TRANSACTIONS OF SECTION G. 



813 



the sizes, clearances in gaup^e plates, best method of reproduction on English lathes 

 of these screws, together with a few sectional drawings showing the shape of thread, 

 rounding off, &c. 



6. A Uniform Factor of Safety for Boilers and Machinery of Steamships 



By JoHX Key. 



The subject of this paper has arisen out of the fact that no uniform code of 

 international regulations has yet been adopted by the various maritime countries 

 and States for the general safety of machinery on board steamships, especially 

 regai'ding the construction and strength of marine boilers and their connections. 



A uniform factor or margin of safety might be devised on broad and intelli- 

 gible grounds, without making any violent change, on the basis, not of the breaking 

 strength, but of the elastic limit of the material — ascertained carefully by a 

 uniform method of conducting tests — that would be accepted by all civilised 

 countries, in order that any steamship passed at one port for a properly certified 

 working pressure might not, when new, be altered or reduced at another, similar 

 in principle to the British system of measurements for ascertaining gross tonnage, 

 and the regulations for preventing collisions at sea. 



The following tabulated statement shows at a glance the working pressures for 

 cylindrical shells of steel boilers allowed by the rules of the various authorities : — 



The water-pressure test allowed by the British Admiralty shall not exceed four- 

 ninths of the ultimate strength of the shell, and the working pressure is fixed at 

 90 lbs. below the test-pressure, which is called their ' constant margin ' of safety 

 for all pressures. 



The Board of Trade allow a factor of safety 4-5, with additions according to the 

 circumstances of each case. 



Lloyd's Committee add ^ inch, and the 



British Corporation add ^ inch to all thicknesses for wear, and their constants 

 vary according to the form of riveted joint. 



Hamburg rules allow a factor of safety 5'0, reduced to 4'7 -^vhen the longi- 

 tudinal seams are drilled and double riveted. 



Bureau Veritas allow a factor of safety of 4*4 after the plates have been corroded 

 away by 0-04 of an inch. 



These authorities all differ in their respective rules for diameter of shafts, thick- 

 ness of plates forming flat surfaces, stress on stays, thickness of plain or corrugated 

 furnace tubes, steam-pipes, and area of safety valves. 



As an example of how unnecessarily we are hampered by want of uniformity 

 even in boiler fittings and connections in the case of water-gauges, the English 

 Board of Trade insist on having cocks or valves next the shell of the boiler, 

 whereas the German Board of Trade will not have such a fitting ; with the conse- 

 quence that ships running to Hamburg are fitted with two standpipes, one with 



