EXCAVATING MACHINERY USED IN LAND DRAINAGE. 5 
engines as suitable for operating machines which have such constantly 
changing loads as is the case with dipper dredges. 
Owing to the constant jar and pound on the hull the vertical engine 
is not so well adapted to excavating machinery as the horizontal 
type. On a large dredge an independent engine unit is used for 
each of the operations of swinging, hoisting, and handling spuds. 
The hoisting and swinging engines are generally of the horizontal, 
double-cylinder type and must be self-contained on a cast-iron or 
structural-steel bed plate. Steam engines are generally designated 
by the dimensions of their cylinders rather than by the horsepower 
they develop. 
Owing to the cost of fuel, the expense of transporting it to the 
dredge, and the impure and muddy water that must be used in some 
cases, the size and type of boiler must be selected with great care. 
The boiler commonly used is the locomotive type with either open 
or water bottom. Vertical boilers have been used in dredges of the 
smallest sizes, but are not economical in the consumption of fuel. 
The grate area of the locomotive-type boilers is ordinarily less than 
that for the same size of Scotch marine boiler. The return-flue 
Scotch marine boiler is used on many dredges and meets with great 
favor. The earlier boilers were designed for a pressure of 100 pounds. 
Later this was increased to 150 pounds and the boiler was worked at 
100 pounds or more pressure. The size of boiler should be at least 
25 per cent greater than that theoretically required to operate the 
engines. Owing to the foul character of the water that must often 
be used, the boiler should have two separate and distinct boiler feeds, 
either injectors or pumps. A great saving of fuel can be effected by 
covering the boiler and steam pipes with asbestos. Either wood or 
coal is used for fuel. 
A-FRAME . 
The A-frame is a tower composed of timber or steel members 
securely anchored on the deck of the hull near the front and joined at 
the top by a cast-steel head or yoke. (See PL I.) The A-frame 
may have either two or four legs. In the latter case the two front 
or main legs are set in a vertical plane. If only two legs are used 
they are inclined slightly forward. The A-frame must be strong] y 
guyed and held rigidly in position, as the severe stresses from the 
outer and loaded end of the boom are carried by the top of this 
tower. Failure of any part of the A-frame may result in serious 
damage to the dredge and even in loss of life. The height is governed 
by the required elevation of the end of the boom, which in turn is 
determined by the depth of excavation and the distance at which the 
excavated material must be placed. On the top of the head block is 
a large pin on which the yoke revolves, this latter being a short beam 
to the ends of which are attached the cables which support the outer 
end of the boom. 
