TRAVEL ON WATER 



305 



taught you that bodies which are submerged or float 

 in water experience an upward push which makes 



them appear to have 

 lost weight. You have 

 also learned from the 

 experiment that the up- 

 thrust or force experi- 

 enced by either a float- 

 ing or a submerged 

 body is equal to the 

 weight of water dis- 

 placed. 



FIG. 509. A FLOATING BODY MS- When a floating body 

 PLACES ITS WEIGHT is weighed in water it 



is found that the water 



supports all the weight of the object. Any substance 

 can be made to float if it can be shaped into such a 

 form that it will displace its own weight of water. 

 Iron, steel, and concrete all will sink in water when 

 in compact forms, and yet if they are so shaped as to 

 have large displacement they can be made to float. 



The displacement of a ship is the weight of water 

 which it will displace when fully loaded with cargo 

 and passengers. Since a floating body displaces its 

 own weight of water, the water displaced will be 

 equal to the total weight of ship and cargo. By know- 

 ing the material from which a ship will be built and 

 the weight of the equipment, engineers can calcu- 

 late the carrying capacity of a ship before it is built. 



FIG. 510. CENTERS OF GRAVITY, BOAT AND CANOE 



What is stability and what are the factors that af- 

 fect it? A canoe can be tipped over much more easily 

 than a flat-bottom rowboat. This is because the row- 

 boat has much greater stability than a canoe. The 

 stability of a boat depends upon several things. If 

 a body has a broad base area, like the rowboat, it has 

 greater stability than one which has a smaller base 

 area. Another important factor in stability is the posi- 



tion of the center of gravity. The center of gravity 

 of any body is the point where all of its weight may 

 be thought of as concentrated. A uniform plank, for 

 example, has its center of gravity in the middle, while 

 a plank thicker at one end has its center of gravity 

 nearer the thick end. In general, the center of gravity 

 of a solid body is the point where it would balance. 

 However, in the case of a chair the center of gravity 

 would fall somewhere in the air below the seat. In 

 a boat the center of gravity would be in the air 

 above the bottom. Study Figure 510. 



The position of the center of gravity is important 

 in stability. If the center of gravity of a boat is high 

 or if it is shifted to one side, the stability is lessened 

 and the boat is more easily tipped over. In the case 



Courtesy Bus-h-Sulzer Bros. 



FIG. 511. THE Aorangi, A DIESEL-POWERED SHIP 



of the canoe with a person sitting in it, the center 

 of gravity is much higher than in the rowboat. This, 

 of course, makes the stability of the canoe much 

 less. Study Figure 510. 



In shipbuilding and in general ship transportation 

 every effort is made to keep the center of gravity low. 

 Racing sailboats have extremely high masts which 

 would tend to raise the center of gravity and make 

 for an unstable condition. This is offset, however, 

 by building a keel which extends far below the bot- 

 tom of the boat and may be weighted. Many freight 

 ships carry ballast of rock or sand in the bottom of 

 the boat to increase their stability. 



Exercise. At a summer camp a boy was sent to a town 

 across the lake with a canoe to transport a heavy iron 

 casting which had been sent to that point for the camp. 

 As the lake was rough, the boy was afraid that if he 

 placed the heavy casting inside it -would tip the canoe over 

 or break the thin supports in the bottom. A man at the 

 dock suggested that the casting be suspended under the 

 canoe in the water by means of a rope for safe trans- 



