Cat hull advice desired

Discussion in 'Multihulls' started by XHeeler, Apr 1, 2009.

  1. XHeeler
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    XHeeler Junior Member

    Hi, long time lurker here. Now starting to pencil out ideas for possible future build of a 11-12 meter coastal cat. My wife loves level, thus the handle.

    With this thread I would like your reaction to the hopefully attached image of an aft hull cross-section. I have widened the beam both inboard and outboard at the level of a mattress, while the vertical sides and semicircle bottom might be the performance choice.

    I am trying to compromise between a narrow hull at the waterline and wider beds in the stern. For performance or building reasons, would you change anything?

    Also, I wonder if bridgedeck clearance is only a height or an area. As in how much can you encroach on the deck/hull joint and minimize clearance area?
     
  2. XHeeler
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    XHeeler Junior Member

    Hopefully attachment will attach
     

    Attached Files:

  3. catsketcher
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    catsketcher Senior Member

    Look at Grainger and Shutle worth

    If you are going to design your own large cat I would say don't. There are issues with flaring the hull above the waterline. If you do decide to go ahead (remember you do not save money by designing your own - you will waste time materials and effort) read Shuttleworth's and Graingers home page

    cheers

    Phil Thompson
     
  4. Richard Woods
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    Richard Woods Woods Designs

    I have to agree with Catsketcher. As I said on another thread, we all risk our lives everytime we go sailing. Would you design your own aeroplane or even your car??

    Thinking about (and thus maybe sketching) what you want in a boat is of course a sensible approach. As is doing as much research as possible. You will find that most good ideas have already been well tested.

    For example I have been using the flare and hull knuckle you have sketched on my designs very successfully for about 20 years. John Shuttleworth and the late Pat Patterson use similar hulls. As did Derek Kelsall with his trimaran designs in the 1960-70's.

    Curious that these are all English designers, no one else seems to have gone this route.

    You can see more about hull shapes, bridgedeck slamming etc on my website (just updated incidentally)

    I hope you find the boat you are looking for.


    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  5. Alik
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    Alik Senior Member

    Section is quite standard, nothing to worry about.

    What is important - do not obstruct the bridgedeck with local recesses for ladders, beds, etc. These things used to drag water on seaway.

    Vertical clearance of bridgedeck - minimum 4-5% of hull length.
     
  6. XHeeler
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    XHeeler Junior Member

    Thanks,

    Definitely not going to design a large cat, but want to prepare concepts of something to take to a naval architect. I want to understand the compromises necessary. I want to draw up ideas for the designer to massage instead of just saying I want some changes to an Atlantic 42 since I want a cockpit forward, pilothouse design. Maybe Mr. White would massage his plans for me. But, I didn't want to go to him without some thought and research.

    I have built a 26 foot monohull without design help, but recognize this is a challenge that will need plans drawn up by a designer. I do want to build her, because there is great pleasure in that.

    I don't think I can afford what excites Mr. Shuttlesworth or Mr. Grainger.

    So is there a good way to put a 5 foot wide queen bed in an aft hull in a 35 plus foot catamaran with performance more like a 4 foot width at the waterline?
     
  7. XHeeler
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    XHeeler Junior Member

    Thanks,

    Definitely not going to design a large cat, but want to prepare concepts of something to take to a naval architect. I want to understand the compromises necessary. I want to draw up ideas for the designer to massage instead of just saying I want some changes to an Atlantic 42 since I want a cockpit forward, pilothouse design. Maybe Mr. White would massage his plans for me. But, I didn't want to go to him without some thought and research.

    I have built a 26 foot monohull without design help, but recognize this is a challenge that will need plans drawn up by a designer. I do want to build her, because there is great pleasure in that.

    I don't think I can afford what excites Mr. Shuttlesworth or Mr. Grainger.

    So is there a good way to put a 5 foot wide queen bed in an aft hull in a 35 plus foot catamaran with performance more like a 4 foot width at the waterline?
     
