design of a fast 27 catamaran

Discussion in 'Multihulls' started by SAILFASTtigre, Apr 18, 2005.

  1. oldsailor
    Joined: Apr 2005
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    oldsailor Junior Member

    Quote:- That is why I was wondering about the LONGITUDINAL CENTER OF BOUYANCY, my first question, widening the bow sections will move the cente of bouyancy forward, and the center of gravity has to be aligned with this LCB for the boat to sit trim. Quote.

    I wouldn't worry to much about the exactness of the CG, as it moves around by quite a considerable amount on a boat as small as this.

    More important is the Centre of Bouyancy. You should design this so that from the lightest displacement to the displacement of the boat loaded to its max designed waterline, ---the Centre of bouyancy varies very little. If it does vary a bit it's not too important so long as the varience is more forward. This is paticularly important in a Catamaran since it relies on the lee bow for stability in gusty or knockdown conditions.

    Cheers. Oldsailor. :cool: :) :D
     
  2. SAILFASTtigre
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    SAILFASTtigre Junior Member

    I don't thing the lbc would vary from light to full displacement.

    it does vary in monohulls as they heel, due to their shape.

    regards
     
  3. fhrussell
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    fhrussell Boatbuilder

    Any forward motion on your project? I am gearing up for a build of a 30' cat of my own design and have been reading this thread with interest.
    Thanks,
    Frank
     
  4. oldsailor
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    oldsailor Junior Member

    All THE BEST FOR THE NEW YEAR to all who are reading this thread.

    I had a Crowther International 23 catamaran. It had four single berths, a toilet and galley. Thats all. It was a very fast cat and a delight to sail. I only had it for a year and sold it when I moved interstate. It was called "Silent Runner" and now resides at the Hindmarsh Yacht Club in SA. Australia. It was fully trailerable with the hulls sliding inwards on the oval section crossbeams.
    This limited it's overall beam.

    Once I moved to Sydney I sailed on a Seawind 24, which was of greater beam and de-mounted by removing the crossbeams for trailering. It carried the rounded hull sections right up into the bows for greater driving power and was(and probably still is) the fastest small cruising Cat I have ever sailed on. One character even sailed one around Australia. No mean feat.

    I still like the Great Barrier Express 28 best of all though. I recently saw one on the beach near my home, owned by a young man who is living aboard and cruising the East coast of Australia.

    Cheers. Patrick. :cool: :) :D
     
  5. Zoro
    Joined: Jan 2006
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    Zoro Size doesn't count!

    Living on a GBE... WOW! Thats minimalist!

    I would like to build a Tennant Wildfire (GBE for the 21st Century!) but alas I have to work on the budget first.
     
  6. oldsailor
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    oldsailor Junior Member

    GENERAL opinion among the worlds best multihull designers is that the Tornado hull design is to this day still the best one for any fast lightweight catamaran.

    Therefore ---I suggest you get hold if a set of Tornado lines, blow them up 140%, increase the Boa to 16" and add a sailplan to suit the sort of sailing you envision.

    Cheers. Patrick. :cool: :) :D
     
  7. Figgy
    Joined: Feb 2006
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    Figgy Senior Member

    I know this an old thread but I have a few questions.
    1.Wont blowing up the lines give you a funny hull shape(beyond increasing the boa)?
    2. Shouldnt the sail plan increase 140% also? If not why?
    I've been considering designing a 30 catamaran, just for speed. Something between an a-class and the cat Afterburner.
    Also, I'm trying to do this in Freeship. How do you get two hulls into the program? I'm still new to it, thanks for the help!
     
  8. oldsailor
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    oldsailor Junior Member

    As you increase the dimensions of a design the stability increases dramatically. You can use a bigger rig as the righting moment increases faster than the increase in the overturning moment.

    I suggest that you take a look at Malcolm Tennants web site and see what sort of sail areas he uses for his cruising VS racing rigs. http://www.tennantdesign.co.nz

    Cheers. Oldsailor. :cool: :) :D
     
  9. Figgy
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    Figgy Senior Member

    Allright, I really think that idea might work. I have the Tornado plans, I just need a place to build it. (I'm moving to Maine soon) Would I be taken seriously tho' if I showed up at the marina with a 30' Tornado? I'm thinking between an Extreme 40 and an A-class. Right in the middle. No frills, and fast as hell!
     
  10. oldsailor
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    oldsailor Junior Member

    SOUNDs Good Figgy.


