Catamarans High Speed Blow Over - Causes & Solutions

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

  1. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,388
    Likes: 1,022, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    No matter, I do believe you had honorable intentions, as always ! :)
     
  2. kidturbo
    Joined: Sep 2013
    Posts: 40
    Likes: 1, Points: 0, Legacy Rep: 24
    Location: Southeastern Ohio US

    kidturbo Junior Member

    daiquiri;
    It's fair to keep your ideas under wrap until your ready, I believe in intellectual property also. My only goal here is realizing a problem exists and finding a reasonable solution. Then those of us who enjoy skipping across the water at break neck speeds, with 3000hp strapped to our asses, while sipping a cold one, can do it safely as possible.. :D

    Tomas;
    Her is a link about that Big Thunder crash. The sport lost two great racers that day.. Not sure if the boat had a roll cage inside the canopy or not, I don't believe one is required. Either way 10'000lbs moving over 100mph is a huge force to recon with, and she landed backwards, hard on her top.
     
  3. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    In a conventional go fast monohull boat, lift is generated by hydrodynamic forces whose center is ahead of the CG. This is either augmented or counteracted by the propeller thrust line, depending on the VCG.

    Savitstky theory on planning design indicates that to go fast, the CG must be located as far back as possible and the prop thrust/lift to keep it in equilibrium.

    When the boat lifts out of the water, the prop loses its ability to keep the boat in equilibrium, the lift ahead of the CG continuous to increase, increasing the angle of attack and will continue to do so until the boat flips over.

    In a conventional tunnel hull design, air is trapped in between the hulls as speed increases. This trapped air, called “stagnated air” has nowhere to go so pressure increases and augment the lift. When the boat lifts out of the water, the same thing happens as in paragraph 3.

    Now, the WIG boat, specifically the anhedral, reverse delta wing of Dr. Lippisch design uses also the stagnated air trapped between the hulls to attain lift. This design uses a conventional wing arrangement as it has a small horizontal tail. When the boat lifts out of the water, the horizontal tail takes over, counteracting the force that tends to flip the boat over. Sure there is an air drag which could be significant but the area can be reduced if the design is meant for the boat only to be aerodynamically stable while out of the water (but still in ground effect). WIG is designed to “fly”, tunnel boats need just enough trapped air to augment its lift to go fast.

    While my knowledge of aerodynamics is amateurish compared to Daiquiri, he is posting the correct principle. A quick check on my books shows that conventional aero design applies and that the Lippish principle can be “married” to Savitsky with compromise.

    Even the canard principle can be used but probably the worst compromise. Highest speed may not be attained but safety is increased.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,388
    Likes: 1,022, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Fix the blowever problem and more power will be installed, and create new dangers, possibly rotation about the longitudinal access. When I was a kid there was a road safety campaign emphasising "the speed that thrills is the speed that kills". Not wrong !
     
  5. kidturbo
    Joined: Sep 2013
    Posts: 40
    Likes: 1, Points: 0, Legacy Rep: 24
    Location: Southeastern Ohio US

    kidturbo Junior Member

    Thanks for that great explanation. I'd say your knowledge of aerodynamics is well above mine. The graceful control of the WIG design is where I believe the modern catamaran should be by now. It's like all the great designers read that APBA rule Daiquiri mentioned above and said "this book says we can't use a horizontal stabilizer, scratch all your brilliant ideas that have one"..

    Check out this WIG design built off a slightly modified hover craft, amazing flight control off of a single control surface. For what a modern high speed cat costs, they should be doing circles and loops.




    Mr Efficiency, you're probably correct about the speed increase soon as this "flight problem" is solved. Will ultimately extend to the highest point propulsion systems and drag will allow. Because on the water is one of the few places left one can satisfy that need for speed, safely and legally. You can buy a 200mph+ car, just very limited by where and when you can legally open it up.

    Do you fly in airplanes? Would you still if they only went 50mph? How about if they if they randomly lost control and crashed with no explanation besides they are just too darn fast?
     
  6. rxcomposite
    Joined: Jan 2005
    Posts: 2,423
    Likes: 403, Points: 83, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    Thanks for appreciating.

    The problem in the cause and effect is that the water "sees" a different profile while the air sees a different one all with the CG not changing. I have attached a graphical presentation.

    The Lippisch catamaran approach (there is a trimaran version) brings the centroid of areas and the CG much closer together. The reverse delta configuration is efficient but is not designed for underwater propulsion.

    The standard delta (Volvo Penta) configuration brings the hull CG and the centroid of lift much closer together and is thus a good candidate for the marriage of aerodynamic and hydrodynamic. Both the CG and the COL is far back. It lacks the horizontal stab though, to further stabilize the boat when "airborne". Perhaps it was not meant to "fly beyond its flight envelope" but it seems a logical approach.
     

    Attached Files:

  7. Jimboat
    Joined: Feb 2002
    Posts: 255
    Likes: 17, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    ...interesting discussion.

    Actually, the tunnel hull/catamaran design is a unique balance of it's hydrodynamic and aerodynamic forces (both lifts and drags). Since it's still a boat, it always needs some portion of the hull setup in contact with the water (either the aftmost inches of sponson pads or the lower unit bullet) to maintain some level of stability. While it's aerodynamic lift is generated the same as a WIG (low aspect ratio wing in ground effect), it's different in that it must maintain water contact, and has it's major contact point fully aft of the craft.

