Design of 16' foot beach catamaran

Discussion in 'Multihulls' started by hashtag_laeuft, Aug 15, 2017.

  1. Manfred.pech
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    Manfred.pech Senior Member

    I think your Hurricane 5.9 is a very fast boat and it does not need help from maderform. Nowadays maderform will be forgotten and the boats will be fitted out with foils. But it is a good idea to apply some of the results to sit on kayaks, surfboards and sup boards. As far as I know maderform has worked on boards too.
     
  2. hashtag_laeuft
    Joined: Aug 2017
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    hashtag_laeuft Junior Member

    Hey everybody ...

    sorry that I wasn't here for a longer time. Meanwhile I was tinkering on a preliminary hullshape for my new catamaran. I decided to lean on some modern forms and until now I came up with this:

    iso.png
    oben.png
    seite.png


    Each hull will be built out of 7 panels + 3 decks and a transome. It should be used for a medium mixed crew of about 150 kg.

    Currently I have the following hydrostatics for fresh water (1000kg/m³) on DWL:
    L: 4,99 m
    B: 0,4 m
    D (immersed): 0,149 m
    Displacement: 128,5 kg (maybe a bit too less?)
    Wetted Area: 1,987 m²
    Cp: 0,566
    Cb: 0,472
    Cm: 0,841

    Do you have any remarks or proposals for improvement?

    Best regards, Nico ...

    iso.png oben.png seite.png
     
  3. patzefran
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    patzefran patzefran

    IMHO, a beach catamaran hull should be designed with total weight / displacement on the leeward hull §
     
  4. Lurch723
    Joined: Aug 2012
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    Lurch723 Junior Member

    Agreed with Patzefran, you look like you have plenty of volume forward and the panel surfaces look good for crew to trapeze from, the wave piercing bows work, and should shed water quickly when moving through waves. I think it all comes down to how much sail area you are intending to drive through the leward hull.
    Try it out.
     
  5. hashtag_laeuft
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    hashtag_laeuft Junior Member

    Hey guys ... thanks for your feedback.

    you are right that I have to check my design in the inclinded condition when one hull has to support the wohle weight of the boat and the crew.
    The given displacement above is that of one hull in upright design condition. The total hull volume is about 650 L.

    So I checked several heel angles with an expected (total) displacement of 273 kg ... (150 kg crew + 123 kg boat). At 15 deg. of heel the hull floats like this (no trim!):

    heel_seite.png heel_vorne.png

    Next step will be to weight all parts of my existing catamaran (17 foot) to do a weight estimation and to find total LCG and VCG. Then I can recheck the model also including the right trim.

    The design should be according to the F16 class regulations, so maximum sail area is limited to: main sail - 15 m²; jib - 3,7 m²; gennaker - 17,5 m².

    I still have some questions and maybe someone can help me with these:

    -> Do you have any hydrostatics or performance informations from similar boats to compare them with my design?
    -> where should the longitudinal center of effort of the sails be located in correlation with the longitudinal center of lateral area, the LCF and LCB?

    Thanks for your advice and best greatings ... Nico
     
  6. hashtag_laeuft
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    hashtag_laeuft Junior Member

    Hi at all ...

    I have some new informations. The Center of Effort of the sails will be located at about LCoE = 1.93 m from the AP. The longitudinal center of lateral resistance will also be at this point with an LCLR = 1.95 m. LCB and LCF will be much more further ahead. I have checked them in three different conditions. First upright an untrimed, secondly with 10 deg. of heel and thirdly with 10 deg. heel and 0.1 m trim. The values are as followed.

    Upright | 10 deg. heel | 10 deg. heel + 0.1 m trim

    LCB: 2.42 | 2.31 | 2.12
    LCF: 2.24 | 2.17 | 2.16

    How will this effect the helm? Should I try to bring them further aft? What's your opinion about that?

    Greatings, Nico ...
     
