Free standing mast on a small boat

Discussion in 'Boat Design' started by jelfiser, Dec 8, 2008.

  1. jelfiser
    Joined: May 2006
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    jelfiser Senior Member

    I'm a Young Designer and i'm working on the structures for a 6 meters cat boat with a free standing mast.
    The mast will be in carbon fiber pultrused

    From the RM 30 deg I've calculated an equivalent force on masthead (8m ) that's 110 kg with a safety factor of 1.5.

    It cames out that a round section of 6000 kg/cm2 carbon fiber with diameter of 8 cm, thick 5 mm would support the load with no problems. it weigts 15 kg with constant section.

    that's a great advantage against aluminium where it comes out a section of 12 cm diameter with a total weight of 48 kg. (!!!)

    Anyway i' ve some doubt about the wishbone rig and i'm feared about the load coming from the boom.
    Also i'm evaluating wich kind of carbon fiber would be better if High Strengt, or hig modulus , or intermediate...

    I didn't found nothin about taht on literature ... may be someone has got any experience bout that ?

  2. messabout
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    messabout Senior Member

    The wishbone boom will place a deflection load on the mast in a higher location than a conventional boom arrangement. From a strictly mathematical viewpoint, it will induce more mast bend. In operation this may not be the case. The wishbone will not exert a higher horizontal pressure on the mast than a boom nearer the base of the mast. Although the wishbone at a higher mast location is in a less advantageous position, it may not exert as much pressure as is first anticipated. One reason is that a boom vang is usually not required on such a sail. Another reason is that the force of the boom is not a horizontal one and the lower part of the mast will have a tension component to counteract the compression component. The wishbone itself provides the downforce on the leech, and it is easily adjustable. Subtract the thrust of the vang and you may actually have less thrust with the wishbone than on the more common boom. The wishbone rig has an additional advantage in that it may not require as much tension on the main sheet as a conventional rig.
  3. Gary Baigent
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    Gary Baigent Senior Member

    mainsheet vang system

    Alternatively you can go with the conventional boom attached very close to the deck base position on your mast and then have end of boom main sheeting attached to a full width mainsheet track - that will reduce rig loads (compared to highly loaded and poorly angled conventional vangs) as the traveller/mainsheet becomes your vang as well - and it is situated correctly under your mainsail leech where IMO, it does the hard work with out loading up your gear or rig (at least it feels that way to me, others may disagree). However if your hull has narrow stern sections, then this option is of little use - you need plenty of beam (a la cat boat design anyway) to make it work properly. You can go even further by having a curved track and gain even more control.
    Another plus is you haven't got a draggy wishbone a third of the way up the mast, right where you want clean air flow.
    ps: why not consider an airfoil shaped cat mast like a Fin?
  4. Gary Baigent
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    Gary Baigent Senior Member

    correction: like a large Finn - which also rotates - designed in 1950 and decades ahead of its time.
  5. HJS
    Joined: Nov 2008
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    HJS Member

    I have sailed with two freestanding mast and a special boom since 1982.
    Mast length 9,7 m grp 18 kg
    It works well.
    Bot today I would try a freestanding mast and a hoytboom


    Attached Files:

  6. Eric Sponberg
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    Eric Sponberg Senior Member


    If you haven't already, go to my website for a complete description of free-standing masts.

    Then, a few points of advice. In most composite pultrusions, the tube has little to no off-axis fiber, and this is no good for a mast. For a round section mast, you need at least 10% +/-45 deg fiber and 10% 90 deg fiber, leaving 80% unidirectional fiber parallel to the mast axis. Placement of these layers is critical (+/- 45 deg fiber on the very inside and the ery outside of the tube) and the laminate must be balanced and mirror image of itself through the mid-plane. For a mast that is not round, but say more elliptically shaped in section, more fiber must go off-axis, and the numbers above double, leaving 60% unidirectional fiber.

    Also, your safety factor is not high enough. Any tubular composite structure under bending load will suffer permanent, broken fiber damage past a loading of half the breaking strength (factor of safety equal to 2.0). Therefore, you should have a minimum factor of safety of 2.5, and conservatively, 3.0. These should be based on the maximum righting moment of the boat, not the righting moment at 30 deg.

    Finally, I am a very strong propopent of wishbone booms, and if you look at my Eagle/Globetrotter designs in the sailboat section, you will see that I like half wishbones. Many catboats and cat ketches use wishbone booms, but all the Herreshoff cat ketches used half wishbones which work quite well.

    I hope that helps.


  7. jelfiser
    Joined: May 2006
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    jelfiser Senior Member

    Firsth of all thank you very much for your suggestion,

    here an image of the design, i have to say that i'm still working on it and many details may change.
    That's a semi rigid inflatable sailboat with a wishbone rig.
    The design is basically focused on simpicity, safety ( collision / boom uninkability... ) low manientance ad trasportability.
    I have to think that they will build more than one, so the the choose should be driven from production.

    To Messabout:
    Yes i agree, my truble was to evaluate some forces, but as it seems it is not so important

    To Gary Baigent

    I wanted the boom high for safety, yes i was thinking also to an airfoil section ... i have still to evaluate varius alternatives, however it seeem that a constant section would be more simple to build( the only one i can do with poltrusion)

    To HJS

    Thank you for the information ... the boat looks great !!

    To Eric

    yes ...once i've read your page, thank you for informations.

    indeed i've the possibility to make the mast with poltrusion and filament winding, i still have to talk with them ( note that they never build up a mast)
    and evaluate cost of different alternatives,
    I immagine that a constant section would be less expensive but also less efficent... I also think that i'll do a finite element analysi ( ansys ? )
    and i 'll post soon some new calcoulation based on your suggestion.

    Attached Files:

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