250 LOGGING 
are made on the boilers for power, wood is not as satisfactory 
as fuel oil or coal and, in some parts of the West, wood has been 
replaced by fuel oil and in other regions where coal can be easily 
obtained it has been substituted frequently for wood. The use 
both of wood and of coal represents a high fire hazard because 
of the heavy spark discharge and this has led to a preference for 
fuel oil when it can be secured at a reasonable price. 
Wood fuel often is cut from merchantable logs which have been 
skidded to the machine. Cull logs are sometimes used, but they 
provide an inferior fuel and their use is limited on that account. 
In the southern pine region, "fat" pine is a common fuel because 
of its high heat value. It is, however, harder on boiler flues 
than most other kinds of wood because of the marked changes 
in the temperature of the fire box. When "fat" wood is first 
thrown on the fire, the early combustion of the volatile gases 
creates an intense heat. Before the wood has been consumed 
to the point where more can be put on the fire, the temperature 
in the fire box will have dropped to a marked extent. The con- 
stant rise and fall of temperature causes a continual contraction 
and expansion of boiler tubes which often leads to tube leakage. 
The amount of wood fuel consumed by a skidder is dependent 
on the length of haul, the size of the logs and the character of 
fuel wood. In general an overhead, a re-haul, and a snaking 
skidder in the South each burn from four to five cords of 2-foot 
wood daily. A 11- by 13-inch yarding engine in the Northwest 
will use daily from 1000 to IGOO feet log scale of timber which is 
equivalent to from four to six 2-foot cords. 
Coal is considered a better fuel than wood, but is fully as 
hazardous from the forest fire point of view and in many places 
is as expensive as fuel oil, hence the latter is preferred. The coal 
requirements for a skidder range from 1 to H tons daily. 
Oil is considered the most satisfactory fuel for yarding engines 
and skidders which are located on a railroad because of the 
low forest fire hazard connected with its use, the ease with which 
it can be placed in the storage tanks, and the ability of the fire- 
man to maintain an adequate steam pressure when hea\'y de- 
mands are made for power. It is claimed that oil burners may 
have from 15 to 25 per cent greater efficiency than wood burn- 
ers because of the ability to always hold a high head of steam. 
A yarding engine will consume from five to eight barrels of fuel oil 
