The conventional drive train
The main diesel engine on board is basically used for propulsion, so solely to drive the propeller. The thruster and, for example, a cargo pump, make use of separate diesel engines. The conventional drive train thus has only a few links, which together, determine the propulsion efficiency. Calculating the required power of the drive train and determining the optimum propeller/diesel engine combination is mainly based on a full load, at a certain speed - typically full power - and deep water. The result: an engine, which within a very limited speed range at full power, can deliver optimum fuel efficiency and an attuned propeller that can convert that available power optimally in terms of propelling torque.
A powerful combination… provided that the power demand is virtually constant within the optimal, but limited, speed range. Therefore on longer stretches, well-loaded and without making continual use of the available energy. If the power demand is beyond that optimum speed range, the decrease in the efficiency of the propeller/engine combination is very fast and very sharp.
Overall, it can be stated that, in an optimum situation, with a propeller/diesel-combination of the energy in the fuel, approximately 22% is converted into thrust. When the ship is more lightly loaded, or for any reason whatsoever beyond the optimum speed range, this efficiency drops significantly. Deviations from the optimum speed and the associated loss of efficiency in daily practice are all too common. The power demand is influenced by very changeable conditions such as water supply (vessel speed, up- and downstream), irregularities on the hull, load factor, water depth, etc. The only control the skipper has is the speed. If the circumstances therefore give rise to 'slowing down' for a short while, then the efficiency of the traditional propeller/engine combination decreases on average from 22% to 17% or even lower. |