PRINCIPLES OF NAVAL ENGINEERING 



weight is a constant percentage of tiie displace- 

 ment, an increase in the fraction of the dis- 

 placement assigned to one military charac- 

 teristic involves the reduction of other fractions 

 of displacement. By increasing the displacement 

 of the ship as a whole, it is possible to in- 

 crease the speed and the radius of action with- 

 out adversely affecting the other required 

 characteristics. 



4. Effect of Seagoing Capabilities. — Larger 

 ships are more seaworthy than smaller ships. 

 However, smaller ships are more maneuver- 

 able because they have smaller turning circle 

 radii than larger ships, where all other factors 

 are proportional. The maneuverability of large 

 ships may be increased somewhat by the use 

 of improved steering gear and large rudders. 



In general, larger ships have the advantage 

 of greater protection because of their greater 

 displacement. From the point of view of under- 

 water attack, larger ships also have an ad- 

 vantage. K compartments are of the same size, 

 the number of compartments increases linearly 

 with the displacement. It is apparent, then, that 

 protection against both surface and subsurface 

 attacks may be more effective on larger ships 

 without impairing other military characteristics. 



Many compromises must be made in de- 

 signing any ship, since action which improves 

 one feature may degrade another. For example, 

 in the design of a conventionally powered ship 

 there is the problem of choosing the hull line 

 for optimum performance at a cruising speed 

 of 20 knots and at a trial speed of 30 knots or 

 more. One may select a hull type which would 

 minimize resistance at top speed, and thus 

 keep the weight of propulsion machinery to 

 a minimum. When this is done, however, re- 

 sistance at cruising speed may be high and the 

 fuel load for a given endurance may be rela- 

 tively great, thus nullifying some of the gain 

 from a light machinery plant. On the other hand, 

 one may choose a hull type favoring cruising 

 power. In this case, fuel load will be lighter 

 but the shaft horsepower required to make 

 trial speed may be greater than before. Now 

 the machinery plant is heavier, cancelling some 

 of the weight gain realized from the lighter 

 fuel load. A compromise based on the inter- 

 relationship of these considerations must usually 

 be adopted. The need for compromise is always 

 present, and the manner in which it is made 

 has an important bearing on the final design of 

 any naval ship. 



SHIP FLOTATION 



When a body floats in still water, the force 

 which supports the body must be equal to the 

 weight. Assume that an object of given volume 

 is placed under water. If the weight of this 

 object is greater than the weight of an equal 

 volume of water, the object will sink. It sinks 

 because the force which buoys it up is less 

 than its own weight. However, if the weight of 

 the submerged object is less than the weight 

 of an equal volume of water, the object will 

 rise. It rises because the force which buoys it 

 up is greater than its own weight. The object 

 will continue to rise until part of it is above 

 the surface of the water. Here it floats at such 

 a depth that the submerged part of the object 

 displaces a volume of water, the weight of 

 which is equal to the weight of the object. 



The principle implied in this discussion is 

 known as Archimedes' law: the weight of a 

 floating body is equal to the w^ght of the fluid 

 displaced . 



The cube of steel shown in part A of figure 

 2-1 is a solid cube of the dimensions shown. 

 If this cube is dropped into salt water, it will 

 sink because it weighs approximately 490 pounds 

 and the weight of the salt water it displaces 

 is approximately 64 pounds. If the cube is 

 hammered out into a watertight flat plate 

 of the dimensions shown in part B of figure 

 2-1, with the edges bent up one foot all around, 

 the box thus formed will float. This box could 

 be made from the same volume of steel as 

 that of the cube. The box will not only float; 

 in calm water, it will carry an additional 1800 

 pounds of weight before sinking. As a box, the 

 metal displaces a greater volume than the same 

 amount of metal does as a 1-foot cube. 



8.44 



Figure 2-1.— Steel cube and box made 

 from same volume of steel. 



16 



