Catamarans High Speed Blow Over - Causes & Solutions

Discussion in 'Hydrodynamics and Aerodynamics' started by kidturbo, Sep 11, 2013.

  1. daiquiri
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    daiquiri Engineering and Design

    Take a look at this drawing, it will explain how it works:

    Wing anti-roll action.gif

    As the boat starts to roll, it gains an angular speed. So, the wing which goes down (in this case, the starboard wing) sees an additional speed component coming from below (in the reference frame of the wing), due to the roll motion. It sums with the boat speed (supposed horizontal), and increases the angle of attack of the resulting apparent wind seen by the wing. Higher AoA = more lift. Hence, the wing which goes down produces more lift.

    On the other side, the port side wing sees a roll-induced speed component coming from above, which in turn decreases the AoA of it's apparent wind. Smaller AoA = less lift. Hence, the wing which rises upwards produces less lift.

    The net result is a roll-resisting moment, as visible in the drawing. By the way, now I am noticing that I have called this moment "restoring" in the drawing, but that is not correct. It will not restore the boat into an upright position because it exists only as long as the roll motion (and the relative induced speed seen by the wings) exists. It just attenuates the roll motions.

    It is a pretty short explanation, but I hope it is was sufficiently clear. A dihedral angle of the wings (the angle relative to a transversal horizontal line) also plays a role, stabilizing the craft in the transversal direction too.

    Cheers
     
  2. powerabout
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    powerabout Senior Member

    its was just a guess as to where the CoM would be on a Cat
    It would get ugly if that wing tip touched the water at speed??
     
  3. rxcomposite
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    rxcomposite Senior Member

    It won't as it is in between the hull. It is not supposed to go beyond the breadth of the craft. In the case of Daiquiri's drawing where it is elevated, a dihedral would help prevent the tip from touching water.
     
  4. powerabout
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    powerabout Senior Member

    Ok go that but unless its at the front between the pointing bits it going to get submerged at some stage for sure?
     
  5. rxcomposite
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    rxcomposite Senior Member

    You mean the canard? It might and will be catasthropic. But that will happen due to bad design or pilot/drivers error.
     
  6. daiquiri
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    daiquiri Engineering and Design

    My drawing refers to Cesa 1882 boat by Buzzi, the one with wings in the first picture of my post #33. The drawing wanted to explain how those wings work as anti-roll device, which is what Powerabout has asked (I believe).
    Cheers
     
  7. rxcomposite
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    rxcomposite Senior Member

    And I responded by saying drag. Not sure also what he was referring to but you have explained it more in depth.

    I was assuming a cylinder. If you stick a horizontal wing in the center, it will resist roll due to drag or resitance induced by the wing to rotate along the longitudinal axis of the cylinder.

    Next question will be what if I stick it above (high wing) or below (low wing)?
     
  8. daiquiri
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    daiquiri Engineering and Design

    The drag only would be far less effective, of course. The secret is in the airfoil shape, which gives a huge anti-roll moment with very little drag. But I am saying obvious things at this point... ;)

    As for your question a high wing doesn't feel the ground effect but can count on a big lever arm relative to the CoG (in case of anti-roll action). And plus, it cannot hit the water surface.
    A low wing works better aerodynamically (ground effect), but the anti-roll lever arm is smaller (if one wants to use them to prevent rolling). And plus, it can hit the water.

    Since monohulls heel during turns, the only viable option for them is a high wing, like in Cesa 1882 case. Cats heel significantly less (almost a flat turn), hence they can use low wings and canards, like the already-mentioned H1 unlimited-class boats do.
     
  9. kidturbo
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    kidturbo Junior Member

    Yeah, I'm lost :eek:

    Not really, I am starting to get some of the principles of the "crude" WIG design they call a tunnel hull or catamaran in the industry. Actually they should be called a high speed pontoon boats. :) Besides the new design "Custom-Tuned Venturi Tunnel" by the manufacture Statement I listed above, these hulls are basically a big sheet of plywood with a pair of sponsors attached that traps some air in ground effect and creates little lift.. And thus they perform as such when it comes to aerodynamics.

