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Stupid catamaran question.

Discussion in 'General Catamaran Discussion' started by Blue Ghost, Apr 20, 2012.

  1. Blue Ghost

    Blue Ghost Member

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    I've always wondered how a multihull pitches over. I understand capsizing, but I'm really in the dark as to how a cat topples forward head first.

    Can any of you naval architects offer any insights?
  2. NYCAP123

    NYCAP123 Senior Member

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    Any boat can pitchpole (bow stuffs and stern continues over), however most will fall to one side or the other into a broach position due to a variety of forces. The two hulls can keep it from rolling to its side, due to the counter-balance of the opposite hull, enabling it to keep rollin forward.
  3. Capt J

    Capt J Senior Member

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    When coming down a large wave, the bow(s) dig into the water and basically stuff themselves under water, and the stern gets lifted until it goes end over end. It depends on the design. I was in a 7ft following sea on a Lagoon 44' Power Cat and coming down a wave (granted at cruise speed) the bow would go down until the wave was at deck level, but not coming over. Then it would lift back up. However, the lagoon has lifting strakes on the front of the pontoons to create lift.......
  4. Blue Ghost

    Blue Ghost Member

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    Interesting.

    I've seen it happen in San Francisco Bay, and I always wondered if it was because the winds tend to be a little stronger here, or if it was just poor sailing.

    After having a look at footage on Youtube, it looks like the wind is pushing more forward than laterally on those cats. And because the design is essentially square, the torque on the mast isn't diverted by a large hull to the water, but translates into forward leverage, forcing the cat's nose to dig into the water.

    Whenever I see a catamaran pitch forward I always want to say "man, how stupid can those guys be?", but have always held off believing that there was something I was missing.

    As you can tell, I'm not a big catamaran sailor. I've only sailed them on lakes, and it's never been the big cats, always a Hobie or something like it.

    Anyway, thanks for the responses. Very helpful.
  5. Pascal

    Pascal Senior Member

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    Well it's pretty simple... If the power in the sails exceed what the hull can take, the boat will pitch pole or flip on its side. A monohull has ballast which will prevent the stern from lifting but a cat doesn't. Also, with a cat the speeds are significantly higher so if the hulls are stuffed into a wake, momentum will help the pitch poking.

    Most cruising cats don't have enough sail area to pitch pole but small beach cats or racing cats can easily.

    And yes, you feel really stupid when that happens... All it takes is a stronger gust combined with a slightly higher wave. And a slow to react helmsman

    Been there, done that, got the dunking... Luckily a H16 is easy to right
  6. Blue Ghost

    Blue Ghost Member

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    Yeah, I've capsized numerous times in a variety of craft, but never experienced the pitch pole thing. Your ballast explanation clears things up quite a bit. It's why you don't see scows pitching over, the next fastest sailing rig out there.

    I got to admit, I'm scared of cats. Even as a young boy when I'd go out on them they'd scare the living stuff out of me when the hull lifted out of the water. Suddenly I'm looking (from five year old's perspective) "straight down". :eek: It ain't natural when you're that age. And of all the dozens and dozens of times I was taken out on cats when I was a kid, I never got used to it.

    The footage I saw showed a two man racing cat humming along at (guessing here)... just over 10 kts? The bow just dug slight into what was relatively calm water, and the guy hanging off the rear on a trapeze shot forward. Wow. Incredible.
  7. Pascal

    Pascal Senior Member

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    Then The weight of the guy swinging forward on the trapeze hands to the forces...

    My GF has been sailing cats for years and finds a hull lifting out to be a natural motion but hates healing on a monohull! It s true that with a small cat hobie 16 and slightly bigger you get a lot if control when flying a hull but it doesn't take much for the bows to dig in and power in the rig to flip it...
  8. tirekicker11

    tirekicker11 Senior Member

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    And not to forget a very low bridgedeck clearance. Aren't all Lagoons powerboats?;)
  9. Capt J

    Capt J Senior Member

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    No, the majority of Lagoon's are sailing cats.
  10. tirekicker11

    tirekicker11 Senior Member

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    I delivered 6 from France to Annapolis and 1 from Hong Kong to Phuket. They all had masts and sails but none of them was a sailing cat.;)
  11. wscott52

    wscott52 Senior Member

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    Two man racing cat was more likely doing around 20 knots if there was any kind of wind. I had a 14' Hobie as a teenager and could get what felt like 20 knots fairly easily. And I always figured if you don't have one hull out of the water you're not pushing it. You did learn how to right it after capsizing pretty fast.
  12. brian eiland

    brian eiland Senior Member

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    Diagonal Stability Factor

    The pitchpole tendency of cats (and tris) is related to their diagonal stability. Some factors contributing to that diagonal stability
    1) Wider beams can result in greater transverse stabilty (greater capsize resistance), but can concurrently contribute to a greater tendancy to bury that lee bow, particularly on racing cats and tris with less reserve bouyancy in their bows.
    2) Taller rigs where the center of effort on the sailing rig tends to produce a greater 'up high' pitching force on the vessel. So when the hull meets a slight increase in drag as it is forced down into the water, the conbination of this drag force to the rear with this extra forward driving force up high in the rig creates a force 'coupling' rotating the bows down. Add a 'little extra gusts' into the equation,....and watch out'.

    You likely see these effects on the racing cats you saw on the bay, and in most beach cats were the sail are to displacement ratios are much higher than in cruising cats.

    Both headsails AND MAINSAILS can contribute to this bow down rotation. I happen to think that the mainsails are a more major contributor than headsails, but that is a subject open to debate as many would have you believe that headsails (and spinnakers) offer no 'lift' to the bows. I've just experienced that 'lift' on a number of occassions in real lift not to believe that it doesn't exist.

    One of the best teachers for sailing multihulls is to go out on some beach cats and push them to their limits. These are easily rightable (at least most), and you will gain an appreciation for their limits; and likely you will become more 'comfortable' with flying a hull, ...and you likely get a bit more fun out of the process as you go along.

    By the way I would suggest a Prindle brand cat rather than a Hobie cat. For one thing it is generally a ligher weigh vessel that can be righted more easily, and its hulls to not flex independantly to as great an extent which makes it less likely to put its lee bow under and pitchpole.

    You will see a LOT of pitchpole activity with those wing-sailed America's Cup cats as they are really pushing the limits on sail power (and rig height) with repeact to their boat weight.....and thats just the smaller versions now. Wait till they get to those 72 footers....WOW they are going to be scary in those conditions in San Fran Bay :eek:

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