  8. XHeeler
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    XHeeler Junior Member

    Sorry duplicate happened with being logged out
    Glad I didn't loose the text though
     
  9. XHeeler
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    XHeeler Junior Member

    Another question- fore hull this time

    Newbie question

    Why don't sailboats, and especially cats, have the prominent bows that saltwater motorboats do? I don't think it's the bow wave since lake boats have the low bow.

    To prevent pitchpoling, why not raise the deck level as the hulls narrow to preserve volume and buoyancy? Hobie cats have a bit of a raise at the bows.

    Or keep the foredeck level, but significantly higher than the bridgedeck, maybe a foot or two. I could have designs on that volume.
     
  10. XHeeler
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    XHeeler Junior Member

    Present concept

    I have sketched out ideas for bridgedeck and pilothouse. The forward cockpit is offcenter to port to open up room in the forward starboard hull for a possible owners cabin.
    I haven't worked out the hulls yet, hence the hull questions.
    This is a brief sketch on Google's free sketchup. I've been working on paper.
    Not meant to impress, but show you where I'm coming from.
     

    Attached Files:

  11. catsketcher
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    catsketcher Senior Member

    be careful with flare

    You must be careful with pushing the hull shape so that it becomes too assymetric fore and aft. Cats differ from tris or monos in that the displacement of each hull can go from 0 when the hull flies to twice the dsiplacement when the other hull flies.

    This means your nice drawings have to be done at various immersions. 5 foot of beam low down in a 35 ft cat seems exceesive and may lead to pushing the bow down when the hull is immersed. This is because the stern sinks slower than the bow so the bow goes down quickly. You could raise the bow but then you increase windage which will have safety implications if you can't head up in a big blow.

    Like all good things the boat must have balance - in accomodation, rig size, hull shape etc. If the bed is raised high enough you certainly can have a 5 foot bed. Just don't try to have it low and at the end of the boat too.

    cheers

    Phil
     
  12. Alik
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    Alik Senior Member

    XHeeler, I would say that scope of questions makes me worry...

    Illusive saving on design done by professional naval architect will later cause expensive re-work of mistakes (not on paper, in materials!) and reduced resale value of boat.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    you need to do a proper global structural analysis first to see if what you want to do is feasible..details come later!
     
  14. Richard Woods
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    Richard Woods Woods Designs

    I assume you want a sailing catamaran, although you don't specifically say so.

    In which case you must decide where you will step the mast and how you will transfer the mast compression loads into the hulls. Usually this means a crossbeam or a bulkhead. And usually the mast is pretty close to half way back from the bow. You also need to think about the genoa sheeting, and what stops the deck lifting under the genoa tracks. Then you can think about the interior layout.

    Also remember that the centre of gravity of the boat must be over the centre of buoyancy if the boat is to float level. So you cannot have a fine stern and the accommodation aft, for example.

    Keep thinking, researching and asking questions. Answers will come from here and elsewhere

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  15. XHeeler
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    XHeeler Junior Member

    Thanks for your input and advice

    Presently my sketchs are for a 40x20 foot cat, and when I've massaged everything and thought through the compromises, I will take this to a designer/architect. My goal for them is to reduce the size of the boat as much as possible to keep as many concepts as possible, to optimize the boat for building by whatever method is the best compromise, improve on the plans for an eventual sale, and do the usual systems design and overall engineering. Not trying to be too fringe here. I understand your need to caution people, as well as my ignorance.

    Now to another idea. I want an aft deck for entertaining when anchored. I have drawn a support structure of stainless steel tubes which are level with the bridgedeck. There are two panels which attach and cover this area, which are stored underway forming a low wall aft between the stairs. These two panels may contain solar panels on one side and sun deck on the other. This would give me a 9x10 foot rear deck deployed and a 3 foot deck when stored.

    Hope the attachments work, just learning this.

    Appreciate any advice.
     

    Attached Files:

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