Engineering boats · Chemistry · Physics · Grades K–12
Archimedes’ Tank
A body in water is lifted by the weight of the water it pushes aside. Use that one rule on a crown, a foil boat, and a ship’s load line.
Both objects have a mass of 1,000 g. Gold is 19.3 g/cm³. Silver is 10.5 g/cm³. Drag across the picture to mix in silver. Step adds 10%.
The hull is 15 cm by 10 cm and 4 cm tall, and it weighs 8 g. Fresh water is 1 g/cm³. Step adds 40 g.
A box-shaped hull 100 m long and 16 m wide. The ship itself is 4,000 t. The painted line is at 8 m of draft. Salt water here is 1.025 t/m³.
What to watch
Archimedes of Syracuse (287–212 B.C.) wrote the rule: the upward force equals the weight of the fluid displaced. Vitruvius, writing in the first century B.C., tells of King Hiero, who suspected a goldsmith had mixed silver into a gold crown. The honest test is a balance. In air the crown matches a lump of pure gold of the same mass. In water the crown, if it is bigger, is lifted more, and the balance tips.
Iron is about 7.8 times as dense as water, so a solid lump sinks. A hull floats when it encloses enough air that the average, iron plus air, is less dense than water. It sinks in until the displaced water weighs as much as the ship.
Samuel Plimsoll pressed Parliament until, in A.D. 1876, British ships had to carry a load line. Load past that line and there is not enough side left above the waves. In A.D. 1787 John Wilkinson launched an iron barge on the River Severn, and it floated.
Original School Launch model of box hulls in still water. It leaves out waves, the shape of a real bow, and the several marks on a modern load line. Nothing is saved or sent.