Boys' Book of Model Boats

Excerpt

WHY A BOAT FLOATS

BEFORE taking up the construction of any of the model power boats described in this book, it will be well for the young boat-builder to become acquainted with such terms as buoyancy, displacement, center of gravity, etc. Knowledge of these subjects is more or less necessary if successful boats are to be made. Aside from this, they are terms that every boy who claims an interest in boats should understand.

"How does a steel boat float?" is a question that many boys ask. The reason they usually designate a steel boat is probably because steel is so much heavier than water. But many things heavier than water can be made to float if they are in the form of a boat. Concrete, for instance, is now being used in ship construction, and this substance, when reinforced with steel rods, is very much heavier than water.

Before learning how a boat floats, what is known as "specific gravity" must be thoroughly understood. Gravity is a force that is continuously "pulling" everything toward the center of the earth. It is gravity that gives a body "weight." Some substances are heavier than others; or, to be more correct, it is said that the specific gravity of one substance is greater than that of another. It will be well to keep in mind that specific gravity merely refers to weight. It is simply a scientific term. The specific gravity of a substance is always expressed by a figure that tells how much heavier any substance is than water, because water has been chosen as a standard.

The specific gravity of water is 1. The specific gravity of gold is 19.26, meaning that it is about 19/ times heavier than water. The specific gravity of a piece of oak is 0.86, which shows that it is not quite so heavy as water. One cubic foot of water weighs 62.42 pounds. It will be understood that a cubic foot of gold would weight 19.26 x 62.42, because it is 19.26 times heavier than water. A cubic foot of oak, however, would weigh only 54 pounds, because it has been found that it has a specific gravity of only 0.86 which is less than water.

A cubic foot of oak (see

), with a weight of 54 pounds, will float when placed in water. The cubic foot of brass (B), however, will not float, because it weights 8.1 times as much as water. For the present, then, it can be said that a substance lighter than water will float in water, but that substances heavier than water, such as iron, lead, gold, silver, etc., will not float. If thecubic foot of oak (A) were placed in water, it would sink to the depth shown atC. When the block sinks into the water, a certain amount of water will be forced away or "displaced"; that is, the block in sinking occupies a space that was previously occupied or filled with water. The oak block sinks to within a short distance of the top because the oak is really just a trifle lighter than water. If a pine block were placed in the water it would sink only to the distance shown atD, since the weight of pine is less than oak, or only 34.6 pounds per cubic foot....