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Author Topic: Electric propultion (ships)  (Read 3542 times)
LB5KE
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« on: January 31, 2012, 03:37:27 PM »

I saw on a program that most larger cruise ships have electric propulsion. The electric motor is driven by generators driven by turbine engines. No i read about the http://en.wikipedia.org/wiki/HMS_Dauntless_%28D33%29 (Destroyer) also having electric motors driving propellers. Does any one know the reason for this?
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M0HCN
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« Reply #1 on: January 31, 2012, 04:23:47 PM »

There are several.

Firstly Cruise ships have an interesting usage pattern for power and propulsion. On board power demand peaks in the early evening (All the entertainments, lighting and such) and can use many Megawatts of total power, but propulsion demand peaks in the very early morning when the ship is sprinting to get to its next port of call....
Thus one (redundant) set of generator plant can be put to both uses, at a considerable improvement in overall efficiency.

Second, the electric propulsion does away with a very complex and expensive gearbox (Big ships usually run very low shaft speeds, turbines not so much), and electric propulsion allows (almost requires) that the prime movers run at constant speed which is enough of an efficiency win that it covers the extra conversions required.  Electric also allows relatively simple switching of power between prime movers and shafts so loosing a generator does not take a shaft out of action where in a directly coupled system loss of a engine has a far bigger effect.

Third, it allows for much more flexibility in the layout of the machinery spaces as the motors can be placed almost right next to the stern glands, without needing much in the way of shaft tunnels and such. Very large multi pole induction machines have speed ranges that are a good match for marine propulsion.   

The downsides are that heavy current electrics in a salt water environment are ah 'Fun', marine engineers do not historically have much heavy electrical power experience (This is slowly changing), and that relatively minor flooding can cause much chaos (See also computer controls for more on this).

The MAIB has a few writeups of incidents that are worth pursuing for some of the issues.

Regards, Dan.
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NA0AA
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« Reply #2 on: January 31, 2012, 07:10:30 PM »

Goes without saying that this type of design was pioneered in submarines, both diesel and nuke designs.  At least one ice breaker, the USCG Glacier had dual screw diesel/electric drive.  In the case of the Glacier, because ice breakers back up a lot in preparation to ram, it's much easier to use electric drive as they can reverse direction much more quickly.  Plus, with up to 12 prime movers [diesel-electric generator sets], you can run only the number of PM's for your needed power demands.

It certainly will simply power management overall I would think.  I wonder if it's safer than high pressure steam?



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STAYVERTICAL
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« Reply #3 on: January 31, 2012, 10:17:01 PM »

They decided to use squirrel cage electric motors because the ship is usually full of nuts!

73s
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SWMAN
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« Reply #4 on: February 01, 2012, 03:48:36 AM »

Dan,
Do you happen to know if this is the same way that the big desile train engines work ? I heard they are very similar ! Just curious.
Thanks 73 Jim W5JJG
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AG6WT
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« Reply #5 on: February 01, 2012, 07:32:28 AM »

Dan,
Do you happen to know if this is the same way that the big desile train engines work ? I heard they are very similar ! Just curious.
Thanks 73 Jim W5JJG

Diesel trains have electric motors in the trucks. The upper part is one or more diesel powered electric generators.
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G3RZP
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« Reply #6 on: February 01, 2012, 09:53:59 AM »

I read somewhere that US Navy had at least one battle wagon around WW2 time with electric propulsion. I think it was because you can spin up motors faster than a steam turbine.
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M0HCN
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« Reply #7 on: February 01, 2012, 11:34:01 AM »

Cold start is MUCH faster for anything that does not involve steam, for all that big marine diesels cans can still take quite some time (You have to heat the engine before starting it otherwise you risk cracking the castings, and may have to build air pressure in the air start accumulators, can take hours, where large steam can take days).

Current state of the fuel economy art is actually really BIG two stroke blown diesels, running directly at shaft speed with hydraulic valve gear which allows the engine to be operated either forward or reverse by changing the valve timing. 
The downside is throttle response and single point of failure (Also single screw ships are often tricky to manoeuvre astern), but the really big container ships seem to be favouring this technology. The things run at less then 100RPM typically.

There have been steam/electric passenger ships built in the past, usually big DC machines at the time, and current naval construction is quite often worked this way, HMS Albion springs immediately to mind as a reasonable modern example. Multiple diesel prime movers driving a really BIG set of inverter drives to the propulsion motors.

