Semi-planing boats

Discussion in 'Boat Design' started by HJS, Jan 22, 2021.

  1. HJS
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    HJS Member

    Semi-planing boats, an ecological opportunity in balance and harmony

    The semi-planing boat is a specific type of boat that deserves more attention. Specialized methods are required to design a semi-planing hull, and therefore it is more complicated than merely forcing a displacement hull to go faster or a planing hull to go slower.

    The stern design is the essential characteristic that distinguishes the semi-planing boat from other types. The area, width, and draft of the transom are critical factors in the semi-planing hull performance.

    The semi-planing boat's optimal speed is around Froude's length number, FnL, 0.7. At this speed, the wave formation is most favorable. L is the wave shaping length, and it is not the same as the length of the waterline. The wave shaping length is intimately connected with the longitudinal metacenter radius. Thin end vessels provide a small metacenter radius. Plump end vessels create a large metacenter radius.

    Several calculation models are available to optimize a semi-planing boat design, including Mercier-Savitsky, Groot, Delft DSDS, and more. They all have their limitations and are thus only a tool. It is necessary to check the basis of the calculations in the studies.

    In addition to the static stability, the hull's dynamic stability should be calculated to verify that it remains stable over the entire speed register. This calculation is essential if the boat has a rounded transition between the bottom and the topside.

    Consideration must be given to how hard the boat is going in the oncoming sea, the g-number, the vertical acceleration, which can be calculated using, for example, Koelbel. The bottom width, position of the total center of gravity, and the longitudinal trim angle affect the seaway movement more than the bottom deadrise.

    To achieve optimal efficiency, one must include the minimum and maximum payloads in all of the calculations mentioned above.

    Semi-planing boats deserves more attention.

    JS
     
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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    No doubt have a place, one limitation that occurs to me is that the cruising speed is often not that much different to that of some wave trains, and that can be a problem when running with those waves, the boat spends a lot of time on the back of waves. That has to increase resistance and reduce fuel economy.
     
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    how about this one? don't think it scores any points for ecological operation, even as designed.

    I thought the whole point of semi-displacement was they CAN maintain high speeds in rougher seas than true planers which would hop and flop.
     

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  4. Mr Efficiency
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    Mr Efficiency Senior Member

    They are basically hulls that can operate effectively at speeds around the typical transition speed from displacement to planing mode, where both displacement hulls, and planing hulls, have very high resistance.
     
  5. fallguy
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    fallguy Senior Member

    How about this one? Two feet wide and flat aft on bottom.

    99FBEDBA-B56C-49A1-B364-FC41018F2B83.jpeg
     
    Last edited: Jan 22, 2021
  6. fallguy
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    fallguy Senior Member

    How do I calculate the fnl, 0.7?

    gonna google it
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Slender hulled cats are a special case, they don't have so much in the way of wave-making resistance to create the "hump" problem that monos have to contend with.
     
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  8. fallguy
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    fallguy Senior Member

    can I find the optimal speed of the cat from dimensions?
     
  9. bajansailor
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    bajansailor Marine Surveyor

    +1 - this transition hump for monos seems to be for the stereotypical type with a L/B ratio around 3 usually - Fallguy's much skinnier hulls (with an L/B ratio around 15?) can cruise MUCH faster while still in displacement mode.

    Re Froude Number, the formula in metric is velocity (in metres per second) divided by the square root of 'g' (9.81 metres per second squared) x L (the waterline length in metres).
     
  10. fallguy
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    fallguy Senior Member

    L/B is mid upper 11s 32'/32"? Or so. I have the numbers if I can find optimum speed. Didn't know I could.
     
  11. bajansailor
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    bajansailor Marine Surveyor

    Is your beam of 32" the maximum beam at the waterline?
    An L/B ratio of 11 or 12 is still very efficient (relatively) compared to an L/B = 3 stereotypical monohull.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    If there is an immersed transom to any degree on your cat, that would increase the "optimum speed", if there was no transom dragging, you'd have to think the best speed for mpg would be quite low, but as a practical matter, you don't want to run at those speeds, and the displacement cat gives you quite a wide band of acceptable mpg figures. That can't be said of most monohulls. The displacement cat hits the wall with frictional drag, more than anything else, the displacement monohull, the sharp increase in wave-making resistance.
     
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  13. HJS
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    HJS Member

    Examples of resistance of different boats.
    JS

    RW FnL.jpg
     
  14. DCockey
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    DCockey Senior Member

    What is the source of the data? What are the dimensions and lines of the hulls?
     

  15. gonzo
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    gonzo Senior Member

    There are monohulls with slender hulls too. It is not limited to multi-hulls. This paper on fast navy vessels is of interest for this thread since it has actual measured vs calculated data: https://apps.dtic.mil/dtic/tr/fulltext/u2/a174027.pdf
     
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