Feasibility of Diesel-Electric propulsion for large planning hull vessels?

Discussion in 'Boat Design' started by BayouBandit0, Jul 14, 2022.

  1. BayouBandit0
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    BayouBandit0 New Member

    I recently enjoyed reading an old thread concerning large planning vessels, where a lot of awesome knowledge was thrown around.
    What is world's biggest planning hull boat and how fast? https://www.boatdesign.net/threads/what-is-worlds-biggest-planning-hull-boat-and-how-fast.64476/

    I was curious as to the practicality of powering a large planning vessel with diesel-electric. Every example of large planning vessels I'm familiar with (Destriero, Fonors, Freedom Class LCS) all sport Gas Turbines to achieve the output necessary to reach their maximum speeds. Are there any examples of Vessels in excess of, lets say 130ft, that can travel in excess of 45 knots that use DE power? What are the limitations that make DE impractical for the application?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Welcome to the form Maurice,

    In a word - WEIGHT!
     
  3. bajansailor
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    bajansailor Marine Surveyor

    I am sure it would be possible - but not much point really if you have to fire up the diesel generator every 20 minutes (or less even) if you insist on travelling at full speed.
    This one in the link below could be a typical example perhaps? How many tonnes of batteries would she need to achieve 38 knots for say 20 minutes?
    Pershing 140 yacht tour: Inside a 10,400hp, 38-knot monster superyacht https://www.mby.com/video/pershing-140-yacht-tour-121863

    Edit - @BayouBandit0 - re the above, I was thinking about if you were operating the vessel off batteries only while in electric mode.
    Or are you talking about diesel engines running all the time to provide power for the electric propulsion motors?
    This is the usual arrangement for large cruise ships nowadays (ok, some of the newer ones now burn LNG rather than fuel oil), and the advantage here is that the diesel engines load can be infinitely varied between the speed demanded to reach the next port, and the hotel load required to keep the guests happy.
    However a large fast motor yacht like Destriero or the Pershing in the link above do not really have this requirement, and it would be easier to just have the diesel engines propelling the boat directly.
     
    Last edited: Jul 15, 2022
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  4. comfisherman
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    comfisherman Senior Member

    Until diesel to electric, electric to thrust becomes more efficient than diesel to propulsion... no.

    Throw a nu scale solid state mini nuke.... then yeah sure.
     
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  5. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Following on the other, better minds, in diesel propulsion the power goes through a transmission to the props. In DE, the engine power turns a generator or generators, the electrical output of which is routed to electric motors, which turn the props. The extra steps are heavy expensive and bulky, and have more power losses than a simple marine gear. As a system it makes sense in particular applications where getting power to the props is awkward, or you want to have huge generating capacity.
     
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  6. fallguy
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    fallguy Senior Member

    The real issue is what AH says; the weight required, or power to weight ratios. For every extra amount of storage you want for the electrical system; the battery weights increase more and more. Duh, right? Well, so look toward the ends of the concept. This in turn requires more power to maintain the speed, ceteris parabis. So, then just reduce the storage! Well, then you are no longer operating electrically as you follow the curve. On the opposite end, if you reduce the range of the vessel to nearly nothing; it can be done easier.

    So, what boats use less power for their weight? Simple. Displacement vessels. They don't really need a lot of extra power to keep a ton or 4 of extra weight, if (the design suits).

    In my opinion, the only way to offset this drama is to achieve storage some other way or lighter batteries by leaps than what we have today. Hydrogen, is of course an easy one to look towards. The creepy thing about using water is the environment, perhaps. Or the explosiveness, perhaps.

    Here is a 'claim' to a smaller system. But I am still wondering if any boat has done it.

    Sport Fishing Hybrid Package https://www.boatinternational.com/virtualboatshow/e-motion/sport-fishing-hybrid-package
     
  7. comfisherman
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    comfisherman Senior Member

    Hybrid seems to only be adopted successful in slow speeds where small increments of adjustment are needed. Or where the house load exceeds propulsion requirements. Until adding a step in the propulsion method is more efficient, high speed heavy vessels are going to use direct propulsion methods.

    Now if we start the discussion around power sources other than diesel it's a different story. If we could revisit using a reactor like the modern modular solid state reactors that utilize waste, electric becomes not only viable but potentially revolutionary. Debate will become what scares a culture more.
     
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  8. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    I wonder what the cradle-to-grave analysis of that hybrid system looks like. It looks like a massive investment in environmental damage, greenwashed in hybrid marketing.

    I'm all for SMR electric propulsion though. That would be awesome.
     
  9. bajansailor
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    bajansailor Marine Surveyor

  10. fallguy
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    fallguy Senior Member

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  11. BayouBandit0
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    BayouBandit0 New Member

    So if there's not a requirement for large amounts amounts of reserve electricity it's more practical to simply use conventional diesel propulsion with a shaft generator for hotel requirements? Or does the action of eliminating the batteries and electric motors to save weight not account for the subsequent need for a shaft generator and associated efficiency draw?
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member

  13. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm liking my Prius Prime and think it could translate into boat. Its an extra 300lbs of battery in a small car with 1.8L lawnmower engine with 98hp, plus big generator/motor so maybe total 400lbs extra. I guess it could make a boat plane but not with too much extra weight.
     
  14. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    An engine that makes 1 hp per pound is doing very well. Ditto for a generator and an electric motor. So a reasonable guess gives 300# of system, 100# of LiIon battery. Given the 40 to 1 weight advantage gasoline has over batteries, the range is roughly equivalent to all that, with a 1 gallon gas tank. Roughly.
     

  15. fallguy
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    fallguy Senior Member

    Am sure you meant well, but this is cryptic John..

    Are you saying 100# of batteries equals a gallon of gas?

    I do enjoy the parallel.

    The Skoota has 128 gallons of fuel full up. Is this equal to 12,800 pounds of batteries?

    My semi-displacement boat only has room for like 1000 pounds xtra or so before losing her lines. Not a planing hull, I can't do half that..
     
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