Flying - What does it really take?

Discussion in 'Multihulls' started by Marvout, Sep 5, 2010.

  1. Marvout
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    Marvout Junior Member

    I was afraid that my McGowan Zip question would turn into a 'flying the main hull' discussion. It wasn't my intent.

    My question - In the real world, presuming I have a 22'-25' Tri that is designed to fly the main hull, what does it really take to make that boat fly the main hull? I almost get the impression that I can just pull out of the birth, whip it up on one hull and balance my way around/past everything else on the water.

    How much wind do I need?
    How constant does it need to be?
    How many crew? Can this be done single handed?
    What water conditions make flying possible/impossible?
    How much water depth do you need?
    How many times do you turtle your boat in learning how to do it?

    I've never done it, let alone even being on a tri. I think I've been on a Hobie with one hull in the air (for a few seconds <grin>).
  2. gonzo
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    gonzo Senior Member

    If you put up enough sail, a couple of knots of wind will do. Your questions don't have enough relevant information to get a significant answer. Either contact the boat designer, or someone has to recalculate everything. Recalculating would probably take a Naval Architect or Engineer about 25-30 hours. It would be cheaper and more fun to take the boat in the water and try it out.
  3. Doug Lord
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    Doug Lord Flight Ready

    If the designer says "the boat is designed to fly the main hull" the next question is "when?". The designer has to answer because ,like Gonzo said, it depends on the SA and how fast the boat is(how much the apparent wind is in a given true wind). I believe that USA would fly in 5-7 knots of true wind and do twice the wind speed or more in those conditions.
    Just a rough calculation with CE above CLR estimated shows that pressure per sq.ft for the boat in your other thread to fly would be above 3lbs- above 20k of wind. Maybe its not designed to fly in "normal" wind for the cruising version??
  4. Marvout
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    Marvout Junior Member

    So it sounds like flying is for the elite and the elusive. It seems it can't be answered with an experience based "I have a boat x and under y wind and z waves, I've been able to fly the main hull. I do find though, that even though the boat is designed to fly the main hull, in my area, I am stuck with dragging it through the water because flying conditions only occur 25% of the sailing time." Or, "I'd love to fly the main hull of my flyer, but I routinely sail alone and only dare do it with at least two other crew."

    Instead we are stuck looking at pretty pictures of mega sized mega million "projects" and fantasize that our little weekend toys are going to look like that too.

    Don't get me wrong. I understand your answers, that it depends on the design. I get it. You can't answer in specifics for all boats because each boat is different. Sweeping generalizations about flying the main hull don't exist. I knew that before I asked. But, you are also implying that each flying boat doesn't have anything in common with another either, and that just can't be true. Sounds like Flying the main hull is a calculator driven fantasy. And it also seems to be a matter of scale, most of the flying pictures are of gargantuan racing machines. ...For the common man with a little boat anyways.

    Let me change my question:
    For those of you who have boats designed to fly the main hull, what percentage of your sailing time are you up on one ama?

  5. captainsideburn
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    captainsideburn Junior Member

    as I understand it you wouldn't really want to fly the main hull in a boat not designed to, as much higher and different stresses than otherwise.
    Below is a seacart 30 on one hull. Not sure if its a boat for the common man though, certainly out of my league at the moment :p
  6. Doug Lord
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    Doug Lord Flight Ready

    Contact Henrik here and ask him:

    It's simple to play around with some rough calculations. The RM of my example was based on the designer's sailing weight of 1400lb @9'=12600 That assumes the crew sitting in the center of the boat. So to find how much wind pressure its going to take to fly the hull you need to estimate the vertical distance between the CE and the CLR(or center of lateral pressure)=the Heeling Arm. And then,roughly: (SA X WP) X HA= Heeling Moment, where SA =sail area and WP =wind pressure in lb.per sq.ft..
    --WP=25knots=3.3 lb./sq.ft.
    (windpressures taken from page 297 of Skenes after converting to knots*)

    For a small sport trimaran(not a cruising boat) I'd probably want to fly the main hull in 10knots of wind(or less) since that is the predominant wind in this area. I'd make sure the rig was easy to depower and to reef.
    Another good question the designer has to answer is what is the sailing angle with the main hull just clearing the water-this varies a lot-like from 10 degrees to 26 degrees for two diffeerent boats that I am familar with. A boat with lifting foils in the ama is likely to sail at less angle than a boat with no foils.
    * there is a more precise way to calculate heeling force presented in Princibles of Yacht design by Larsson and Eliasson. This method uses flat plate pressures and is conservative in real life. Its been my experience that you can sail in a bit higher wind than predicted by this method. Also, apparent wind would be significantly greater. But the method is good for reasonable estimates upwind.
    My new 12' tri design is designed to fly the main hull almost all the time-from 5 knots of wind on up. I probably would not go out in less than 5 so the answer is 100% of the time.
  7. Doug Lord
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    Doug Lord Flight Ready

