Energy distribution and losses; shaft power and brake power
Only a small percentage of the energy in the fuel is ultimately converted to Useful power. Most of the energy
in the fuel is dissipated to the surroundings as heat:
1 to 2% is lost to surroundings from hot surfaces
some heat is rejected with the exhaust:
about 10% in steam plants
25 to 30% in diesel plants
65 to 70% in gas turbine plants
some heat is rejected to the sea, via cooling water:
in steam plants, mostly at the condenser, up to 60%
in diesel plants, at engine coolers, about 30%.
in gas turbine plants. less than 1%, at lubricating-oil coolers
The balance is brake power output of the plant or engine, and, in steam plants, energy supplied to ship's
services:
about 30% in steam plants
up to 45% in diesel plants
about 30% in gas turbine plants
This value is the thermal efficiency of the plant, and is inversely related to the fuel consumption: as marine
propulsion plants, diesel engines are the most efficient, and have the lowest fuel consumption.
Except for the low-speed diesel, the brake power is provided at too high a speed for driving the propeller, and
is therefore reduced in speed, usually by gearing. About 1 to 2% of the brake power is dissipated in the
gearing. What is passed on is the shaft power.
There are further losses in bearings, perhaps another 1%, but there is a large loss at the propeller itself,
which, under the best circumstances, might convert only about 60 to 70% of the shaft power to thrust. As a
result, the thrust power will be 30% or less of the fuel energy in a diesel plant, and 20% or less in a steam
plant or gas turbine plant.