buoyancy and water displacement

Thanks to Alex Gabrielli and Mrs. Moffitt for explaining our science activities.

Mrs. Moffitt began our Titanic Science unit with lessons on Buoyancy and Water Displacement. The reason why ships float is because the water that the ships displace (amount of water that rises because of the mass of the object) weighs more than the weight of the ship, which means it is buoyant (will stay afloat). We began by testing various items made out of different materials for their ability to float. Next, we made boats out of clay to decide which design is the most buoyant. We also made boats out of aluminum foil to learn about water displacement. We placed marbles in the boats one at a time. When the weight of the marbles exceeded the displacement of water, the ship would sink.

To test the Titanic, the engineers actually had to measure the mass of the parts separately, along with the amount of water displacement. Another key property of buoyancy is the weight that is distributed. If the object has a surface that distributes its weight, then this allows the combined force of the water along with any capital (man-made) forces, such as bulkheads, to allow the ship to float more easily than if it were compact and light. The bulkheads did not need to be empty, and many were filled with cargo. On a side note, after people looked at the original shipping log, they found out that the cargo was not worth much and had a small estimated value.

The bulkheads were designed to be watertight, because the doors would shut if water leaked in to any one compartment. The expectation was that something would make one hole in the side of the ship. With waterproof bulkheads extending up through several decks of the ship, a single hole might cause one or two compartments to flood, but the remaining ones would remain dry. While this would increase the weight of the ship, the ship would still displace enough water to allow it to float. No one expected something that would cause an opening or openings to extend through several compartments at one time.

Our next experiment involved trying to make unsinkable ships out of two-liter bottles with bulkheads made from the bottoms of two more bottles. We used duct tape to attach the bulkheads, and placed enough clay inside to weigh the ship down so it would take on water with the cap off. We did a trial without bulkheads, and most of the ships went down in less than a minute. On the second trial with the bulkheads, we had much more success, with one group successfully building a ship that we could not sink! Many of the group’s boats lasted for at least two hours!

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