Electric rail traction tends to distribute the motors across many driven wheels (at least in UK passenger service) with a low frequency generator making up the bulk of the locomotive proper. The advantage is the removal of the need for a gearbox and clutch capable of handling the very high starting torque.

73, Dan.
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N2EY
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« Reply #8 on: February 01, 2012, 02:27:03 PM »

Current state of the fuel economy art is actually really BIG two stroke blown diesels, running directly at shaft speed with hydraulic valve gear which allows the engine to be operated either forward or reverse by changing the valve timing. 

Like this:

http://www.autoblog.com/2011/07/22/worlds-largest-diesel-engine-makes-109-000-horsepower/

Electric rail traction tends to distribute the motors across many driven wheels (at least in UK passenger service) with a low frequency generator making up the bulk of the locomotive proper. The advantage is the removal of the need for a gearbox and clutch capable of handling the very high starting torque.

Diesel-electric locomotives date back to at least the 1930s. There's really no practical way to use a gearbox and clutch in such applications anyway.

The first generation diesel-electrics used a DC generator and DC motors. Then the DC generator was replaced by an alternator and rectifiers, because the latter are more efficient and require less maintenance. Current design is an alternator, controlled inverter and AC motors, eliminating brushes and all the problems involved, improving slip control and many other advantages such as dynamic braking.

To give you an idea how efficient they are, one US freight railroad totalled up all the diesel fuel their locomotives used in a year, the total freight carried that year, and found they moved 1 ton of freight 422 miles per gallon of diesel fuel. That includes empty backhauls, switching, idling, etc.

73 de Jim, N2EY
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AJ4CU
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« Reply #9 on: February 08, 2012, 07:38:38 PM »

Hello all,

Does anyone remember the really cool lightships that used to mark dangerous shoals up and down the US coast? Most all of them were diesel electric...
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K2OWK
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« Reply #10 on: February 08, 2012, 08:58:17 PM »

Don't forget another reason for the use of diesel electric trains (locomotives) such as the Long Island Railroad. When operating in New York City it uses the electric rail system available. When it leaves the city it uses the diesel electric system. This is a very economical operation.

73s

K2OWK
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W8JX
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« Reply #11 on: February 08, 2012, 10:09:47 PM »

The main reason of a diesel electric train is the power, controllablity and reliability of electric traction motors. Without this there would be no diesel/electric locomotives and we might still be using steam. Nothing was ever wrong with power of a steam locomotive but maintenance costs were very high. A few are actually studying a modern clean steam designs with fuel prices climbing and us sitting on basically unlimited coal.
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KB4QAA
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« Reply #12 on: February 08, 2012, 11:31:25 PM »

I read somewhere that US Navy had at least one battle wagon around WW2 time with electric propulsion. I think it was because you can spin up motors faster than a steam turbine.
They did indeed!   The US Navy has entire classes of auxiliary and warships that were electric drive in the 1920's and 1930'.
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W8JX
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« Reply #13 on: February 09, 2012, 05:32:14 AM »

I read somewhere that US Navy had at least one battle wagon around WW2 time with electric propulsion. I think it was because you can spin up motors faster than a steam turbine.
They did indeed!   The US Navy has entire classes of auxiliary and warships that were electric drive in the 1920's and 1930'.

Yes but they were powered by steam powered turbines running generators. The turbines could be kept spun up and excess power not needed switched to a load bank so in effect it could idle at full power and switch from load banks to drive switch on demand and not wait for steam to build up. Very inefficient though at lower speeds. Some large aircraft such as post WW2 B36 used this concept in that generators always ran and full output and load bank in aircraft converted excess power to heat and vented it into slipstream in flight.
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AG6WT
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« Reply #14 on: February 09, 2012, 07:24:55 AM »

The main reason of a diesel electric train is the power, controllablity and reliability of electric traction motors. Without this there would be no diesel/electric locomotives and we might still be using steam. Nothing was ever wrong with power of a steam locomotive but maintenance costs were very high. A few are actually studying a modern clean steam designs with fuel prices climbing and us sitting on basically unlimited coal.

Let's hope the next president is coal friendly. The current one has avowed to shut down existing and future coal fueled power plants through draconian regulation and taxation.
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