    When it comes to designs, my current project boat is based off late 80's early 90's racing technology, it's a big 38' cat, 12' beam with a deep center pod. I would consider it a tri-hull, but they call it a cat. Research shows it was the ONLY 38' produced by "Team Warlock" for pleasure use, and this design somehow went on to become known as the Express Cat model by other manufactures. At least they resemble this hull closer than any other design. Speaking with some present boat builders in the states, I leaned that the design doesn't present a lot of lift, and is more stable in rough water than present cat's. Also it's limited on speed due to the design and lack of lifting surface. However it is still susceptible to blow overs at higher speeds / angles.

    All the newer "late 90's and up" cat's have a very pronounced tunnel and no center pods. These are where I think this research into aerodynamic control surfaces will benefit the most. Yes designers could make some changes to COG and lifting surfaces that could reduce the blow over risk on newer models, but esthetics seems to be pretty high on that list of demands. New buyers want a sharp looking boat that also cruises at 140 or higher. Not a problem if you grew up running cat's, but many have not and get into trouble, while in an unforgiving design...

    Hope that gives you a little insight into my view of the subject. Most owners I know would be open to slight modifications that can reduce the departure risks without starting from scratch. An add on safety system lets call it. For the designers, hopefully some of this reaches them and they realize that sheet of plywood needs some shape and control surfaces to obtain stable flight. The racing rules can change, history shows they do it all the time. So *** Buzzi did so well years ago, lets toss it out there and see if it sticks.

    I'm back to reading, thanks again for all the insight.

    :cool:
     
  10. rxcomposite
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    rxcomposite Senior Member

    Hot thread and informative but it is 3 AM here. Going to bed. Drank too much coffee.;)
     
  11. powerabout
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    powerabout Senior Member

    Many thanks, that makes sense
     
  12. Petros
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    Petros Senior Member

    wow, lots of happened overnight on this thread.

    I think a properly designed canard can be made to work, it would not even have to be a movable surface.

    The canard helps maintain lift is ground effects, keeping the carft level, it should also assist in dampening pitch as well since it is so far forward. When the angle of the hull gets to a certain point the canard will stall, or start to stall with some stratigically places vortex generators you will only stall part of the surface, either way front end lift is reduced the pitch angle decreases. This should help keep the boat within ground effects all of the time.

    But if sized properly, and set at the correct angle of attack relative to the "main lifting surface" of the aft hull area, it could also stop the run away reaction of an unstable flight condition when fully airborne and out of ground effects (what causes the flip over). If the boat should jump high out of the air out of ground effects, the aerodynamic center shifts forward from about 50 percent to about 25 percent, thus putting the CG too far aft and it goes nose high and out of control. With a large enough canard, and having it stall, it effectively will shift the aerodynamic center aft, preventing the pitch up in the first place. No need for any movable surfaces, at least on the canard.

    this could also be assisted with a large trailing edge flap at the aft end of the "main lifting surface" or the deck. So when out of ground effects, the canard stalls and a trailing edge flap goes down, the combination of both of these should created a powerful nose down pitching moment. when the canard recovers lift at a lower angle of attack, the nose down forces will be reduced and the pitch rate slowed, the trailing edge flap will come back up. A trailing edge flap would only have to move about 5 to 10 degrees to be effective, it does not take much to change pitching moment. Of course that means a movable device, adding to the complications.

    It would be nice if it could be done with just fixed surfaces, no moving parts.

    In some of those clips there were crashes that were caused by the hull rolling to far, so there has to be some roll stabilizing as well. Some fins canted outward at a steep angle should take care of that. I am thinking something stylish, like the large fins used on Cadillacs in the early sixties.
     
  13. powerabout
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    powerabout Senior Member

    Where's the canard going to go on a cat where you can be sure it will never hit the water?
     
  14. Petros
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    Petros Senior Member

    you can not, it is on a boat, a very fast one. That means there is not one part of the boat where you can be sure it will never hit the water.

    I was thinking as far forward as possible, between the hulls, as high as possible. That is where the rules allow it, they call it the "pickle fork" area. That is about the best we can do. Also, as large an air space as possible between the trailing edge of the canard, and the leading edge of the main deck area (the main lifting element). If the canard can be elevated above the deck elevation, so much the better. It will get cleaner air there.
     

  15. Jimboat
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    Jimboat Senior Member

    Here is a recent magazine article published on "Anatomy of a Blow-Over".

    [​IMG]
     
    Last edited: Sep 21, 2013
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