306 



TRANSPORT A TION 



portation. Did the suggestion have any scientific basis? 

 Was it a good suggestion? State reasons for your answer 

 one way or the other. 



How are modern ships propelled? While the steam 

 engine is still much used as a source of power, the 

 steam turbine is coming more and more into use on 

 modern ships. Since turbines are not reversible, they 



Courtesy Sperry Gyroscope Company 

 FIG. 512. GYRO COMPASS 



must either be in pairs or, as is usually the case, used 

 to turn electric generators which furnish current to 

 electric motors attached to the propellers. These can 

 be easily reversed. 



Coal as a fuel on ships is being replaced by oil 

 not only because it is cleaner but also because of the 

 saving in space needed for fuel storage. Many large 

 ocean-going vessels are equipped with oil burners. 



Oil is also used as fuel on those vessels which are 

 driven by Diesel engines. These have been used as 

 a source of power on vessels up to about 30,000 tons 

 displacement. The Diesel engine differs from the or- 

 dinary gasoline engine in that it uses the heat de- 

 veloped in compression to explode the fuel rather 

 than an electric spark. Figure 511 shows a cut of 

 a modern ocean-going vessel equipped with Diesel 

 engines. 



For ocean travel, the side and stern drive wheels 

 have long since been replaced by the screw propeller. 



These can be turned much more rapidly and thus can 

 drive the boat with greater speed. Some of the most 

 modern of ocean liners have two, three, and some- 

 times four screw propellers. 



In Unit XI you learned that the earth is a large 

 magnet and that as a result compasses point north- 

 ward. Until recent years ocean transportation was 

 dependent upon the magnetic compass to point the 

 direction for voyages. 



Exercise. Explain briefly why the magnetic compass 

 does not point true geographic north. If you are unable 

 to do this, turn to page 246 and review the section on 

 the magnetic compass. 



Exercise. Often the magnetic compass on steel vessels 

 is found to be in error. Can you infer a cause for this 

 condition? Suggest a simple way in which it might be 

 corrected. 



How is direction told at sea? If you have ever 

 played with a gyroscope top, you are familiar with 

 the tricks that it can perform because of the rapidly 

 turning weighted wheel. A body such as this when 

 spinning rapidly tends to resist any force which at- 

 tempts to change the line of the axis of rotation. 

 This principle has been applied in stabilizing ships 

 to prevent rolling in rough water. Another application 

 is made of the action of the gyroscope in the gyro- 

 scopic compass, invented by Sperry, an American. In 

 combination with the spin of the earth on its axis 

 this instrument acts to point true north. The mariner's 

 compass, on the other hand, points magnetic north 

 and is, therefore, subject to the variations of the 

 earth's magnetism and those caused by iron and steel 

 used in the construction of the ship. Figure 512 shows 

 a picture of a gyro compass. 



How is the submarine enabled to sink and rise in 

 water? This type of boat has been used chiefly as an 

 instrument of war and probably will continue so, in- 

 asmuch as its value as a means of transportation seems 

 limited. The first submarine is believed to have been 

 built by a Dutchman by the name of van Drebble. 

 In 1624 he exhibited his craft in the Thames River 

 at London. It is said to have been built of wood and 

 propelled by oars. Robert Fulton experimented with 

 a cigar-shaped submarine before building the first 

 steamboat. He succeeded in staying under water in 

 his craft for five hours. 



The modern submarine dates from 1877 when John 

 P. Holland, an American, built the first under-water 

 craft which seemed to have prospects of developing 

 into an effective weapon of war. Since that time it has 

 been developed until now a modern submarine may 

 be three hundred fifty feet in length. Figure 513 shows 

 a modern submarine. 



The submarine is able to submerge or rise to the 

 surface at the will of the commander of the boat. 



