COGAG

COGAG stands for Combined Gas And Gas and refers to a naval propulsion arrangement that uses two gas turbine engines connected to a single propeller shaft through a reduction gearbox with engaging clutches. Each turbine can be operated independently or together, allowing different power and speed modes without changing the propulsion shaft configuration.

In a typical COGAG system, one turbine is sized for cruising efficiency while the other provides additional power for high-speed or rapid acceleration. The clutches and gearbox enable selecting one turbine, both turbines, or a reduced configuration as required by the mission or sea state. This setup offers a compact, high-power solution by keeping propulsion on a single shaft, simplifying mechanical drives relative to multi-shaft layouts.

Advantages of COGAG include high power density, fast throttle response and acceleration, and built-in redundancy since the ship can operate on a single turbine if the other is unavailable. It also enables relatively straightforward integration with existing shipboard electrical and control systems. Disadvantages include mechanical complexity, higher maintenance requirements, a heavier and more expensive gearbox, and the need for sophisticated control logic to synchronize turbine operation and clutch engagement.

COGAG is one of several gas-turbine based propulsion schemes used on naval vessels. Related arrangements include CODAG (diesel and gas) and COGAS (gas and steam), which offer alternative trade-offs in efficiency, reliability, and machinery layout.