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Why do marine engines weigh so much for that power? An airplane uses ~100MW (depending on how you want to calculate it) but obviously its engines don't have a

Why do marine engines weigh so much for that power? An airplane uses ~100MW (depending on how you want to calculate it) but obviously its engines don't have a combined weight of 200 tonne.

dt5702 7y ago
The entirety of the drive train is huge. You have to turn two propellers which have to propel a ship weighing thousands of tonnes. To produce the necessary torque you need to have some big components. The supporting infrastructure you need to heat and circulate the fuel, then to supply it to the engines is another thing too. Gas turbines are much smaller but would be unsuitable for this particular application as they have good efficiency only at high loads. The range of loads this is used for would mean they are unsuitable.
- deleted 7y ago
  [deleted]
- walrus01 7y ago
  A number of very large cruise ships now have the drive entirely decoupled from the fuel burning engines. Same idea as a diesel electric locomotive. Big engine burns heavy fuel oil, turns generator, power goes to electric azipods.
  - fulafel 7y ago
    Smaller new vessels do this too.
perlgeek 7y ago
Aircraft engines are designed to be light, and sacrifice low cost and the ability to burn cheap diesel fuel for that. Ship engines are the way round: their weight doesn't matter much, but it matters that they use the cheapest fuel possible.
- sah2ed 7y ago
  > Aircraft engines are designed to be light, and sacrifice low cost and the ability to burn cheap diesel fuel for that. Small correction: aircraft engines generally use kerosene-based fuels (Jet A-1, Jet B). Also cost-wise, aircraft-grade kerosene costs more relative to other uses of kerosene since those applications can tolerate kerosene of lower quality than aircraft-grade kero. https://en.wikipedia.org/wiki/Aviation_fuel https://en.wikipedia.org/wiki/Aviation_fuel
  - perlgeek 7y ago
    That's what I meant by "sacrifice ... the ability to burn cheap diesel fuel". Probably worded ambiguously :(
    - davedx 7y ago
      Your sentence was fine, and I don't know if anyone thinks jet engines burn cheap diesel fuel, do they?
      - deleted 7y ago
        [deleted]
- pjc50 7y ago
  Exactly. In fact, for some applications GE are using modified aircraft turbines on ships: https://fas.org/man/dod-101/sys/ship/eng/lm2500.htm https://fas.org/man/dod-101/sys/ship/eng/lm2500.htm Those are for military use, where speed matters but money is basically unlimited.
  - rkangel 7y ago
    Money is unlimited, but fuel efficiency isn't just about money for the military. More fuel usage means more supply lines. That's less of an issue for navies than armies but still important.
- seminatl 7y ago

Why do marine engines weigh so much for that power? An airplane uses ~100MW (depending on how you want to calculate it) but obviously its engines don't have a combined weight of 200 tonne.

dt5702 7y ago

The entirety of the drive train is huge. You have to turn two propellers which have to propel a ship weighing thousands of tonnes. To produce the necessary torque you need to have some big components. The supporting infrastructure you need to heat and circulate the fuel, then to supply it to the engines is another thing too. Gas turbines are much smaller but would be unsuitable for this particular application as they have good efficiency only at high loads. The range of loads this is used for would mean they are unsuitable.

deleted 7y ago
[deleted]
walrus01 7y ago
A number of very large cruise ships now have the drive entirely decoupled from the fuel burning engines. Same idea as a diesel electric locomotive. Big engine burns heavy fuel oil, turns generator, power goes to electric azipods.
- fulafel 7y ago
  Smaller new vessels do this too.

perlgeek 7y ago

Aircraft engines are designed to be light, and sacrifice low cost and the ability to burn cheap diesel fuel for that. Ship engines are the way round: their weight doesn't matter much, but it matters that they use the cheapest fuel possible.

sah2ed 7y ago
> Aircraft engines are designed to be light, and sacrifice low cost and the ability to burn cheap diesel fuel for that. Small correction: aircraft engines generally use kerosene-based fuels (Jet A-1, Jet B). Also cost-wise, aircraft-grade kerosene costs more relative to other uses of kerosene since those applications can tolerate kerosene of lower quality than aircraft-grade kero. https://en.wikipedia.org/wiki/Aviation_fuel https://en.wikipedia.org/wiki/Aviation_fuel
- perlgeek 7y ago
  That's what I meant by "sacrifice ... the ability to burn cheap diesel fuel". Probably worded ambiguously :(
  - davedx 7y ago
    Your sentence was fine, and I don't know if anyone thinks jet engines burn cheap diesel fuel, do they?
    - deleted 7y ago
      [deleted]
pjc50 7y ago
Exactly. In fact, for some applications GE are using modified aircraft turbines on ships: https://fas.org/man/dod-101/sys/ship/eng/lm2500.htm https://fas.org/man/dod-101/sys/ship/eng/lm2500.htm Those are for military use, where speed matters but money is basically unlimited.
- rkangel 7y ago
  Money is unlimited, but fuel efficiency isn't just about money for the military. More fuel usage means more supply lines. That's less of an issue for navies than armies but still important.
seminatl 7y ago