Stability/Seakeeping of Deep V catamarans

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

  1. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    It's obvious you don't understand the deeper toughts behind this boats. There is no whiplash simply because you are never at the ends of the boat. The designer made shure of that, you can not stand on them and you certainly can not crawl into them.
    When on this boats you are supposed to lay down in the middle of the deck enjoying the beauty of nature and entertaining the crew. The rocking motion of the boat is a desired feature for this kind of active sailing.
    That is why the sails are boomless, and the boat underrigged, it increases the freedom of enjoying nature under windvane. The boat takes care of itself and the crew of each other. In case of heavy weather you reef and retire to the hull. The hulls are cramped on purpose. One has to lay down, you don't get trown around and beeing wedged in there with one or two crew insures it is warm.
     
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  2. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Waterbear,

    You're barking up the wrong tree my friend.
    The angle (verticalness) of the free board is going to make a bigger difference to pitching than the depth of the V.
    But it's really terribly complex and not reducible to one or two statements.

    I don't know why you're asking, theoretical perhaps, but if you want to build a cat and are concerned about porpoising then build a proven design that doesn't. Simple.
    If you're stuck on designing and building your own, then go for it, you'll find out what Ad Hoc is trying to explain to you.
     
  3. waterbear
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    waterbear Junior Member

    I'm not dumb enough to design my own boat, unless it was something I could build in a very short period of time and treat as a learning project.

    I knew a guy who decided to design and build his own 60 foot long catamaran, without consulting a naval architect or a single book. The hulls were flat on the inboard side and curved outboard, so the waterplane looked like a wing section. There appeared to be too much volume in the rear, so had it ever made it to the water it would have sat very low at the bow. In some areas there were long expanses of plywood without any stringers or bracing. In others there were heavy, expensive aluminum beams strong enough to hold up a bridge. He intended to build a two story structure on top, complete with a house sized kitchen and granite counter tops.

    He passed away after working on it for many years, and it was later demolished by a marine salvage company. The guy operating the backhoe said the boat came apart with little effort.

    I was depressing watching the process. If I designed my own boat it wouldn't be as ridiculous, but the same things would be happening. The boat would be overbuilt in some ways, and under built in others. I would be making many design choices without understanding all the implications. The boat would be slower, heavier, less seaworthy and more expensive than something designed by a competent NA.
     
  4. waterbear
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    waterbear Junior Member

    No surprise here.

    I looked, but I didn't feel like digging though a forum not knowing what I'm searching for.

    Do you mean total water plane area? Or the longitudinal distribution of area in the water plane?
     
  5. waterbear
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    waterbear Junior Member

    Exactly.

    [​IMG]
     
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  6. waterbear
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    waterbear Junior Member

    Theoretical. I've long been fascinated with Mr W's designs, but at the same time I have doubts about his competence and design choices. My impression is his boats are proven and safe, but also sub optimal in many ways.

    If I were to ever become mentally ill and decide to build a catamaran it would be something else, eg a Richard Woods design. Just comparing the crossbeams: Despite claims of simplicity, Mr W's beams are either tapered I-beams or tapered triangular sections, both unnecessarily complex. Richard Woods' beams are simple and logical: square tubular beams, not tapered.

    Let me simplify it for you:

    You're being chased by a hoard of Zombies and you arrive at the shore. A strong gale is blowing and you spot two catamarans, 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]

    Which boat do you use to make your escape?
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Look again at the people on board, Ride 'em cowboy !
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Is this supposed to be a recommendation of this type of boat ? Sounds more like a demerit list. Boats that pitch excessively, are a pain in the bum wherever you are situated, the change in tilt angle is the same everywhere, even if vertical accelerations are not.
     
    Last edited: May 7, 2019
  9. waterbear
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    waterbear Junior Member

    I think he's trying to say the pitching amplifies the motion of the sea, making you feel closer to nature.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I see, each to his own, boating as a recreational activity needs a modicum of comfort, the novelty of being belted around wears off.
     
  11. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Oh, marine architecture in the zombie apocalypse, now we're talking!

    In that hypothetical case I would pick boat A because I like being closer to nature ;)
     
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  12. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    I'm sorry if I offended someone. My intention was to expose the marketing hype that to this day surrond one designer and his boats.
    Just to be clear I do not endorse his boats or his business model. I do recognize his achievements as a multi pioneer and businessman becase the facts speek for themselfs.
     
  13. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    Poe's Law - even if you didn't intend to - you probably made the perfect sales pitch for someone who just loves the style of these boats :)
     
    Last edited: May 8, 2019
  14. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Not sure why you won't name Mr W, as I clearly have no idea who the mystery builder/designer is and therefore have no idea what you're talking about.

    To answer your "simple" question, the first one of course.
    It has the same draft as the other hull at less than half the displacement so is a much lighter boat and lighter is faster.
    Also the sides (freeboard) are less vertical so more reserve buoyancy.
    Now I know you said the boats were the same displacement but that contradicts Mr Woods diagram
    if that is in fact the waterline shown.
     
    Last edited: May 8, 2019

  15. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Finally, the penny drops:

    WHARRAM!
     
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