Which is the most economical kind of small vessel?

Discussion in 'Boat Design' started by Guest, Jul 24, 2003.

  1. Guest

    Guest Guest

    Which kind of vessel do you people think is the most economical:

    1: Classical large cargo-ship type. Slow-moving, displacement type.

    2: Hydrofoil

    3: Hovercraft

    4: Regular speed-boat

    5: Ground-effect aircraft

    6: Submarine

    7: Other?

    With "economical" I mean for the vessel to be able to transport 500kg of cargo across the atlantic. Speed is not important.

    They have different advantages:

    Hovercraft/ground effect aircraft: No viscous drag from water, and no wave making drag.

    Hydrofoil/Speedboat: Less waves and water drag.

    Submarine: Little wave-making drag.


    I think my bet is on the submarine.
     
  2. Guest

    Guest Guest

    clearification

    Maybe I should have written that I mean energy-conservative when I say "economical".
     
  3. gonzo
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    gonzo Senior Member

    500 Kg of cargo is minimal compared to the load of fuel, water, fuel and other necessities for the crew. A submarine would have to be nuclear powered to stay below for such a long time. Are you putting any other restrictions to your query? I think that a displacement sailboat is the most economical; particularly for a small load.
     
  4. tspeer
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    tspeer Senior Member

    There's actually been a lot of research on this topic, most of it starting with the work of Von Karman and Gabrielli (Gabrielli, G. and von Karman, T. (1950). What price speed? Mech. Eng. 72, 775-781). They found there seems to be a limit to transport performance of all kinds of locomotion. This is variously expressed as speed * Lift/Drag = constant or speed*fuel consumption*payload = constant.

    Here are some examples applied to
    submarines: http://www.acq.osd.mil/dsb/suboffuture.pdf
    SES ships: http://www.se-technology.com/wig/html/main.php?open=commercial&code=0

    Since you specified no time limit, the winner will be the long, narrow displacement hull. It's hard to beat this for moving bulk cargo, because the drag goes to zero at zero speed and the L/D goes to infinity. If fuel costs are greater than payroll costs, you can make it even cheaper by using sail power. Yep, you're right back to the age of the clipper ship.

    I put my money on #7.
     
  5. SailDesign
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    SailDesign Old Phart! Stay upwind..

    The problem with "time not an otion" is that it becomes a theoretical exercise. If you had to move X tons of material every day, then the equation is very differnet to that if you have to move X pounds every day.
    As gonzo says, crew resources are an important factor, and the cost of making fresh water vs. carrying sufficient for the trip may have an effect on the "economy".
    You should look at "cost per ton-mile" as a relative factor. Add in ALL costs, crew wages, crew life-support (food, etc.), cost of vessel, lifespan of vessel, machinery purchase price and maintenance costs, and so on. Then divide this figure by the number of tons you could hve carried times the number of miles you would have travelled (remember to allow for maintenance days/weeks) and you will have an overall cost per ton-mile for that vessel.
    It can be scary....

    Steve
     
  6. gonzo
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    gonzo Senior Member

    In the theorethical realm, it would be possible to float the cargo across the ocean. The currents would eventually carry it across where the cargo could be retrieved. Of course the current pattern is limiting, but in a world without thieves this system would work.
     
  7. Guest

    Guest Guest

    I really appreciate all these replies! Much more than I had expected.

    I wasn't thinking economy as in "money", just as in fuel-economy, energy-efficient. How many joules have to be expended to transport 500kg from some specified location in europe to some specified location on the american east-coast?

    If using a submarine, it would be allowed to go to the surface at any time, to replenish air-supply.

    I've done some simulation with the michlet program to determine which is best, submarine or surface vessel. It seems that skin drag is greater on the submarine because of its greater surface area / volume.

    Maybe we should put a time-limit on the journey, so that a message-in-a-bottle design is disqualified :)... Say a maximum time of 6 months.

    I'm pretty sure an unmanned craft will be more fuel-efficient (if it doesn't break down halfway over :)).

    According to
    http://www.trektracker.com/WS_HomePg1.htm
    it's 2500 miles across the atlantic (somewhere).

    According to the michlet-program a 3 metric tonne craft travelling at 1.6m/s needs a thrust of 0.18kN.

    2500 miles = 4000km.

    4000e3m*180N = 0.72GigaJoules (=approx 200litres of diesel (approx 50 gallons), at 10% engine efficiency)

    Anyone think it can be done with substantially less energy?
     
  8. Portager
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    Portager Senior Member

    I think your calculations fail to consider the effect of waves. The displacement vessel will be more efficient than the submarine in dead calm conditions, however when you add in sea states and winds the balance reverses.

