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Why not use diesel to run the engines directly?


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If I understand it correctly, cruise ships use fuel to generate electricity, which in turn is used to run the engines that move the ship.

 

Why the step of generating electricity in between? To me it seems to would just add more reasons for failure, more space to be used, and especially useless heat created while going from fuel to electricity to propulsion. Maybe the extra fuel costs are limited to just a very small percentage but even in that case I can't see a reason why electrical propulsion could do a better job than the diesel engine itself. So why's that? :)

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If I understand it correctly, cruise ships use fuel to generate electricity, which in turn is used to run the engines that move the ship.

 

Why the step of generating electricity in between? To me it seems to would just add more reasons for failure, more space to be used, and especially useless heat created while going from fuel to electricity to propulsion. Maybe the extra fuel costs are limited to just a very small percentage but even in that case I can't see a reason why electrical propulsion could do a better job than the diesel engine itself. So why's that? :)

 

1 - Electricity gives them better control of the ships speed

2 - The heat is use to generate fresh water and heat the laundry system

3 - The bulk of everything is runs electricity.

4 - Most ships have 4 to 6 diesels to generate electricity.

5 - Direct coupling diesels to the propellers is very expensive to do and maintain.

6 - Are more fuel efficient when run at one speed or load

 

There are many other reasons but I can't remember call.

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I was gonna say that they couldn't find anyone big enough to step on the clutch to shift gears...:) but on a more serious note...

 

Diesel-electric is a very mature technology. Over the years it has only gotten better.

 

Control is the main issue, as stated in the above post.

 

Virtually all drives for very large equipment are diesel-electric. Locomotives, container ships, navy vessels (tho some are nuclear), excavating machinery, to name a few.

 

Point of fact....if direct drive was cheaper, industry would be using it.

Edited by thinfool
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Another form of propulsion used on some cruise ships and navy ships is gas turbine. A jet engine that drives a generator that powers an electric motor to power and drive the ship. The advantage of gas turbines is much smaller size and higher power output compared with huge Diesel engines. The down side is that they are not as fuel efficient as Diesels.

 

Going back to 1935, the French Line's "Normandie" used turbo-electric propulsion, with steam turbines running generators to power electric motors to propel the ship. While she eventually lost the Blue Ribband to Queen Mary she was using far less fuel, and was far more efficient as a ship of similar size.

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Another form of propulsion used on some cruise ships and navy ships is gas turbine. A jet engine that drives a generator that powers an electric motor to power and drive the ship. The advantage of gas turbines is much smaller size and higher power output compared with huge Diesel engines. The down side is that they are not as fuel efficient as Diesels.

 

Going back to 1935, the French Line's "Normandie" used turbo-electric propulsion, with steam turbines running generators to power electric motors to propel the ship. While she eventually lost the Blue Ribband to Queen Mary she was using far less fuel, and was far more efficient as a ship of similar size.

 

 

The Destroyer I was on in the Navy had 7 gas turbines. 3 "small" ones used for electric power and 4 big ones used for propulsion. The propulsion ones (2 for each shaft) were connected to the shaft by reduction gears. You needed to have a reversable pitch prop to change direction as the shafts only spun one way.

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RCI tried gas turbines on two of their ships a few year ago as an experiment. Maintenance costs where high and the crews had a hard time doing maintenance.

 

Sent from my SGH-I317M using Forums mobile app

Edited by Kamloops50
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Thank you all for your elaborate answers. It find it quite amazing that a site like this has experts on every detail regarding cruising, not just experts on tipping behavior or formal nights, and so many people willing to take the time to answer each and every question.

 

Point of fact....if direct drive was cheaper, industry would be using it.

 

I'm totally sure of that :)

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I worked in the shipyard for 12 years during the 1970's & 80's. I worked on many World War II era oil tankers that were powered by electric motors. These tankers were designed during World War II when steel was at a premium. Steam turbines were used to turn generators and the electricity was used to turn electric motors that drove the propellers. By doing it this way, the ships did not need the heavy reduction gears necessary to take the high-speed turbine revolutions and slow them down to drive a propeller. I was also involved with converting the QE2 to diesel electric drive. The main purpose there was fuel savings and reliability. They could run as few as 1 diesel generator to provide power to the electric drive. As more speed was necessary more generators were put online. The QE2 has 9 diesel generators for propulsion, so even if 8 generators were not capable of running, the ship would be able to maneuver at a very slow speed. The biggest advantage to diesel electric drive is reliability.

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  • 4 weeks later...
RCI tried gas turbines on two of their ships a few year ago as an experiment. Maintenance costs where high and the crews had a hard time doing maintenance.

 

Sent from my SGH-I317M using Forums mobile app

 

It turned out to be a lot more than an experiment! Actually RCI has 4 ships, the Radiance class that are powered solely by gas turbines and Celebrity has 4 ships in the Millennium class powered by the same gas turbines as well. Originally the gas turbines supplied all electrical power for propulsion and hotel use. Using the gas turbines to supply hotel power while in port was so inefficient that a few years ago Royal retro fitted all of the Radiance class ships with diesel generators to operate for hotel use while in port. I think they did the same for the Millennium class ships too.

 

Princess on the other hand was a bit more conservative in their use of gas turbines. The Island and the Coral have 1 gas turbine installed, they are used in conjunction with diesel engines to provide electricity for propulsion and hotel needs.

