32 STRUCTURE OF VESSELS AS AFFECTED 



words, if further precautions in the building, navigation, and upkeep of vessels 

 for the purpose of promoting safety are to be taken, they should be along such 

 lines and so limited in extent that the commercial success of the vessels is not 

 jeopardized. As the safety of vessels is a matter which is not apparent to the 

 general public, it seems proper that it should be made the subject of national or 

 international regulations as far as regulations can conduce to increased safety. 

 The dangers against which precautions should be taken in connection with 

 the structure of a steel vessel may be classified as follows: Leaks, storms, ice, 

 grounding, collision, and fire. 



LEAKS. 



Intact outside plating to above the water surface is such a common feature 

 of new steel vessels that no comment on this subject is needed; except, perhaps, 

 that the government should require that the outside plating of old vessels should 

 be subject to examination at intervals and should be kept up to a certain stand- 

 ard of strength and tightness, and that underwater openings should be properly 

 protected. 



STORMS. 



The precautions against storms should include the elements of strength, 

 watertightness, stability, and control. 



Strength. — The rules of the classification societies are based partly on expe- 

 rience and partly on theory, and in general may be said to provide ample strength 

 of structure. Indeed there are many instances of vessels in service which have 

 scantlings much below the standard of the classification societies, and which have 

 shown no signs of weakness. It is generally accepted that if this matter is covered 

 by legislation in this country, a suitable method would be the assignment to every 

 vessel of a maximum load line which should be determined from a consideration of 

 the scantlings of the vessel and her route in service. It is suggested that this 

 maximum load line could be lowered as the age of the vessel increases, in a sim- 

 ilar manner to the reduction of pressure on a boiler, to allow for deterioration. 

 The stresses in the structure of a vessel seem to increase rapidly with increase of 

 draught and displacement, and although the amount of the stress would depend on 

 the shape of the vessel and the distribution of the cargo and other weights, it is 

 considered that no fairer method of obtaining the measure of precaution as re- 

 gards strength of structure to weather storms can be devised than by what is 

 known as the assignment of freeboard. 



To illustrate the effect of increase in draught, the case is presented of an oil 

 tanker, 389 feet x 49% feet x 30 feet. When loaded to a draught of 20 feet, dis- 

 placement 8,720 tons, the compressive stress in the upper deck figures out at 63^ 

 tons per square inch in hollow of wave 385 feet x 1914 feet. When loaded to a 

 draught of 24 feet 3 inches, displacement 10,705 tons, the corresponding stress in- 

 creases to gy% tons per square inch, or an increase in stress of 40 per cent for an 

 increase in draught of 21 per cent. 



