Wing mast prebend

Discussion in 'Multihulls' started by MichaelRoberts, Jan 15, 2019.

  1. MichaelRoberts
    Joined: Sep 2015
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    Location: Australia

    MichaelRoberts Junior Member

    I would be very grateful for your advice about a 20 m wing mast I am building for my catamaran.

    The question is will I put some pre-bend into it - say about 100 mm.

    With no pre-bend, the main sail luff is straight and the sail camber will not change when the mast rotates relative to the boom. There is no back stay.

    On the other hand if the mast is prebent, the middle of the sail will be pulled taut as the mast rotates. Probably good for going up wind in 15 to 20 knots but maybe not on a reach.

    A bit of background:
    1. height 20 m
    2. sits on a lateral hinge on a stub on a thrust race - so the rake can be adjusted
    3. at 9 m up the cross section is 550 mm chord x 180 mm thick - it tapers to the base and the top
    4. cross section is a truncated Clarke Y - like a D section - as discussed in a previous thread
    5. construction - 40 CNC cut ribs, stringers in notches, lateral webs, ply shell then 5 mm uniaxial carbon-epoxy laminate with interleaved glass bi-axial for torsional strength.
    6. designed maximum compression 14 tonnes, Eular buckling load > 40 tonnes,
    I would really appreciate some expert opinions - I'm going to start laminating in a couple of weeks then it will be too late to bend it.

    Thank you
     
  2. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    100 mm over 20 m is peanuts, A class cat use currently 100 / 140 mm over 9 m !
     
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It will be the opposite. As the boom rotates from center, the middle of the sail will become looser and increase camber.
     
  4. MichaelRoberts
    Joined: Sep 2015
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    Location: Australia

    MichaelRoberts Junior Member

    You're absolutely right Gonzo - what was I thinking - must be breathing too much hot epoxy.

    If the sail outline were projected onto a vertical plane through the axes of the pre-bent mast and boom the luff would be an arc when the mast is not rotated and as the mast rotates the arc flattens eventually becoming a straight line when mast is at 90 degress to the boom. Consequently more camber/belly in the middle of the sail.

    Another question - what is a reasonable weight? My calculations indicate 210 kg. I'm thinking if the mast length doubles the weight should increase by four times (area) or maybe eight (volume). So when I see the VMG mast coming in at around 60 kg for a 9 m mast I feel OK about my 200 kg. This thing is not for racing, just survival in the Pacific.

    All comments welcome.

    Thanks
    Michael
    20190116_180416.jpg
     
  5. Doug Halsey
    Joined: Feb 2007
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    Doug Halsey Senior Member

    The Clark Y section is asymmetrical. Why complicate things needlessly?
     
    revintage likes this.
  6. MichaelRoberts
    Joined: Sep 2015
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    Location: Australia

    MichaelRoberts Junior Member

    Hi Doug
    Clarke Y - I should have explained - I used the upper surface and mirrored this about the chord line - so the section is symmetrical - here is a pic of some of the CNC cut ribs
     

    Attached Files:

  7. patzefran
    Joined: Feb 2011
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    Location: france

    patzefran patzefran

    Not the good way to derive the correct symmetrical profile. Instead to mirror the upper part he should have used the thickness between the upper and lower profile.
    Anyway, the NACA 00XX series is derived from the Clark Y profile !
     
  8. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    In Lorient (France) the company LORIMAT which is currently making the longest teardrop masts for big trimaran, and has one of the longest autoclave in Europe.

    To cook their mast in the autoclave, they put a little block of some material, in the middle, under the luff of the mast mold to give it some prebend:
    The mast mold with carbon layers inside, ready for polymerisation, will bend under its own weight when on the frame for the autoclave.
    No idea how much is it in %¨of mast lenght, it cannot be more than what the mold can stand.
    Cheers

    LORIMA http://www.lorima-carbon-mast.com/en/index/
     

  9. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Prebend with diminushing mast chord going to the bottom can improve marginally the efficiency of the bottom of the sail.
    Especially if the sail is decksweeping.
     
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