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While in the Coast Guard in the '70s, I served on a 311' which was a former WWII Navy ship. It had four 16-cylinder direct drive diesel engines in two engine rooms. Two engines in one engineroom drove the starboard shaft, and the other two in the other engineroom drove the port shaft. To go from forward to reverse the engines had to be brought to a stop by the throttleman, and then started again in the opposite direction. Some ports required numerous changes which really was a workout manhandling the throttles. We could run combinations of engines from two to four, if required for maintenance.

 

With a diesel-electric ship, they essentially turn a knob. It allows numerous combinations of engines.

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Diesel (and gas) engines run efficiently only over a narrow range of RPMs. However, ships have to be able to travel at various speeds. In addition, they need a lot of electricity for other services.

 

The solution is to have engines, typically gas turbine engines, similar to jet engines, running at optimal RPM. These are very efficient, and can be used to generate electricity at low fuel costs. Standard diesel engines can also be used, and most ships also have smaller diesel engines for use while in port.

 

Electric Azipods are extremely efficient as the propeller faces forward and has no turbulence from the connecting structures. This would be difficult or impossible with a direct drive system. Azipods can be rotated 360 degrees to provide thrust in any direction for docking or maintaining station. This would be difficult or impossible with a direct drive system.

 

Most large scale propulsion systems such as freight trains, and cargo ships use the more efficient electric drive supported by fossil fuel power generation systems. Separating the power generation from power usage allows the engines to run at maximum efficiency, while providing flexibility and fine control over thrust direction and power.

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  • 2 weeks later...
Thank you all for your elaborate answers. It find it quite amazing that a site like this has experts on every detail regarding cruising, not just experts on tipping behavior or formal nights, and so many people willing to take the time to answer each and every question.

 

 

 

 

 

 

 

I'm totally sure of that :)

 

 

Engineers need vacations too!

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Internal combustion engines have a narrow range of RPMs where they are most efficient. The diesel-electric configuration allows the engines to run a optimal RPMs most of the time thus saving fuel. This also allows for engines to be easily taken offline for maintenance and servicing without impacting propulsion or ship services. When energy needs are lower, fewer engines can be run. When more speed is needed, extra engines can be turned on....all running at optimal RPMs.

 

Also, many engines on ships are turbine engines which do not operate well at low RPMs. This would cause problems during low speed operations, or when accelerating.

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Not to mention long stroke diesels have reached efficiency levels of 50 to 55%, in other words, 50 to 55% of the heat produced by burning fuel is converted to work. They then use the waste exhaust heat to flash evaporate sea water into fresh water and heat the hot water used on the ship.

 

Turbines have great efficiency at full power, but use nearly as much fuel at idle as they do as full power, That's why you'll see airliners only use one engine to taxi now, and start the second engine just before turning onto the runway. Gas turbine cruise ships (Radiance and Millenium) used waste exhaust heat to run a steam generator that powered another steam turbine, as well as making fresh water and hot water.

 

Another thing, diesels can burn bunker oil, #5 or #6 diesel, which is essentially the tar left over from distilling crude oil, while gas turbines need relatively highly refined kerosene distillates. Bunker oil has to actually be heated to 150F to make it thin enough to inject into the engine cylinders, and is very cheap compared to kerosene.

 

Direct drive is more efficient, that's why cargo ships use it, a huge engine and screw that turn at 105 RPM, but cargo ships drive a straight line between ports, at constant speed.

 

http://newatlas.com/go/3263/

Edited by wraithe
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navy vessels (tho some are nuclear)

 

US Navy vessels are either steam turbines coupled directly to the shafts and propellors. Steam can be generated by boilers burning Bunker fuel (VERY thick oil, almost asphalt thick) or by nuclear.

 

Others are gas turbine powered, think turbo prop airplane.

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Another form of propulsion used on some cruise ships and navy ships is gas turbine. A jet engine that drives a generator that powers an electric motor to power and drive the ship. The advantage of gas turbines is much smaller size and higher power output compared with huge Diesel engines. The down side is that they are not as fuel efficient as Diesels.

 

What cruise ship uses gas turbines?

 

And US Navy gas turbine ships are typically direct drive, gas turbine, through transmission to shafts and props. Not gas turbine to electric.

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What cruise ship uses gas turbines?

 

 

 

And US Navy gas turbine ships are typically direct drive, gas turbine, through transmission to shafts and props. Not gas turbine to electric.

 

 

 

RCL and Celebrity have a few ships that use gas turbines. Many ships use Asa turbine as an auxiliary power source.

 

 

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What cruise ship uses gas turbines?

 

And US Navy gas turbine ships are typically direct drive, gas turbine, through transmission to shafts and props. Not gas turbine to electric.

 

RCCL Radiance class and Celebrity Millenium class use hybrid, gas turbines with steam generated from the exhaust running another turbine. The turbines all generate eectricity.

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Compare the price of IFO 380 ( The fuel diesel cruise ships burn) to the cost of MGO (the fuel that gas turbine ships burn ) and you will see why both Royal and Celeberty changed back to diesel power for their new ships (oasis class and solistce class ships).The website ship and bunker will tell you current prices of the 2 fuels for comparision.

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Just got off the Celebrity Millennium a few days ago. Was claimed by the ship officers that the ship was quiet due to being powered by 2 gas turbines (claimed to be the marine equivalent of a 757 engine, so either a R-R or less likely, P&W engines). The other was a 16 cylinder Wartsilla diesel. All I heard for most of the cruise (a TPAC) was the rather-distinct thump of a large diesel, and this was in the top deck.

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