open bridgedeck catamaran design

Discussion in 'Multihulls' started by BobH, Apr 17, 2015.

  1. BobH
    Joined: Apr 2015
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    BobH Junior Member

    re-rigged

    OK, single mast...simple.
     

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  2. saltdragon
    Joined: Oct 2014
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    saltdragon Junior Member

    Gosh! a Hi-tech Kankama rig :) that will keep Phil happy.
     
  3. BobH
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    BobH Junior Member

    Right...I've got the sailplan for Robin Chamberlin's Cirrostratus 10. I do like it. I also really like what I see in this forum post on CruisersForum. I think the soft rigging would be a perfect combo with the three tiered cutter rig.
     
  4. catsketcher
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    catsketcher Senior Member

    I like it more now but then again I will like it any time it has a wishbone, cutter rig and a low cabin.

    Think about the exposed companionway hatches into each hull. I had these because my first cockpit shelter was, like yours, an extended windscreen. I was okay with walking around the open hatches but my 6 year old son slipped one day and fell down onto the cabin sole below - lots of blood and crying. More stitches and a guilty father.

    This was one major reason I extended the cockpit shelter - to protect people and stop water from falling into the hulls from on deck. After seeing my son plummet whilst in the lake I was anxious not to do this again offshore.

    Think of how you can stop water and people falling into the hulls whilst still keeping the cockpit usable. My eventual option was to have a pretty normal looking cabin with very good visibility. Look at Seawind 1000s for how they get a covered cockpit and good visibility.

    cheers

    Phil
     
  5. BobH
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    BobH Junior Member

    I got you...I have a four-yr-old. He'll be helping build by the time I'm done, I suppose. But the concern I have is that with 34 ft LOA and good bridgedeck clearance I'd have a really high cabin top. It just doesn't seem possible...and I want that clearance, and I want to keep the overall size of this project within practical limits of what I can complete in a reasonable amount of time and money....I just can't spend all his college money on my dream. I have looked at a lot of designs...but I hear over and over about bridgedeck slamming in heavy seas...the clearance on most designs under 36 ft is just too low by all accounts...all to get a cabin.
     
  6. catsketcher
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    catsketcher Senior Member

    So Bob you are in the design spiral - what do you need? Good bridgedeck clearance - 600mm minimum and good headroom in the hulls with payload for 1500 kg of cruising gear. Then you dictate your length unless you have a marina berth already.

    If you not increase anything else in size (beam, height) then extending the boat to 37ft will cost about $1000 and make the boat worth much more, sail faster, pitch less and carry more payload easier. If you can get over 34ft then you can have a cabin/dodger and get what you want. Can you economise on something else? The diesels would be first on my list. This looks like a nice sailer and you will be motoring with outboards at 6 knots using about 1.2 litres per hour. Just bolt on and go.

    If you do need to stay at 34ft and don't want a cabin/dodger (which is fine in many ways) then small blisters over the companionways may suit. Work out the ergonomics of moving forward from the cockpit in the dark, wet nasty night. Also I would move the intermediate stays in to the inward chainplates so you don't cop a faceful of stay when using the sidedeck.

    I also would move the inner forestay aft to about 50% of the J. Where you have it will be a pain to tack with. Searunners are like this and whilst I dearly love them you almost need someone to run the genoa around the inner forestay because the inner forestay is far forward. I would move it aft, put a storm jib on it and use it only in heavy weather.

    cheers

    Phil
     
  7. BobH
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    BobH Junior Member

    limited space

    The bridgedeck is at 32 inches...812mm...maybe 30 inches fully loaded...the only accommodation I fit above deck is beds. Regarding clearance: from what I read more is better (at the cost of space for accommodations)...I may be aiming at too much. It does seem that the consensus for a boat this size is more like 600mm. Changing that would be a huge shift in the design limits. It may be that I'm designing for conditions the boat will never see...I took features from
    http://www.multihulldesigns.com/designs_stock/36ccat.html

    I already have a building space 36' by 36' (a metal Quonset hut)...with our climate it needs to be a permanent insulated structure...the winds here would destroy a temporary structure in no time (the barn next door had its back wall blown out when they left the east end open less than a year after construction), and the temperature average for winter is around 20F at night (extremes to -20F)...we get a couple weeks of 100+ in the summer, 90+ for the rest, with brutal sun. If the space were not insulated it would be unusable half the year.

    Moved the lowers inboard and made more room for the foresail.
     

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    Last edited: May 21, 2015
  8. catsketcher
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    catsketcher Senior Member

    Having more bridgedeck clearance is almost always better, mine is 860 down to 660 under the trench. I would ensure your bunks have 1000mm over them so you can sit nicely on them.

