Solar Power and Scale Factor

Discussion in 'Boat Design' started by alan craig, Jan 25, 2017.

  1. DennisRB
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    DennisRB Senior Member

    You seem to agree the ships will be grossly under powered. My post demonstrates that the underpowering is worse with the large ship as per the scaling laws. It has 25% less power per ton. It has less power per unit of wsa.

    Therefore if trying to make a solar powered vessel that actually works, the statement holds true that smaller is better.
     
  2. fredrosse
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    fredrosse USACE Steam

    Bigger is Better

    A statement that the smaller boat is "Better" needs to have some criteria for comparison. The criteria I chose is the common denominator for seawise shipping, faster delivery of tons of cargo.

    A small solar boat would have the advantage of being only a small financial/personel loss when she went down in bad weather. Is that the criteria?

    Large or small, it is clear that a solar powered ship (for main propulsion) is absurd, but that is not the original poster's question.
     
  3. Emerson White
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    Emerson White Junior Member

    Fredrosse, so your hypothetical ships are only powered when the sun is shining on them directly, and go completely unpowered from mid evening until mid morning?
     
  4. fredrosse
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    fredrosse USACE Steam

    "your hypothetical ships are only powered when the sun is shining on them directly, and go completely unpowered from mid evening until mid morning?"

    Considering them as just "day trippers", or in continuous running day and night with some sort of storage of electric energy, is not a relevant factor here, the answer is always the same, bigger is better from the standpoint of transporting cargo.

    Of course higher power if only used during sunlight hours, but as stated in my post on 08:14AM, "Assuming 0.3HP per square Meter of collector area (or use whatever number you like)," Doing the math for whatever solar output per square meter of available surface collector area you choose,the result is always the same.
     
  5. DennisRB
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    DennisRB Senior Member

    The question was regarding scaling. You agree the issue is not enough power. My arguments relate to the power issue and how it is affected by scaling. The bigger the ship gets, the worse the power issue becomes to the point where the powering level is absurdly small. Comparing cargo capability of a ships with unquestionably under powered solar systems is irrelevant. You cant say ship A is better than ship B at transporting cargo because its bigger, when the limiting issue is power and the bigger ship is even more limited by the power constraint.

    Making the ship bigger means it carries more cargo (obvious) but it means its even more underpowered which is the whole argument to why solar ships cant work in the first place. Solar does not work because there is not enough power, and making the ship bigger means it is even more under powered. The OP said nothing about seaworthiness.
     
  6. Emerson White
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    Emerson White Junior Member

    Fredrosse, the problem I'm having is that it just looks like you compared two boats that don't function. Once you've got your boats completely divorced from reality I am left with doubts about the reliability of your thought experiment.
     
  7. Rurudyne
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    Rurudyne Senior Member

    On the plus side, if we can get them divorced from reality they can get serious funding and subsidies from Congress. ;)

    (Not a partisan statement, btw, tragic that)
     
  8. alan craig
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    alan craig Senior Member

    DennisRB has the basically correct answer to my original question, which had absolutely nothing to do with commercial efficiency or energy density. I knew that much already; but if you scale a vessel by x2 I don't think you necessarily need X8 (2power3) to power it due to (I think) Froude number.
    That was really the gist of my original question but never mind, there has been some interesting talk.
     
  9. DennisRB
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    DennisRB Senior Member

    Powering requirements seem to be roughly based on displacement. But of course this also changes with scaling too.

    I am struggling to find total displacements vs power for different sized ships.

    Here I found something. As can be seen, the larger ship has less power installed per ton. Which is what we would expect. But is it enough to counter the effect of scaling down the panel area?

    [​IMG]

    The 12000 ton ship has 0.62kw per ton.
    The 6000 ton ship has 0.77kw per ton.

    That happens to be 25% less power per ton for the ship twice the displacement. I worked out before from Freds examples that his ship which was twice the size would have 25% less power per ton due to less panel area. So that seems to work out. Interesting :)

    In the examples shown all the ships are doing 25K so the smaller ships would be running at a higher Froude number which means more power needed to overcome wave making.

    The solar ship would however be so slow in these sizes that only skin friction would matter and we have seen that WSA goes up faster than panel area so the smaller ship would be at an advantage in this case.
     
  10. Rurudyne
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    Rurudyne Senior Member

    The bigger ship, though traveling at similar speed, is moving at a lower Fn which should account for the difference.

    I believe the 300m ship is traveling at something like 0.8 of "hull speed" (rather the square root of her length in feet) whereas the larger is about 0.71.

    This actually helps to further demonstrate the difficulty of "going slow" as a tactic to make solar ships work as these ships are already at a point on the gentler slopes of the power curve, given their size, while still cruising at these speeds.

    A ship designed to go slow may be okay at sea but without reserve power may be simply overwhelmed by ordinary currents. Here I'm recalling tales of Dream Boats of yore having to hide out behind islands waiting for favorable tides ... imagine having to jockey for space with some monster solar ship having to do the same?
     
  11. Stumble
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    Stumble Senior Member

    Actually most large cargo ships (container or bulk) travel at less than 1/2 their 'hull speed'. The industry standard is to travel at 20kn for container ships, and 16kn for tankers. For a 1,000' long vessel with a hull speed in the +40kn range to travel at 20kn.... well it's already in a pretty efficient area of the power curve.


    The Emma Maersk has a hull speed (1.34*lwl^.5) of 48kn (cruises at 20kn) or 41% hull speed.
    The Exxon Valdez has a hull speed of 42.1kn (cruises at 16.25) or 39% of hull speed.


    The reason larger cargo ships have less hp/ton is that they simply cannot achieve hull speed. They don't need to, don't care too, and will never want to burn the fuel required to get them there. Instead they are designed around the industry average speed of 20kn (it was 25 until just a few years ago). So long as a ship can operate at that speed the ability to go any faster just isn't required, and is actually a negative (loss of cargo space for bigger engines).
     
  12. Rurudyne
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    Rurudyne Senior Member

    That's why I said "rather the square root of her length in speed" and put hull speed in quotation marks.

    Sorry that I wasn't clear.

    The point that I was trying to make is these boats are already on the shallower part of the power requirements curve and going much slower isn't going to get you as much as if they weren't (but actually going fast).
     
  13. DennisRB
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    DennisRB Senior Member

    So they are travelling at around half hull speed. Even on most cruising yachts going, slower than that wont save fuel. But the engines on these are usually big enough to do hull speed and more. So will be well oversize for going at half hull speed which is inefficient.

    These ships are fairly underpowered when you look at it this way. I guess this is why they struggle in storm conditions.
     
  14. Stumble
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    Stumble Senior Member

    The Emma Maersk has 150,000 installed hp which gives her a top speed of about 25.5kn. The powerplant at the time of her construction was the most powerful Diesel engine in the world. But she cruises at ~20kn. With a hull speed closing in on 45kn, by yacht standards she is pretty underpowered but even if they wanted to install that much hp, there isn't an engine in the world that could do it, or the cargo capacity for the fuel really.

    Think of it like this, she would need, what, 600,000hp to hit hull speed or thereabouts? Or about two and a half times the power installed on the Nimitz Carriers.
     

  15. DennisRB
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    DennisRB Senior Member

    I think there should be nuclear powered commercial shipping.
     
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