    If you look at trimarans under 20' very ,very few are designed to fly the main hull. I wouldn't call those that do "elite" but maybe they are "elusive". If you look at the numbers a little you can figure out why: one reason is that if you design the thing to have enough RM for big wind then you have too much RM for light air unless you do something different with the design. Using a conventional layout with a wide beam(approx. square) on a small boat flying in light air is almost impossible. More and more new "sport tri's"over 20' LOA are coming along claiming to fly the main hull. I'd ask some questions:
    a. at what wind speed?
    b. at what sailing angle?
    c. at what sailing weight,incl crew?
    c. are there any enhancements to the pitch control of the boat when flying the main hull?
    d. Is the SA depowerable and reefable easily and quickly?
    e. does this correspond to the type of sailing you want to do and the conditions you sail in?
    f. do you realize how close to capsize you may be when flying the main hull?
    This is interesting, again looking at the numbers of various boats might help. But this is almost like looking at a 505 and an F18 and saying: "Look they each have a trapeze". Of course the F18 is much faster and more powerful and it has two trapezes. But it is true that they each have a trapeze in common.
    You seem to want simple answer's to complex considerations. For instance, predominant wind at a location will determine when a boat flies the main hull.
    If the boat is designed so that it will fly the main hull in those conditions the percentage of time the boat flies will be great. If another boat is designed to fly the main hull but in heavier air ,or with fewer crew or with less sa, well......
  8. tspeer
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    tspeer Senior Member

    Since you're asking the question, I take it you're not experienced in mulithulls. Before you ask about how to fly a hull, I think you should have a sound answer to the quesiton, "How am I going to right the boat after a capsize?" Because you will capsize while learning the technique.

    Beach cat sailors capsize a lot learning to sail their boats, but their boats are also easily righted. You are going to need assistance to right a tri of that size, so you better have a chase boat with a crew that knows what to do.

    Shuttleworth's stability index will help you estimate how much wind it will take:
    Where :
    D = displacement (lbs).
    CE = height of the center of effort above the center of gravity (C.G.) in feet.
    SF = windspeed in MPH that the boat has to reduce sail.
    SA = sail area in square feet.
    B = beam between the cenerlines of the outer hulls in feet.

    That depends on how good a sailor you are. With crew, you can send them to leeward and "do the wild thing". You can single-hand a boat that size, and the fewer crew to windward, the easier it is to fly the hull. But you better have the mainsheet in your hand and be able to play it actively.

    That will depend on your boat and you.

    Minimum: enough not to drag the board
    Desired: more than the height of the mast

    As many times as you go over. A tri will turtle every time.
    Fanie likes this.
  9. Doug Lord
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    Doug Lord Flight Ready

    Thanks for the info on the Stability index,Tom. I tried McGowans 23 and two small tri's using the formula above:
    NOTE: red=guestimate
    For McGowans 23
    LOA 23'
    Beam 20'
    Main + jib=302 sq.ft.
    CE-CG= 11'
    Sailing Weight= 1400lb(assumed this includes crew)
    Dis. between centerlines of outer two hulls=9.5'
    .5 X 9.5 X 1400=6650
    302 X 11=3322
    6650/3322= 2.00
    9.48 X 2 = 18.97 mph
    Just for the heck of it thought I'd try it on an existing small tri and a small tri design that is underway:
    LOA 14'
    Beam 12'
    Main + Jib 123 sq.ft.
    CE-CG= 8.4'
    Sailing Weight= 395lb.(175lb crew)
    Dis. between centerline's of outer two hulls= 5.75'
    .5 X 5.75 X 395= 1135.6
    123 X 8.4= 1033.2
    1135.6/1033.2= 1.1
    1.1 X 9.48= 10.43mph
    LOA 12'(+ gantry)
    Beam 16.5'
    Main + Jib 163 sq.ft.
    CE-CG 10.5'
    Sailing Weight 395lb.(240lb. crew)
    Dis. between centerlines's of outer two hulls= 7.5'
    .5 X 7.5 X 395= 1481.3
    163 X 10.5= 1711.5
    1481.3/1711.5= .87
    .87 X 9.48= 8.25 mph

    The numbers for the small tri's don't appear to be applicable.....
  10. Fanie
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    Fanie Fanie

    When I first came to the forum I knew nothing about sailing, and one distinction I was told is when you sail the big trimaran it does everything slowish. If you come down in size, things tend to speed up, hence a capsize would happen somewhat faster and more difficult to control on a small tri.

    Often one has an 'idea' of how it would be based on one's perseption, but it is often different in real life. Best is to find a tri and go for a ride to get a feel for it.

  11. Doug Lord
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    Doug Lord Flight Ready


    When I first did this I got it wrong. It is based on Shuttleworths formula in post #8 and I used the wrong measurement for the "beam between the centerlines of the outside hulls". Below is the correct info:

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