1915] SUBMARINE BoaTs 133 
dangers by this volatile oil soon made it necessary to introduce heavy- 
oil motors, although they were in several respects inferior to the gaso- 
line motors. While the latter are easy to start, special means are re- 
quired for starting the former, and the consumption of fuel in heavy-oil 
motors, as for instance of the Koerting type, was about twice as great 
as in the gasoline motors. The last step in the development was the 
introduction of the Diesel engine, which although rather heavy and 
likewise burning heavy oil, has a consumption of fuel somewhat less 
than that of the gasoline engine. The nominal radius of action of 
recent boats of the largest size, driven by Diesel motors, is given as from 
3000 to 5000 miles. The speed on the surface has attained 16 kts. in 
several boats and the designed speed in some boats now under con- 
struction is 18 or 20 kts. 
The Diesel engine is the motor which to-day finds most favour in sub- 
marine boats, but with the increasing size of boats and the claims to 
higher speed it becomes increasingly difficult to produce motors of 
sufficient power. Units of from 800 to 1200 H.P. are under construc- 
tion and there are usually two and in some boats three propellers. Many 
difficulties are met with and failures have occurred, whence steam 
power has been preferred in some boats as for instance in the French 
submersibles Gustave Zédé and Nereide of 1000 ts. displacement, which 
are to make 20 kts. Steam machinery has the advantages of reliability 
and durability, but it occupies much space and the problem of getting 
rid of the heat is difficult to solve. The radius of action obtainable 
with steam power on a given weight of fuel is much smaller than with 
Diesel motors. 
The weight of Diesel engines as fitted in submarine boats is about 
65 lbs. per H.P. as compared with about 50 lbs. per H.P. for gasoline 
engines and from 50 to 60 Ibs. per H.P. for steam machinery inclusive 
of propellers and shafts. 
For under-water propulsion electric power derived from a storage 
battery of lead accumulators still offers the best solution. Since the 
first appearance of these cells they have been improved upon in many 
technical details, and are now reliable and durable. They will stand 
complete charging and discharging more than 400 times, and under 
ordinary peace-service conditions if carefully handled they may be 
expected to last about 5 or 6 years. The weight per H.P.-hour includ- 
ing outfit is by discharge in 34% hours about 80 lbs., practically the 
same as in early accumulators. Lead cells permit great variations in 
power and are at their best at low rates of discharge, a most valuable 
quality for submerged work. They can be stowed low in the boat and 
add thus considerably to the stability. They occupy about 0.4 cb. ft. 