    The biggest design challenge is that the overall "dynamic CofG" (moments of all deadweight, hydrodynamic and aerodynamic forces) changes significantly throughout the velocity range of the hull. This is since the wetted surfaces (length/location) reduce throughout the velocity range. Consequently, the Static CG (deadweight) is only at or close to the Dynamic CofG at a singular velocity.

    So, the tunnel hull, by it's very nature, is always "inherently unstable" - requiring constant driver input to maintain sufaces on the water.

    The best designs try to limit the "difference" between Static CG and Dynamic CG and minimize the rate of change that this difference sees through the critical velocities.

    [​IMG]
     
  8. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Now, finally we have a person here who makes his living from tunnel hulls. :) Jimboat, I am sure you would have a lots of things to tell about this subject. First question, if you don't mind: could you please define the term "Dynamic CoG" a bit more precisely? It is not really clear from your brief explanation how you calculate it.
    Cheers
     
  9. Jimboat
    Joined: Feb 2002
    Posts: 255
    Likes: 17, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    Location of Dynamic CG (XCGDynamic) is the center of balanced moments of all aerodynamic and hydrodynamic forces (including lower unit thrust and drag) while the hull is under the specified running conditions, referenced fore (+) of the transom. (So, it's just the location at which all moments would be balanced). So, ideally, XCGStatic would like to be at same location, for stable 'flight'.
     
  10. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    So, if the boat was hypothetically a simple airfoil with just aerodynamic forces acting on it, what you call XCGdyn would be the center of pressure, if I understand it correctly?
     
  11. Jimboat
    Joined: Feb 2002
    Posts: 255
    Likes: 17, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    yup! (except that Cp would not include Thrust, also). XCGDyn includes ALL forces acting. It comes from the balance of moments in satisfying stability in flight, actually (which also includes Thrust and all drags, for aircraft) - modified to account for the additional hydrodynamic forces in a boat.
     
  12. DCockey
    Joined: Oct 2009
    Posts: 5,023
    Likes: 512, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    I'm confused. If the boat is moving at a constant velocity and not accelerating then:
    The sum of all forces boat must be zero.
    - and -
    The sum of all moments about any point must be zero including the moments from forces not acting through the selected point.

    In Daquiri's airfoil example if the airfoil is not accelerating then there must be an additional force and moment acting on the airfoil.
     
  13. Jimboat
    Joined: Feb 2002
    Posts: 255
    Likes: 17, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    Check out this article.
     
  14. kidturbo
    Joined: Sep 2013
    Posts: 40
    Likes: 1, Points: 0, Legacy Rep: 24
    Location: Southeastern Ohio US

    kidturbo Junior Member

    Thank you very much for joining this topic.

    Believe I'm starting to get some grasp of the concepts that go into these hull designs. Your graph helps answer my question as to why a 15 meter boat like "My Way" looks glued to the water at 200mph, but showed serious problems recovering from a simple wake jump going half that speed.

    Since static CG can change significantly as fuel is burned off, passenger load variations made, or from later rigging adjustments, how much "play" is taken into consideration during the hull design? Are these hull specs remaining well within the trim control of of today's surface piercing drives running high speeds? In layman terms, it seems the hydrodynamic control surfaces are being over powered by the aerodynamic lift generated.

    Last question. In two of the examples I've listed of incidents at speed, prop roost from other boats looks to have played a significant role in starting the blow overs. I understand the introduction of such outside forces is probably beyond the best WIG designs considerations. So what actions do you feel could be taken to best overcome these forces quickly enough to halt the departure?

    Theories like cutting the power to increase drag were once considered the norm for recovery. But these old school approaches using throttle or trim are no longer fast enough responses to counteract the lifting forces at present speeds. Even a seasoned operator is currently helpless once the bullets leave the surface.
     

  15. Jimboat
    Joined: Feb 2002
    Posts: 255
    Likes: 17, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    I certainly do agree with all of your comments. The driver can most always adjust trim angle to retain balance. This is a constant adjustment in a tunnel boat at higher speeds. The most significant changes usually come from crossing waves or unexpected wind changes. The designers job is match the power/weights of hull with designed-in aerodynamic and hydrodynamic lift surfaces. When well-matched, the tunnel hull is a very comfortable ride.

    You are completely right! Driving through another boat's roostertail is often enough to sufficiently upset the balance of a tunnel boat at speed, that the driver can not react quickly enough to adjust. There are lots of examples of high speed blow-overs caused by just this situation.

    In some conditions, cutting power can make things worse, since it can further disrupt the balance. (The propeller and lowerunit are the only remaining control surface!)

    Yes, right again! Once the lower unit bullet and propeller leave the water surface, the boat is an airplane without the control surfaces to make any adjustments at all.
     
Loading...
Similar Threads
  1. redviking
    Replies:
    13
    Views:
    5,518
  2. dustman
    Replies:
    70
    Views:
    2,889
  3. dustman
    Replies:
    10
    Views:
    1,229
  4. Dave_S
    Replies:
    7
    Views:
    950
  5. Inquisitor
    Replies:
    5
    Views:
    1,550
  6. tea_floss
    Replies:
    8
    Views:
    2,466
  7. tea_floss
    Replies:
    0
    Views:
    1,192
  8. Antoine Haettel
    Replies:
    1
    Views:
    1,891
  9. Luke Frisbee
    Replies:
    26
    Views:
    1,213
  10. dustman
    Replies:
    78
    Views:
    3,720
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.