    Diesel electric submarine can and have crossed the Atlantic submerged using diesel power and their snorkel.

    I think you could have a smaller craft than 3 tons to carry 1000 kg of cargo, although it will depend a lot on the cargo density, which you haven’t specified yet. Low density cargo like mail and electronic equipment in boxes with crush space and light weight packing material will be a bigger problem than high density cargo, especially for the submarine, which must have sufficient mass to sink the cargo.

    If you’re interested in the optimum unmanned vehicle, then I think a semi-submersible would be the optimum. Rockwell built one for the US Navy for a remote controlled mine hunter. It worked quite well when it was in the water, but launch and recovery was a pain. It was a torpedo shape with a mast that penetrated the surface for the diesel engine air intake/antenna and a keel for ballast. A significant advantage of this design is it can be made nearly immune to storm conditions.

    If you are going to send an unmanned vehicle across the Ocean, you are going to need really good communications and reliable navigation. I’d consider Iridium and HF radio since they are the only truly global system. Running lights and a beacon that says, “Vessel running blind” would be a good idea also.

    Regards;
    Mike Schooley
     
  9. tspeer
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    tspeer Senior Member

    Actually, this problem is solved for real every day. Just look at the kinds of boats people use for cruising. They have pretty much the same criteria - has to be economical and seaworthy, time is not the most pressing constraint, but they still want to cover long distances carrying a reasonable load.

    The vast majority settle on the sailing yacht.

    Given how difficult it is to operate underwater, I think the advantage of the sub is it can cruise beneath the waves - provided it doesn't have to draw air from the surface. So a sub would make sense if your operating area is the Southern Ocean.
     
  10. dionysis
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    dionysis Senior Member

    I don't know whether this is beside the point, but will you include the energy expended in producing the vehicle? A sub, yacht or any moderately complicated vessel can take a lot of energy to build, both in manufacture and materials.
     
  11. tspeer
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    tspeer Senior Member

    If you want to include the energy of building the vessel, then you have to start dealing with the question, "Over how many trips will the building energy be amortized?"

    Wood construction is pretty energy efficient to produce, and I suspect modern composite materials are, too. Bottom of the list has got to be aluminum.

    Energy = cost. The whole point of economics is how to optimize over many conflicting and competing criteria.

    So the most energy efficient solution for carrying a modest payload over large distances still comes back to the classic wooden sailing yacht. And the vast majority of the energy required to build and operate it comes from renewable sources!
     
  12. dionysis
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    dionysis Senior Member

    Yes, they are grand!
     
  13. Guest

    Guest Guest

    Portager: Very interesting!

    As you might have guessed, I'm really interested in making a small autonomous vessel. The remote controlled mine-hunter that you described is almost exactly the kind of vessel that I'd like to build!

    Having a keel seems like a reasonable design decision.

    What is HF-radio?

    I was thinking Sattelite HAM-radio. As I understand it's got really great coverage, and doesn't require large transmission power.

    How about this idea:
    Include a very small diesel engine. Include lead-batteries and an electric motor. The craft runs for apx 20hours submerged, then surfaces and starts diesel engine to recharge for about 3hours. The the cycle is repeated. Let the snorkel be a long pipe sticking out of the front of the craft. Make the center of gravity of the craft be exactly at the center of buyancy. Then it's possible to make the craft stand up in the water simply by displacing a weight to the rear of the vessel. This way the snorkel doesn't have to protrude upwards from the craft and it can be more hydrodynamical. And it might be easier to design than to design for the snorkel to be raisable.
     
  14. Portager
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    Portager Senior Member

    Interesting idea, but unless you need to dive deep, I think it would be easier to use a fixed vertical mast with a water trap. If the engine can breathe and run all the time you can use a smaller engine and eliminate the batteries.

    How would you maintain neutral buoyancy as you consume fuel? The conventional approach is to provide ballast tanks which are empty when the tanks are full and you flood as you consume fuel, of course you need additional mass to make the overall system neutrally buoyant. I would use a bladder tank for the fuel and as fuel is consumed from the bladder allow sea water to fill the void in the tank. By adding a little air to compensate for the difference in density of fuel and sea water you maintain neutral buoyancy with minimal additional mass.

    Regards;
    Mike Schooley
     

  15. dionysis
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    dionysis Senior Member

    if you have an "all the time up air pipe" then you have the problem of the sub having to follow the contour of waves and swell. This will consume quite a bit of energy, in terms of friction and induced drag of plane rudders, in going up and down.
     
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