Stability/Seakeeping of Deep V catamarans

Discussion in 'Stability' started by waterbear, May 3, 2019.

  1. waterbear
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    waterbear Junior Member

    Right, I had some feelings about deep V catamaran designs, not based on anything scientific or empirical. That's why I came here: to ask someone more knowledgeable.
     
  2. waterbear
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    waterbear Junior Member

    Here, I made a GIF to show what I'm talking about.

    [​IMG]
     
  3. waterbear
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    waterbear Junior Member

    Here's another

    [​IMG]
     
  4. waterbear
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    waterbear Junior Member

    Here's another.

    [​IMG]
     
  5. waterbear
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    waterbear Junior Member

    More seriously, these boats remind me of cars without shock absorbers. Maybe it's poor pitch damping? Does poor pitch damping detract from seakeeping?
     
  6. Dejay
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    Dejay Senior Newbie

    Damnit haha I stepped right into that one :)

    Good use of the gifs to illustrate your question. I'm curious about that too now. I guess the triangle shape wastes less energy diving into the water so it leads to smooth motions but not much dampening.
    Maybe the frequency and shape of the wave generated when diving into the water is so that they amplify the motion.
    Or maybe there is a relationship between the waterplane area or displacement volume fore/aft when pitching and the inertia of the pendulum motion. You could do the math for the longitudinal metacentric height and put it in a spreadsheet with a graph to see how that looks like.
    Or it's just that it's too symmetrical fore/aft.

    PS: I probably should stop speculating, I don't know a lot. Just curious about this too since I'm interested in double ended boats.
     
    Last edited: May 6, 2019
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Even pitching is complicated, but significant factors seem to be fore and aft symmetry in producing pendulum effects, hull slimness, sea condition, direction of wave trains, distribution of weight, and the list goes on. You'd get very tired of a pitching catamaran, quickly.
     
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    Well, depending upon your interest and willingness to delve deeper into the subject of seakeeping beyond one line sound bites, you can make a simple start here and here which has a tad more info. Then if you wish to look at pitching alone you can try here too., where i briefly explain the basics behind science. (Just ignore the background heckler RW).

    :p:eek:
     
  9. Rumars
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    Rumars Senior Member

    While seakeeping is complicated the reasons the featured boats behave like they do are known. Short overhangs, low buoyancy, deep V, deep rocker, symetrical fore and aft hulls. The problem with deep V hulls is usually lenght. Short lenghts force you to deep rocker otherwise the hulls are to directionaly stable and tacking is difficult. This brings short overhangs low volume pointy ends. Since you need some volume inside the hull you make it wider in the middle exactly where the deepest rocker is.
    If the hull is long enough, rocker becomes much more gentle, the overhangs become much longer and volume distribution is different. If you cut off the pointy stern you can change volume distribution even more. All that can significantly cut pitching.

    Why did the designer chose this hull form? Buy the book and find out. It's your choice if you drink the cool-aid. In my opinion it was the shape he could safely and simply make at his skill level and materials. Pitching just got described as "the natural motion of the sea".
     
  10. waterbear
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    waterbear Junior Member

    Just looking at the videos makes me ill.

    Ok, deep V hulls require some compromises in the shape that lead to pitching, especially in shorter craft. Again, I'm curious if these compromises that lead to pitching also make for a less seaworthy boat, and whether other properties of the deep V have any seakeeping benefits

    At the risk of sounding repetitive: say we have two boats, both with the same length and beam (let's say 30ft loa), both open deck, same mass, windage, cg and construction, except:

    -Boat A has has a deep V section, with plenty of rocker and fine ends, like so:

    [​IMG]

    -Boat B has a dory section with fore/aft asymmetry, fuller ends and a transom, like so:

    [​IMG]

    Now let's recreate the 1979 fastnet race using a few hundred of these vessels. Would we expect the crew of boat A to need rescuing at a significantly higher rate than boat B? Or vice versa? Based on comments so far I'm guessing not.

    Btw the above images were pinched from Richard Woods' site, not the more controversial Mr. W.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    The worst possible combination for pitching, shape wise, is fine ends, symmetrical ends, lots of rocker, slender hulls. Anyone we know ? :eek: Trouble with the dory shape, slamming.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    And yet....it all goes pear shaped.

    Can I assume you didn't read the links posted above then! :rolleyes:

    The waterplane areas remain basically the same, ERGO, little change in the motions.
    All that is being influenced by U or V shape is the dampening, and to a much much lesser extend, the added mass.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    The GIF boat with the brown sail, the crew have their own rodeo !
     
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  14. Dejay
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    Dejay Senior Newbie

    From what I've read the difference in motions that make the crew seasick or not would have an influence on seaworthiness.

    After reading a bit in the other threads (thanks for the links!) the flaring of the hulls seems to have the most significant influence. Since the waves create the exiting moment and the strong flair of the V or the dory shape have a non-linear response due to the buoyancy increase. And the height of the waves are external and the response of the craft delayed so you end up with stronger excitations from waves for the V / Dory. So even if you have stronger dampening and asymmetry you still get more motion than with a real U "wavepiercing" shape with more vertical hull sides.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Whiplash doesn't count ? :(
     
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