Boys' Book of Model Boats

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 191/4 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 Fig. 1), 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 the cubic foot of oak (A) were placed in water, it would sink to the depth shown at C. 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 at D, since the weight of pine is less than oak, or only 34.6 pounds per cubic foot. A pine block will, then, displace only about 34.6 pounds of water, which leaves nearly half of the block out of the water. Thus, it will be seen that for a given volume (size) a cubic foot of wood will sink to a depth corresponding to its weight. Different kinds of wood have different weights.

If a cubic foot of brass is placed in water, it will sink rapidly to the bottom, because the brass is much heavier than water. How is it, then, that an iron or concrete ship will float? If the cubic foot of brass is rolled or flattened out in a sheet, and formed or pressed into the shape of a boat hull, as shown in Fig. 2, it will float when placed upon the surface of the water. Why is it that brass is caused to float in this way, when it sank so rapidly in the form of a solid square?

It will be remembered that the pine and oak block were caused to float because they displaced a greater weight of water than their own weight. This is just what causes the brass boat-hull to float. If the amount of water actually displaced by the hull could be weighed, it would be found that the weight of the water would be greater than the weight of the hull. It will be understood that the space occupied by the brass boat-hull is far greater than the space occupied by the block of brass before it was rolled out and formed into a hull. What is true of brass holds true of iron, steel, etc. A block of steel will not float, because the water it displaces does not weigh nearly as much as the block. If this block, however, were rolled out into a sheet and the sheet formed into a hollow hull, the hull would float, because it would displace a volume of water that would more than total the weight of the steel in the hull.

In the case of the brass boat-hull, it would be found that a greater portion of the hull would remain out of the water. The hull, then, could be loaded until the top of it came within a safe distance from the water. As the load is increased, the hull sinks deeper and deeper. The capacity of big boats is reckoned in tons. If a boat had a carrying capacity of ten tons it would sink to what is called its "load water-line" (L.W.L.) when carrying ten tons. As a load or cargo is removed from a vessel it rises out of the water.

What if the hull of a boat has a hole in it? If the hole is below the water-line, water will leak in and in time completely fill the inside of the hull, causing the boat to sink. Also, if too great a load or cargo were placed in a boat, it would sink. It must be understood that water leaking into a boat increases its load, and if it is not stopped it will cause the boat to sink.

The center of gravity of a boat is a very important matter. First, attention will be directed to the meaning of "center of gravity." If a one-foot ruler is made to balance (as shown in Fig. 3) at the six-inch mark, the point at which it balances will be very close to the center of gravity. The real center, however, will be in the middle of the wood of which the rule is composed. It should constantly be kept in mind that this "center of gravity" is a purely imaginary point.

Look at Fig. 4. If wires are arranged in a wooden frame, as shown, the point where the wires cross will be the center of gravity if the square formed by the wooden strips is solid. Every body, no matter what its shape, has a center of gravity. The center of gravity is really an imaginary point in a body, at the center of its mass. Oftentimes engineers are heard saying that the center of gravity of a certain object is too high or too low. Fig. 5 shows the center of gravity in a boat. If the center of gravity in a boat is too high (as illustrated in Fig. 6) the boat is said to be topheavy and unsafe. When a boat is topheavy or its center of gravity is too high, the boat is liable to capsize. In fact, some very serious marine accidents have been caused by this fault.

The center of gravity (or center of weight) in a boat should be as low as possible. A boat with a low center of gravity will be very stable in the water and difficult to capsize. This is true of model boats just as much as it is true of large boats. The model boat builder must keep the weight of his boat as near the bottom as possible. For instance, if a heavy cabin were built on a frail little hull, the boat would be very unstable and would probably capsize easily.

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