  7. patzefran
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    patzefran patzefran

    IMO, LCB and LCF choice should be determined according to LCG and the maximum needed pitching restoring moment, not the effect on the helm.
    Pitch restoring moment determine the maximum reaching/downwind velocity. The helm effectiveness is related to LCLR and the distance between hull + daggerboard CLR and
    rudder CLR
     
  8. hashtag_laeuft
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    hashtag_laeuft Junior Member

    Ok ... the LCG could be varied by the crew weight LCG = 1.24 - 2.12. But when reaching the forces of the sails will also push the bow down and create a trim moment. I never heard about that pitch restoring moment, but I will try to find some informations about that. Du you have any good source for that?

    The LCLR of the bare hull is located at 1.96 m from the transome. The pressure point of the daggerboard will be at about 1.9 m.
     
  9. patzefran
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    patzefran patzefran

    When the boat pitch under the sail's thrust, LCB moves forward, restoring moment is (XLCB-XLCG)*WEIGHT, it is why you should put adequate depth and volume of hull forward,
    to promote forward motion of LCB with trim down of the bow.
     
  10. hashtag_laeuft
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    hashtag_laeuft Junior Member

    Beams - round, square or round+square ?

    Hi at all,

    I have another question. I thought about what profile for the beams would be the best. Intuitively I woud use a round profile because of it's better mechanical properties than a square profile. The momentum of inerga and the torsion resistance of a round profile is quite higher than these of a square one.

    My goal is to hold the twist between the tow hulls at a minimum. I saw on some bigger catamarans for example the GC32 that they use a square profile for their beams but I can't explain me why. Is it because it guarantees a better fit between the beams and the hulls or are there other reasons to use a more square profile?

    Can anyone explain this to me, please ... or give me a hint to understand that?

    Thanks to all ....
     
  11. patzefran
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    patzefran patzefran

    "The momentum of inerga and the torsion resistance of a round profile is quite higher than these of a square one"

    Not right ! it is just the opposite, the momentum of inertia of a round tube is isotropic, but a square tube of the same dimension and thickness has higher
    momentum perpendicular to his faces. just because there is more material away from neutral axis.
     
  12. upchurchmr
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    upchurchmr Senior Member

    A round tube of the same weight as a square tube has higher torsional resistance. A square tube of the same maximum size (height and width) as the diameter of a round tube has more inertia, but higher weight.

    Assuming equal thickness.

    Many times the shape used has more to do with manufacturing convenience than the "best". The method of attachment to the hulls may have more influence on the resistance to pitching of the hulls when comparing one to the other.
     
  13. hashtag_laeuft
    Joined: Aug 2017
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    hashtag_laeuft Junior Member

    Yeah that's it ... in relation to the same sectional area.
    So the best would be to have a square profile where the beams are attached to the hulls to provide a tight fit and then a round profile between the hulls, right?

    I'll think about that ...
     
  14. BobBill
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    BobBill Senior Member

    I did not read the compete thread. First thought was that I would not build a cat, I would consider an outrigger...

    Hull shapes be interesting. Me would look to A-cats first to see fast and light...but that is moi and moi knows squat. Just first thoughts as I began to peruse.
     

  15. Lurch723
    Joined: Aug 2012
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    Lurch723 Junior Member

    The main benifit to using a square type of spar over a round spar section is you can bias and favour one axis over the other for strength. So if your fore and aft loads are low relative to your lateral bending moments you can easily add stiffness into these areas. A box section essentially behaves like having an additional set of spar caps but on its side to cope with the drag loads.

    Now a tube gives you one set of stiffness data but carries it three sixty degrees so for a given radius you may be carrying more build than necessary in a given axis. The box section on the other hand can be built to one hundred percent required stiffness and then on the other axis be built down to fifty percent, kind of strength and weight where you want it. But your week points in fabrication are the corners where all loads are focused so careful design is needed to transfer these loads.
     
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