Combined diesel-electric and gas
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COmbined Diesel-eLectric And Gas (CODLAG) is a modification of the combined diesel and gas propulsion system for ships.
A CODLAG system employs electric motors which are connected to the propeller shafts (usually 2). The motors are powered by diesel generators. For higher speeds, a gas turbine powers the shafts via a cross-connecting gearbox; for cruise speed, the drive train of the turbine is disengaged with clutches.
This arrangement combines the diesel engines used for propulsion and for electric power generation, greatly reducing service cost, since it reduces the number of different diesel engines and electric motors require considerably less maintenance. Also electric motors work efficiently over a wide range of revolutions and can be connected directly to the propeller shaft so that simpler gearboxes can be used to combine the mechanical output of turbine and diesel-electric systems.
Another advantage of the diesel-electric transmission is that without the need of a mechanical connection, the diesel generators can be decoupled acousticly from the hull of the ship, making it less noisy. This has been used extensively by military submarines but surface naval vessels like anti-submarine vessels will benefit as well. Usually CODLAG ships are equipped with rechargeable batteries like diesel-electric submarines as well, allowing it to manoeuvre in silence without any heavy machinery running.
A system which employs the gas turbines as turbo generators, i.e. like the diesel engines without mechanical transmission to the propellers, is not classified as CODLAG. Some passenger ships like the RMS Queen Mary 2 use this configuration with a pool of diesel generators for the base load and turbo generators for peak power.
[edit] List of CODLAG ships
- Type 23 frigate (Royal Navy)
- F125 class frigate (German Navy)
- GTS Finnjet (Finnish cruiseferry)
