some distance above the waterline. A ship with a moon pool may have marginally less increase in buoyant force when the submergence depth is increased than if the moon pool wasn’t there, but it is still capable of displacing a greater weight of water than the weight of the ship before its watertight integrity is compromised.

This is like drilling a hole in a piece of Styrofoam. You still expect it to float, right?

Ever see one of those circular life preservers tossed into the water? Like that.

The boat has sections that dip under water. If Those parts push away more water than the boat weighs, the boat floats. It doesn’t matter it it’s a rectangle or ring, or whatever shape. You can have a place in the middle where water comes up (a moon pool).

What sinks a boat is when a hole opens somewhere that lets water fill up a spot below the water filled with air, so that part of the boat isn’t pushing away as much water.

Fill your sink. Then take an empty glass and turn it upside down. Push it into the water. Notice the glass doesnt fill up completely.

The air pressure pushes on the water enough so it doesn’t fill up the glass (or boat)

There is air above the moon pool which is contained by the ship. The water pushes up through the moon pool bottom into the air. This compresses the air a little which makes it push back harder. The air stops the water coming in.

This works because the air cannot escape down through the water or up (because the ship is in the way). If you put a hole in the top of the moon pool area, the air wouldn’t be compressed so the water would be able to push up until it reaches sea level, which would fill the moon pool with water.

Edit: there are different kind of moon pools, my explanation is for those that have pockets of air below sea level, eg: used for underwater work. Same principle as a diving bell. You can also have open topped moon pools which are literally just building a boat around a pool.