Egyptian tomb painting from 1450 B.C. showing officer with sounding pole. Officer is telling crew to come ahead slow. Engineers with cat-o’-nine-tails assure proper response from the “engines.” Courtesy NOAA
Who were the first boat-builders? No one really knows.
The oldest known boats are dugout canoes constructed in China and South Korea around 6,000 B.C. But it’s possible that the almost-human species Homo erectus used some type of boat 800,000 years ago! Since H. erectus are known to have made tools from bamboo, they may also have made rafts from the same material. But regardless of the materials and who used them, the basic principles that allowed the first boats to float are the same principles that operate on the most modern supertanker.
Have a contest to see who can build a hull that will carry the most “cargo” (the greatest number of pennies), starting with the same materials. You can modify the contest to see whose can build the fastest sailing boat. You may want to experiment to find out whether it is better to use one large sail or several small sails to increase sailing speed, and whether triangular shaped sails or square sails give the best performance.
Water tends to maintain a level surface. When you put an object into water, gravity pulls the object down and displaces some of the water, which means some of the water is pushed aside.
Now the surface of the water is no longer level. Gravity pulls the displaced water down, and causes an upward force on the object. This upward force is equal to the weight of the water that the object displaces, and is called buoyancy. Buoyancy depends upon the volume of liquid displaced as well as the density of the liquid. Density is the mass of a certain volume of liquid, usually stated as grams per milliliter. This is why it is easier to float in the ocean than in fresh water. Seawater is more dense than fresh water, so your buoyancy is greater in the ocean.
The amount of fluid that an object displaces depends upon the volume of the object: more volume means more fluid displaced, which means more buoyancy. Increasing the volume of an object also increases its surface area, which in turn increases the effect of friction as the object moves through the fluid. Boat designers have to consider buoyancy as well as friction when deciding on the shape of a boat’s hull. A boat designed for speed must have enough displacement to stay afloat, but surface area has to be minimized to decrease the effects of friction. On the other hand, an object designed to carry a heavy weight, such as a cargo ship, must be designed with greater displacement, as well as greater power to overcome the effects of increased friction.
Boat hulls are designed to displace a volume of water that weighs more than the boat and its cargo. If the force of displaced water pushing on the hull is greater than the force of gravity pulling the boat down, then the boat floats! But if you could take a sea-worthy boat hull, break it down, and bundle the pieces together, the bundle will sink. Why? Because there is no longer sufficient water displacement to counteract the force of gravity on the pieces of the hull.
Visit www.sedl.org/scimath/compass/v02n03/boat.html and www.thirteen.org/edonline/nttidb/lessons/jx/buoyjx.html for more boat building challenge ideas.
Portrait of Archimedes, by Domenico Fetti, about 1620
The idea of buoyancy was summed up by a Greek mathematician named Archimedes: any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. Today, this definition is called the Archimedes Principle.
Archimedes is considered one of the three greatest mathematicians of all time. The other two are Newton and Gauss. Archimedes was born in 287 B.C., in Syracuse, Greece. He was a master at mathematics and spent most of his time thinking about new problems to solve.
Many of these problems came from Hiero, the king of Syracuse. Archimedes came up with his famous principle while trying to solve this problem.
The king ordered a gold crown and gave the goldsmith the exact amount of metal to make it. When Hiero received it, the crown had the correct weight but the king suspected that some silver had been substituted for the gold. He did not know how to prove it, so he asked Archimedes for help.
One day while thinking this over, Archimedes went for a bath and water overflowed the tub. He recognized that there was a relationship between the amount of water that overflowed the tub and the amount of his body that was submerged. This observation gave him the means to solve the problem. He was so excited that he ran naked through the streets of Syracuse shouting “I have found it!”. The goldsmith was brought to justice and Archimedes never took another bath…(just kidding!).