236 HEAVY-OIL ENGINES FOR MARINE PROPULSION. 
them for use on board ship. As a result a number of firms have produced 
engines which are amply strong, and yet lighter in weight than the lightest 
steam plants afloat. The lightest engine which has been built weighs only 
20 pounds per horse-power. In this engine the bedplate and housing were 
aluminum, the crank-shaft and connecting rods of special high-grade steel, 
the water jackets of copper, and the cylinders and pistons of cast iron. 
Certain four-cycle marine engines have been constructed up to 1,000 
horse-power, and weighing from 40 to 60 pounds per horse-power. In these 
engines cast steel bedplates, and cast, or built-up, steel housings have been 
used. In these, cylinders and pistons are the only cast iron parts. ‘These 
engines have demonstrated that they are amply strong and durable. 
Many two-cycle marine engines, ranging from 150 up to 2,500 horse- 
power have been built, in which the weight is not more than 40 pounds per 
horse-power. In these, high grade bronze castings are employed for bed- 
plates and housings. 
Piston Speeds.—The lowest speeds used are about 600 feet per minute 
in very heavy slow-running engines. ‘The highest speed in the lightest types 
is 1,100 feet per minute. For ordinary work it is perfectly safe to use a 
piston speed of 1,000 feet per minute, as has been found to be the case in 
steam and gas engines. . 
Lubrication.—in practically all marine oil engines some form of forced 
lubrication is used. Babbitted bearings are used throughout. Little or no 
trouble has been found with bearings. Due to the high pressures used, the 
bearings are somewhat larger than in steam engines of the same power. 
Piston Packing —The same form of ordinary split rings are used as in 
ordinary gas engine practice. On account of the higher pressure a greater 
number of rings are employed in oil engines. 
Stuffing Boxes —For double-acting engines of this type a special form 
of metallic packing for stuffing boxes has to be provided. The usual forms 
of metallic packing used in modern steam engines are inadmissable, both 
on account of the high pressure and temperature. This problem has, 
however, already been successfully solved. 
To sum up the only problems which have had to be solved are those due 
to the high pressure and temperature in the cylinders. All these problems 
have been successfully solved so that there are now fully a score of different 
designs in successful operation. From this it should not be assumed that 
these practical problems are easily and cheaply solved. Their solution 
represents years of expensive experimental work on the part of manu- 
facturers who naturally retain for their own use the practical knowledge 
thus derived. Ordinary steam engine and gas engine practice cannot be 
followed without some modification. 