    If you really want some more length then your rudder design gives scope for this. I would rather have kick ups instead of your barrel rudders but you can get both. In Chris White's book he shows a cat hull stern where the entire aft section of a hull pivots up. Your hull could pivot at the deck from behind the aft beam - giving a fuse to the barrels and also a way of detaching the stern during construction. I am thinking of something similar for a project of my own.

    You could build one hull across the shed - diagonal 50ft. Make it the length dicated by shape required to carry the load - it will never be cheaper to make the boat longer than in the shed. Then make the stern hinge so the rudder don't break when they hit something and take it off after fairing etc. Store it along one side (or outside) till you have finished the second hull and place the aft sections somewhere else in the shed.

    Start with weight then design the rest.

    cheers

    Phil
     
  9. BobH
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    BobH Junior Member

    latest (hopefully, final sail plan)

    I have arrived here.
     

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  10. waikikin
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    waikikin Senior Member

    Hi BobH,

    now if you just bring the cockpit aft & close in the targa you will have arrived close to many other cats;)

    Jeff.
     
  11. John Perry
    Joined: Nov 2003
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    John Perry Senior Member

    The wishbone boom, actually a half wishbone I think, is a nice idea and one that I used on a much smaller boat many years ago. Catsketcher (Phill's) posts confirm that it can work well on this type of catamaran.

    I thought it worth pointing out though that a conventional mainsheet with a track can contribute significantly to maintaining forestay tension, with the wishbone boom you loose that effect almost completely. That could be particularly important with a rotating mast (although I realise that you are not proposing that), since I think it can be difficult to maintain forestay tension with a rotating mast supported only by cap shrouds.

    From your drawings it would seem that you are including running backstays, I would guess that these could make up for the lack of a conventional mainsheet.

    Your design looks promising to me and it is significantly different from the run of the mill cruising catamaran in that it does not attempt to provide a big lounge/galley on the bridgedeck, so allowing good clearance under the bridge deck and a low profile. Probably also a bit lighter not having so much cabin and furniture on the bridgedeck. One minor point is that if I am interpreting the drawings correctly, there does not seem to be much provision for natural light in the hulls which are the main accommodation - just small portholes and aft facing window.
     
  12. catsketcher
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    catsketcher Senior Member

    Good points about rotating masts and forestay tension. I don't find that Kankama's forestay sags more than an equivalent cat with typical set up. I do remember getting lots of sag on my rotating rig set up on the Twiggy.

    As for the windows - that really piqued my interest. Here in Australia I am not a fan of large windows. Kankama has mad many changes of her 15 year life and the reduction in the size of the side windows should be the last major alteration.

    When she was a non bridgedeck cat I thought having large windows would be helpful. Easy to see out of and bright. Now with the addition of a cabin and a few years in the tropics my advice is to keep side windows in the hulls as small as possible.

    In the tropics the Sun is ever present, overwhelmingly so and so you need to reduce its impact on your life. having somewhere inside where the light levels are reduced is essential. The windows let light in which helps heat the interior. They also are heavy, expensive and prone to leaks. On top of this privacy can be an issue with large windows. My advice - big windows in a see through bridgedeck cabin and small windows in the hulls.

    I also sometimes like to get away from the incessant landscape of big water and big sky. Nice sombre interior where you can forget that it is nasty outside is good too.

    cheers

    Phil
     
  13. BobH
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    BobH Junior Member

    rig

    The rig design draws on several sources. In particular, the cutter/wishbone from Chamberlain (brought to my attention here by Phil's advice) and the opinions regarding cat rigs expressed by Kelsall on his site (http://www.kelsall.com/TechnicalArticles/TheCatRig.pdf). Kelsall's recommendation for bringing the shrouds forward and going with runners to allow a greater arc of movement for the main while providing good tension in the forestay makes sense.
     
  14. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Don't Use the Main ?

    All of this effort to 'tame' the mainsail :eek::D

    I might suggest some read thru this discussion on another forum,
    Don't Use the Main
    http://www.cruisersforum.com/forums/f48/dont-use-the-main-115978.html
    See, as sailors get a little older (or even some younger guys) try to do away with hoisting the mainsail.


    You might find one of my more recent replies interesting ...posting #73 (with illustrations)
    http://www.cruisersforum.com/forums/f48/dont-use-the-main-115978-5.html#post1853665
    ...just a couple excerpts...
     

  15. brian eiland
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    brian eiland Senior Member

    I do need to keep some of these thoughts in mind as I go about looking at new ideas to incorporate into a new catamaran design as an alternative to that monohull Jimmy Buffett is currently having built.
    http://www.boatdesign.net/forums/motorsailers/jimmy-buffett-just-bought-new-sail-gamefishing-boat-motorsailer-54630.html
     
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