Marine gas turbines use the same basic idea as jet engines, but instead of pushing an aircraft through the air, they spin a shaft that drives a ship's propeller. They are valued on warships and fast ferries because they deliver high power from a relatively compact and lightweight engine. This gives a vessel rapid acceleration, high top speed, and the ability to respond quickly to changing mission needs.
The tradeoff is that gas turbines usually burn more fuel than diesel engines at low power.
Key Facts
- Gas turbine cycle: air intake, compression, combustion, expansion through turbine, exhaust.
- Power relation: P = τω, where P is power, τ is torque, and ω is angular speed.
- Propeller shaft speed is reduced by a gearbox because turbines spin much faster than propellers.
- Thermal efficiency: η = useful output energy / fuel input energy.
- Combined plants such as CODOG use diesel engines for cruising and gas turbines for high speed.
- For a ship moving at constant speed, useful propulsive power is approximately P = Fv.
Vocabulary
- Gas turbine
- A heat engine that compresses air, burns fuel in it, and uses the hot expanding gas to spin turbine blades.
- Reduction gearbox
- A gear system that lowers the very high turbine rotation speed to a slower speed suitable for the propeller shaft.
- Propeller shaft
- A rotating shaft that carries mechanical power from the engine or gearbox to the ship's propeller.
- CODOG
- Combined diesel or gas propulsion is a system where a ship uses diesel engines for efficient cruising or gas turbines for high speed, but not both at the same time on the same shaft.
- Specific fuel consumption
- A measure of how much fuel an engine uses to produce a given amount of power for a given time.
Common Mistakes to Avoid
- Thinking the turbine exhaust directly pushes the ship, which is wrong because most marine gas turbines drive a shaft through a gearbox rather than using jet thrust.
- Ignoring the reduction gearbox, which is wrong because a turbine can spin at thousands of revolutions per minute while a large propeller must turn much more slowly to work efficiently.
- Assuming gas turbines are always the most fuel efficient choice, which is wrong because diesel engines are often more efficient during slow cruising and low power operation.
- Treating power and speed as proportional, which is wrong because the power needed by a ship usually rises much faster than speed due to increasing drag.
Practice Questions
- 1 A gas turbine delivers 24 MW to a reduction gearbox that is 95 percent efficient. How much power reaches the propeller shaft?
- 2 A turbine shaft spins at 9000 rpm and the reduction gearbox ratio is 30:1. What is the propeller shaft speed in rpm?
- 3 A patrol ship can cruise on diesel engines or switch to a gas turbine for sprint speed. Explain why the designers might choose this combined arrangement instead of using only gas turbines.