Pod cat square rigger cold weather cruiser project

Discussion in 'Multihulls' started by Autodafe, Feb 4, 2012.

  1. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    How did you go about designing your crossbeams?
     
  2. SeaweedSam
    Joined: Feb 2012
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    SeaweedSam New Member

    Quite a machine indeed. Cleverly done interior features . Curious why you didn't "proa-tize" the design since it appears that this could simplify your rig .Make the masts freestanding and rid yourself of that crossbeam aloft and the standing rigging too. Theres plenty of science already out there on freestanding Junk rigs. BTW , The flexible taper aloft of FS spars save the rig in sudden gusting conditions and allow less reefing. This would all save you some KGs. Since you opted for no cockpit, minimal sail handling mechanics and the steerable kickup CBs ( I love em') then the logic says PROA. Definitely not hating it and can't wait to see the ongoing development Sam
     
  3. Autodafe
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    Autodafe Senior Member

    The loads I calculated as per post #25.

    I didn't want torsional stiffness, I wanted maximum "depth" to give strength and stiffness in vertical bending without excessive windage or weight, so open truss seemed like a good idea.
    This also appealed because they provide effective guard rails at the front and back with (almost) no additional weight or windage over the basic structure.

    I then drew half a dozen different planar trusses with different numbers of elements (and hence angles) and did a manual load analysis on each to work out the required sections, and hence weight and windage. Then a Euler buckling check on each member at max load.

    The one I selected isn't the lightest possible. I ended up selecting one without many elements mainly because that makes the sections quite substantial and more resistant to physical damage.

    George
     
  4. Autodafe
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    Autodafe Senior Member

    Thanks for the compliments.

    I seriously thought about both proa and free standing masts.

    Largely I just prefer the bilateral symmetry of a cat for aesthetic reasons.

    I aslo have more faith in the behaviour of a cat in extreme weather than a proa. It may not be true, but it seems to me that by having more stability in one direction than the other (the big proa advantage) you are more susceptible to events from unknown directions such as microbursts, confused seas and rouge waves.

    I'm a freestanding convert, but having chosen a catamaran with low hulls and a biplane rig for other reasons, then free standing just doesn't add up structurally. My staying angles are low enough that the rig loads are lower than the sea loads, so I can't save structure weight that way. The low hulls mean bearing loads would be very high for freestanding.

    The strut up high has it's drawbacks, but upsides too:
    -somewhere to put the nav lights and aerials (not as trivial as it sounds with rotating masts)
    -it has enough buoyancy that the boat will stop at ~105degrees rather than capsizing fully in a pitchpole (assuming the rig doesn't come apart).

    George
     
  5. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Are the crossbeams continuous from hull to hull, or are they each in two pieces and attach at the sides of the pod? It looks like the aft beam could be continuous, and the fore beam made of two sections. How is wracking in the horizontal plane resisted? Are the solid decks structural; do you have diagonal wires under the trampoline? Does the pod contribute to the cross beam structure, or is it just supported by the cross beams?

    In the event of a pitchpole do you anticipate the catamaran will remain supported by the mast strut and the nose of the pod and the bows of the hulls, or do you expect the catamaran to come to rest on its side, supported by the mast strut to prevent it from completely turning turtle? Do I understand that you will attempt to right the catamaran over the sides, not over the bow or stern. Here is a link to Tony Bigras' idea of how to right a completely inverted catamaran over the bows. http://www.ideaintegrator.com/boats/o7/flipper.htm

    The sheets on conventional fore-aft sails can typically be quickly released if a strong gust hits. Can your square sails be quickly luffed? Can the sails be controlled completely from inside the pod (raising, reefing, trim)?

    Since you plan to sail shorthanded I assume you will use some sort of self-steering mechanism. Will you use an autopilot or a wind-vane system?

    Will the electric motor be permanently installed in a hull(s), or will you use some sort of electric outboard that will only be deployed when needed? Will you also have a bow thruster to aid in maneuverability?

