12 BULLETIN 300, U. S. DEPARTMENT OF AGRICULTURE. 
front end of the hull should always be of double thickness to prevent 
damage and possible sinking should the dipper strike the hull. 
In the larger sizes built at the present time the practice is to make 
the hull of the same width, top and bottom. On some of the smaller 
machines, especially those with steel hulls, the top is made wider 
than the bottom. Hulls must be very carefully calked, since in op- 
erating the.dredge the strains will tend to loosen poor calking. 
Hulls are always built upon blocking at the place where the pro- 
posed work is to begin and usually are launched sidewise into the 
stream or pit. If there is no natural channel available for launch- 
ing the hull, an artificial pit must be excavated. This may be done 
by means of teams and slips, the excavated material being deposited 
on the sides of the pit for holding the water. Where the ground 
at the launching site is so wet as to preclude the use of teams and 
slip scrapers, a small tower and scraper device may be used. In 
one case, with such an arrangement, the scraper was operated by a 
small tractor. Some operators blast out their pits with dynamite. 
It is not advisable to launch a hull in less than 24 feet of water. 
It often is necessary to dismantle a dredge in order to move it from 
one project to another. A wooden hull is frequently used on more 
than one job if the shipping distance between projects is not too 
ereat, but a hull usually requires some new timbers when it is rebuilt. 
If the hull is to be used two or more years without rebuilding, long- 
leaf yellow pine is probably the best material for construction. 
Should the hull be rebuilt every other year or so, Douglas fir is more 
desirable. To build a wooden hull of new lumber usually takes 
about one-third longer than to rebuild an old hull. This is due to 
the fact that the new timbers must be cut to dimensions and all bolt 
holes drilled. Some contractors use electric drills operated by gen- 
erators run by gasoline engines and thus save much time in drilling 
bolt holes. 
Steel hulls usually are assembled much more quickly than wooden 
hulls, but require more cars for shipping. 
Some manufacturers furnish pontoon hulls of either wood or steel. 
the pontoons being built and calked at the factory. The sections can 
easily be shipped and the hull assembled in a short time, the pon- 
toons being placed crosswise of the hull. They not only make a - 
rigid hull, but comprise separate compartments, making it practically 
impossible for such a dredge to sink. The draft of a dipper-dredge 
hull is approximately one-half the depth of the hull. 
The machinery for a dredge ordinarily is placed on the main deck 
of the hull. Sometimes, however, it is placed below the main deck 
in order to gain head room. The boiler and coal bins are sometimes 
placed on a deck from 1 to 3 feet lower than the main deck. 
