Is this a Planning, Displacement, or Combination Hull

Discussion in 'Hydrodynamics and Aerodynamics' started by leop, Jan 9, 2014.

  1. leop
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    leop Junior Member

    I realize that this picture is not that great. However I was wondering if someone could tell me if this is a Planning, Displacement, or Combination Hull and why?

    My guess is that it is a planing hull becuase it is rather flat at the stern? Is that a correct assumption or totally wrong? Does the lack of a keel that extends all the way to her transom say anything?

    [​IMG]
     
  2. NavalSArtichoke
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    NavalSArtichoke Senior Member

    There's lots of boats which have a flat bottom at the stern. Doesn't necessarily make 'em planing hulls, though. The flat bottom is kinda a by-product of having such a wide transom stern.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Definitely not a displacement hull, I'd say. It is designed for more speed than that, it is not so much the flatness of the bottom crossways, but the straightness lengthways that dictates that. But when you have a large, heavy boat not going overly fast, "planing" is probably a bit misleading really, the kg per unit of bottom surface area is much greater than for smaller, lighter boats that really do skim the surface, consequently the dynamic lift effect is much less. Also, the props on that boat aren't really that big diameter wise, which is more in keeping with greater speed emphasis.
     
  4. PAR
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    PAR Yacht Designer/Builder

    I'm not sure what a "combination" hull is, but judging be her size and the very vague details visible, she's very likely a semi displacement vessel.
     
  5. NavalSArtichoke
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    NavalSArtichoke Senior Member

    I don't know how you can really tell what kind of hull you have from one blurry photograph. The small prop size is ambiguous at best; they could be small because the boat operates in shallow water, or the boat may have relatively low power.
     
  6. daiquiri
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    daiquiri Engineering and Design

    Considering the absence of lifting strakes, a long skeg and a very low deadrise at the transom, I would second PAR's guess.

    BTW, I presume that the term "combination hull" is a misnomer which was intended to mean "semi-displacement".
     
  7. tom28571
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    tom28571 Senior Member

    I second the remarks about the boat likely being semi-displacement although that term embraces a really large range of shapes. The low deadrise stern may be no more than a desire for shallower draft. The straight buttocks do say that it is intended to go faster than "hull speed" if there is enough power.

    Most people who own such boats these days run at a bit less than displacement speed because of the fuel bill for going faster.
     
  8. leop
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    leop Junior Member

    Yup, combination was meant to convey semi-displacement.

    She was originally equipped with twin 6-71 Detroit Diesels, and later repowered with 4-53 DD sometime during the 70's. I have no idea if the props were resized for the smaller motors or just left as is.

    Her builder was Stephens Brothers, and the year is 1948. Her sister ship is shown in the attached photograph of that time. The boat appears to be traveling pretty fast, but then again - fuel was much cheaper in 1948.

    The reasons I ask all of this is becuase she needs engine work and I wonder whether I should rebuild the 4-53's becuase they have plenty of power for her hull speed, or go with the larger 6-71 DD her builders intended or do something entirely different.

    I want to pick the engines before she come out of the water and hence I had to rely on that old blurry 70's photo.

    Opinions about engines? Yes fuel price is a concern, but i never stop smiling when my other boat, 1948 Chris Craft passes a brand new ski boat with a mercruiser 350 in it. It would be nice to be able to get the old girl to get up and go once in a while - though she will admittedly be run slow 99% of the time due to fuel costs.

    Thank you all for your answers. I am off to study them now and understand some of the new words I see here.

    Sincerely,

    Leo

    [​IMG]
     
  9. leop
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    leop Junior Member

    Is the boat's tumble home an esthetic design element or does it serve a hydrodynamic purpose?
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    If you intend to "go slow" 99 % of the time, you have answered your own question about needing bigger engines, you don't !
     
  11. leop
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    leop Junior Member

    That's must be why they call you Mr. Efficiency :)
     
    Last edited: Jan 12, 2014
  12. Milehog
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    Milehog Clever Quip

    Are the DD 53's a higher rpm engine than the 71's? Can you find the transmission gear ratios?
    IMO the 71's would have a better sound than the 53's.
     
  13. tom28571
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    tom28571 Senior Member

    There is no tumblehome. The transom is reversed with a curve and it is never is in the water (hopefully) so it can have no hydrodynamic purpose. You might say it makes the boat bottom longer and lighter than a vertical transom but I suspect it is purely for aesthetics.
     
  14. daiquiri
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    daiquiri Engineering and Design

    It also decreases the available deck area in the aft and makes boarding and docking by stern more difficult. But yes, it looks sexy. ;)
     

  15. lewisboats
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    lewisboats Obsessed Member

    And who doesn't like a smooth flank ending in a nicely curved transom?
     
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