BOILERS FOR MARINE PURPOSES.

6 9

exposed to the action of the fire; for undoubtedly at the tops andsides of the furnaces the water is most rapidly evaporated, andconsequently a better steam-producing boiler will be obtained,—aresult that all should strive to reach, even although it may be atconsiderably more cost in manufacturing.

Thickness of Plates for Round Boilers .—As these boilers, for thecompound marine engine, are made large in diameter, carrying highsteam pressure, the best quality of iron must be used, or B, B, equalto Yorkshire plates, the breaking strain being 57,120 Ibs. per squareinch; and taking one-fifth of the breaking strain of the plate orrivetted seams as the constant for this quality of iron, we have—

-= thickness of the shell.

11424

Thus, supposing we have a boiler 142 inches in diameter, and thesteam pressure 70 lbs. per square inch—

142 x jo_ _ g y inch11424 '

The seams should be double-rivetted, which are nearly equal instrength to the solid plates. For the thickness of the plates for theother parts of boilers we give two examples by English and Scotchfirms, the steam pressure being 70 lbs. per square inch in each

case:—

London Made,inches.

Diameter of Boiler.142

Thickness of Shell. |-

Do.

End Plate..

s

Do.

Tube Plate.

3

Do.

Furnace Plate ..

• *

Do.

Back of Furnace Plate.

T(T

Leith Made.

inches.. 120

XI

.. TT

1 X

. TT

X X

. TT

7

. TT

7

. IT

Strength of Flue Tubes to resist crushing. Example:—

• 43 2 inch thickness of plate x constant 7000007.25 feet length of tube x 36 inches, diameter of tubeult. strength 495 — 6th = 82 lbs. working pressure.

In a paper on the Strength of Boilers, by Mr. J. Milton, surveyorto Lloyd’s, * 1 the question of factors of safety is considered; and itis shown that the ordinary cylindrical boiler is the only really reli-able marine boiler at present in use, and that as the shell plates hadreached up to a thickness of I inch, the weight of such a boilerbecame an important item in the load carried by the ship. Hence,

1 See Trans. Insi. Naval Architects, session xviii.