    Thanks for humoring my endless questions, and for your very thought-out answers.
     
  6. Autodafe
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    Autodafe Senior Member

    I N,
    Both crossbeams are continuous.
    The front of the pod is to be built flat below the windscreen, and the curved nose section under the window is actually a foam/frp fairing attached to the beam. Everything under the front bench and in front of the sheeting winches is part of the front cross beam.

    The solid deck area is the primary load path for horizontal wracking.

    I expect that whether the boat ends up on its side or bows will depend on the dynamic details of the crash; either will be a stable resting state.
    The righting system is designed to be able to flip over the bows as well as sideways.

    In order to luff the sails I need to manually rotate them (or the boat), which is done using winches from inside the pod, but it will take 10-20sec due to a high reduction ratio.
    So I will have to be pretty conservative with reefing. The low CE given by a short biplane rig helps here.
    On the plus side the rig can be luffed on any heading.

    With the current design I will have to go to the masts for raising and reefing the sails, but as mentioned sheeting is done from inside.
    All sail control could be run into the pod if required, but with some additional complexity due to the 360deg rotation of the mast. For the moment I prefer the light weight and simplicity of leaving the halyard and downhauls on the masts.

    For self steering I'll start with an autopilot. I would quite like to try putting a wind-vane on one of the rudders when I have time to tinker.

    I'm thinking of two electric outboards, but I'm having mixed feelings about where to mount them. If I put them off the sterns I can use them in areas with lots of floating debris or ice, but they'll be hard to keep immersed in a chop. If I hang them off the wing decks they'll be pretty exposed.
    I want to be able to retract them to get the best sailing performance and shallow draft when required (which rules out fixed props), and I also want to be able to use them as water generators to top up the batteries while sailing (which rules out jet drives).

    No objection to questions (or even criticism). They help me clarify my thoughts.

    Cheers,
    George
     
  7. Autodafe
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    Autodafe Senior Member

    Just checked out your link for the righting system. Very similar to what I was thinking, only I hope to get away without flooding any compartments due to the buoyant rig and pod on my boat.
     
  8. Othmar
    Joined: Dec 2006
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    Othmar Multihulls ...

    Hi all, want only comment, that I made the design of FUTURE 10 and 12 about 20 years ago. Nothing really new since then happened ;)
    Othmar
     
  9. spidennis
    Joined: Feb 2007
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    spidennis Chief Sawdust Sweeper

    Othmar,
    Your design caught my eye! Had you built any of them?

    and since this design uses the biplane rig, I invite you over to my new thread on just this subject in this forum.

    http://www.boatdesign.net/forums/multihulls/biplane-rigs-41811.html
     
  10. Autodafe
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    Autodafe Senior Member

    Good to see you're about here Othmar.
    Nice looking designs.
     
  11. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    George,
    Why type of tender will you use, and how will you stow it on passage?
     
  12. Autodafe
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    Autodafe Senior Member

    I quite like the look of this
    http://www.duckworksmagazine.com/08/designs/catskiff/index.htm
    And I may get around to building something similar, but at the moment it's a 12' "tinny".

    I will have davits off the back of the pod, but I haven't designed the structure yet, so I haven't included it on the plans.
    I'm tossing up whether to hoist the tender level and upright, or to hoist one side only so it hangs on edge. Hanging on edge would increase the windage, but stop it filling with rain water or spray.
    Maybe I'll set up for both.
     
  13. Milan
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    Milan Senior Member

    That cat was not designed for ocean cruising. Look at the "acomodation".
     

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  14. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Attached Files:


  15. Autodafe
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    Autodafe Senior Member

    I'd think the lack of load carrying capability for water and food would be far more of an issue than the spartan accommodation for me.

    Either way, IIRC, the Dashews themselves describe it as an ocean cruiser/racer and I have no reason to dispute their terminology.

    George
     
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