Electrically driven ships?

Electrically driven ships?

We are probably all aware of the rise of electrically driven vehicles, including cars and even some prototype planes.

What if I told you that the majority of ships we cruise on are actually driven by large electrical motors!

So these motors aren’t driven by batteries and we certainly don’t have a cable attached to our last port of call. You might have heard of the term diesel electric drive before, this has been gaining popularity over the last couple of decades and more so on cruise ships than cargo vessels.

So what does diesel electric drive mean.

We have huge electric motors which drive the ship, normally 2 or 4 depending on the ship, which can either be located within pods hanging beneath the ship or within the hull driving propeller shafts.  These motors turn the propellers which thrust us towards our next sunny destination.

The power to supply these motors is generated by diesel engines, hence the term diesel electric.  All engines found on-board are just generating electrical power, we basically operate a floating power station.  Some of this power is used to supply the lights and entertainment systems and all services on-board, but the majority of the power is consumed by the electrical motors driving the vessel through the sea.

This might seem like a roundabout way of propelling a cruise ship but there are many advantages.

Say for example we are on an older conventional drive ship with propellers driven by the engines via a gear box.  We are island hopping around the Caribbean which normally means the vessel is sailing at a reduced speed.  The load on the engine might only be 40%.  Marine engines tend to be designed to run at about 85%, of course they can run at various loads but the turbo charger, fuel injection equipment, valve timing etc. is all optimised for operating conditions at 85%.  An engine operating at 40% will therefore not be as efficient and result in higher fuel consumption per nautical mile.

A modern cruise ship for example might have 6 diesel generators.  These produce power for the electrical motors, if we are sailing at reduced speed, rather than running the 6 generators at a low load we can just stop 3 and keep 3 running at a more optimum load therefore increasing efficiently, With 6 generators we can optimize the engine load at virtually all ship speeds.

As the engines on-board are just coupled to electrical alternators rather than propeller shafts or a gear box it gives us a greater flexibility of engine layout.  Allows us to optimize the given space which can increase space for cabins and therefore revenue.

The vessel is also very easy to manoeuvre if electrically driven. The control systems can alter the electrical frequency which changes the speed of the motors and even swap one of the 3 supply phases to quickly reverse the motor when required.

The motors used range in power up to 20MW (26820hp) which is more than the power of 200 ford fiestas.  Keep in mind modern ships will have at least 2 of these motors it really is a lot of power.

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