Is this a Planning, Displacement, or Combination Hull

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

  1. PAR
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    PAR Yacht Designer/Builder

    That's the way my other half is shaped . . .
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    This boat is a semi-ancient 51 footer that was on the market for $7.5k, at that rate if it just stays afloat you have got a result, without worrying about anything else.
     
  3. leop
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    leop Junior Member

    Yes that's the boat, though i paid a lot less than that for it.

    Actually the hull is not in bad condition. I spent four hours on hands and knees with an awl going over every square inch of her. No cracked ribs, a few rotten planks but all above the water line. The only water she takes in is from one of her stuffing boxes that was left dribbling even after the motors died. I dove on her and bottom and it was in decent condition - for a wooden boat that is. I'll hual her out when I figure out what to do with the engines and then I'll get an a better view then i did diving.
     
  4. leop
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    leop Junior Member

    I believe that the 53's turn a bit faster than the 71's but not by very much. The transmissions are 2:1 if I recall correctly.

    I am headed to the marina tomorrow, and then I'll check them again.
     
  5. leop
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    leop Junior Member

    You can not see it in the two prior pictures I posted, but when I sight down the hull from bow to stern I can see the hull curve in toward the boat's center line starting in the last six feet or so. Kind of like an old Chris Craft barrel-back runabout, but not so pronounced. Is that curving in toward the center line not referred to as tumblehome?

    This new picture shows it a bit better - though still not very well. I thought that it might aid in reducing spray, perhaps by allowing the bow wave (probably wrong term) an easier path back into the hole the boat creates as it moves forward?

    [​IMG]
     
  6. PAR
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    PAR Yacht Designer/Builder

    Tumblehome in modern craft is an aesthetic consideration only. In working craft during the golden age of sail and the very early portions of power, there was a use for tumblehome, which offered the crew more room for their feet while at the rail, tending gear.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    In a classic powerboat at speed it helps create an elegant roostertail . . . that probably was "planning" (sic)
     
  8. leop
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    leop Junior Member

    I take it that your use of (sic) means that the boat was not really planning, or is "planning" spelled wrong, or was the tumblehome not actually responsible for the elegant roostertail?

    I gather that the science of hydrodynamics has come a long way since 1948 and that many of the beliefs of those days have not proven to be true? Or did the designers of those times know pretty much what was going on and simply put aesthetics ahead of functionality/efficiency? I know that this boat does not have much interior room compared to a modern 51 footer, I believe becuase of the aesthetics of her design.
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    AFAIK sic is not a hydrodynamic term, I was referring to the spelling. Esthetics scored at least as high in a designers objectives back then as it does today. And those tumblehome sterns do look great, don't they? Those boats were strictly for swank, there was probably lots of interior room, mostly devoted to the motors though, not for seats. The kind of folk that owned and drove those boats probably weren't interested in having hoards of visitors on their shiny mahogany treasures . . .
     
  10. leop
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    leop Junior Member

    Thanks for your clarification.

    Yes those old sterns do look great. I am pretty sure that if it was not for this boat's stern that she would have been scraped out long ago.

    It's funny that you mention swank. The boat was commissioned by Paul Koss a businessman in San Francisco who held the patent to the paper toilet seat covers you see in public toilets. He was know as a huge show off and named both his boats after himself. There are lots of references to him and the boat in the SF papers of the time.

    The second owner was the owner of the law firm that represented Shell Oil Company. It is thought that he entertained many local politicians on the boat in the course of getting a deal cut with them to allow Shell to build an oil refinery in the city of Richmond California. That Oil refinery is still hated to this day by that town's people and it's been widely rumored that had it not been for a few bribes and entertainment by Shell Oil that it would not have been approved even back then.

    So how do you spell Planning?

    Thanks for your clarifications. Much to learn for me.
     
  11. daiquiri
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    daiquiri Engineering and Design

    I spell it like "planing". :p :p

    Interesting stories about that boat. Sure, they do raise a bit it's market value, don't they? ;)
     
  12. leop
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    leop Junior Member

    Better Picture of the hull.

    This is a better picture of a sister yacht to mine. Her name is Zanzabar. She was built the same year, same builder, same hull with the only difference being that Zanzabar was built with a conventional transom.

    Not sure if this can help positively identify her as a semi-displacement hull or planing hull though? If it does however please let me know.

    Also - how do you calculate the hull speed for a semi-displacement hull? How do you calculate the proper horsepower for a semi-displacement Hull. I know the equations for a displacement hull - and read that the best rule for a planning hull is "the more the better" but don't see much on the subject for semi-displacement hulls

    Leo

    [​IMG]
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Suffice to say, running that boat beyond displacement speeds will cost a bomb in fuel outlays, your initial small outlay will seem a minor compensation. It is pretty well axiomatic in power boats that speed costs, either in terms of fuel $ or loss of comfort. Try tootling around at sub-"displacement" speeds, you might actually enjoy it, and the fuel economy aspect won't go over a cliff.
     

  15. PAR
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    PAR Yacht Designer/Builder

    The best advice you could take to heart is to hire a professional to work out the re-power requirements, based on the design and your SOR.
     
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