    Cheers. Oldsailor. :cool: :) :D
     
  11. fhrussell
    Joined: Dec 2004
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    fhrussell Boatbuilder

    Figgy,
    Blowing up the lines of the Tornado hulls shouldn't be an issue, but you should study the rig dimensions carefully. Depending on your displacement, your sail area should be considered.
    The 40' CSK catamaran, Machete, borrowed heavily from the Tornado design and was even built with tortured-ply. I believe she's in a yard in Laguna if you have a chance to see her.
    If you are looking for more info on a 30' cat and design scantlings, you can contact me at fhrussell@yahoo.com. When are you moving to Maine?
    Best,
    Frank
     

  12. Pilgrim
    Joined: Jun 2006
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    Location: Lanzarote de Canarias

    Pilgrim New Member

    If it's good enough for Nigel.....

    ...then it should work well for you!

    The all-conquering 'Formula TAG' (later, ENZA, etc, etc) was allegedly described by the eminent Mr Irens as being an 'enlarged' Tornado, conceptually. Stretched later on, too.

    Of course, it all depends on the intended use, but from what you say this might suit you well, just make sure that square-cube effects do not work against you as you scale up! Weight is the enemy, and for a one-off design a plywood composite is still one of the best approaches - wood is tough. An epoxy-glass skin saves the wood from water and weather, and is very low maintenance. Polyester or vinylester resin works OK if the budget is tight, but the gelcoat needs to be applied just right.

    FWIW our cat is a Wharram Tiki 30, which is (almost) relevant to the thread since our WLL is 25 ft, and we displace around 2 Tons fully laden for cruising. Empty weight well under 1 ton, so the payload ratio is excellent - this is one of the benefits of a modern well-rounded deep Vee, with a knuckle above the waterline.

    Other benefits include an excellent high speed ride - just gets better and better as you go past 15 knots - ask the offshore powerboats. When beaching, the vee takes the ground just fine - we have over a foot of structure between the sacrificial strip on the keel and the floor of the hull. So we have parked on reefs with impunity, so long as they are reasonably level. Berths for six, deep water cruising for three with a watermaker, since payload is the limitation when crossing oceans.

    Vee-like hulls tend to triangular structural sections, which may explain why our home built 30 ft cruiser weighs almost the same as the (claimed) weight of a professionally built 'carbon composite' CoolCat at 26ft. Triangulation improves stiffness of a structure and that can allow the structure to be lighter.

    The downside? Wetted surface is greater on a vee, this penalises you in light airs if you do not carry a reacher set flying.
    We do.... and it's a *monster* on an 18 ft prod :D

    Prod is also handy for lowering/raising the mast one-handed.

    Lateral plane provision can be a pain.... Twin daggerboards do the job well, but a word of caution if you plan to spend time afloat - the daggerboards and cases seem to attract marine life, and I have seen a few stuck boards being prised out with ominous crunching sounds. One needed a power saw to cut it free. Big rebuild.

    The JWD daggerboard case is perfect - it is a vee box, with only the bottom slot a tight fit on the board, and that usually has neoprene or similar flaps to improve the sealing. A light removable frame holds the top of the board so you can reach in and clean the slot when the board is out. You'll need to one day.

    Central boards hung from the beam structure can ventilate air if great care is not taken to reduce the local lift coefficient near the surface, add fences, etc, etc. Hard to brace tidily too, but one board is lighter than two, and none is lightest:p

    Round bilge hulls cannot be left on a drying mooring in some places, since the flat hull can sit on a sharp stone a day too long, and you can come back to find your boat awash. Seen a couple of Woods cats go that way on the Millbrook mud, very sad.

    Rig?
    For top performance to a price you cannot beat the Tiki Wingsail, imho.

    The windsurfer style wraparound luff causes the mast to 'disappear' inside the aerodynamic envelope of the sail (like a 747 spar) so the mast can be cylindrical for strength (and cheapness). It can also be fatter for no drag penalty, since it is inside the sail. This means it can be stiff enough (especially with a foot or two of mast bury at the foot) to not need more than two shrouds and a forestay. Again, less drag. An aluminium flagpole supplier should be able to help you.... ;)

    Our 34 ft mast is around 6" diameter and 1/10" thick marine aluminium.

    Best if both ends of the mast are doubled in thickness for two or three feet - this is done by using some extra mast tube, slitting it lengthwise, closing the slit, reshaping and ramming the thing into the mast tube with some epoxy, then riveting it. 1965 monocoque F1 car technology....

    The Tiki main is 4-sided, with a 'big batten' so that you can go square top (or even leave the top panel out for vortex lift like some of the windsurfers). OK, you might call the 'big batten' a gaff if you liked, but it surely works!

    Result is a boomless rig that can be reefed going downwind (big sleeve luff and weight of big batten bring it down like trousers on a broomstick) and will point remarkably well, since it has a self-aligning wingmast, in effect.

    We have put 90 degree tacks on the GPS plot when footing at 7 knots in a 10 knot breeze. Stunning for a deep vee cat that draws 2 feet and will carry a family around the world. Then bear away a bit and see 11 knots!

    You may not want vee hulls for your needs, but you would love a Tiki rig, I am sure.

    I'm ranting on again.....:rolleyes:

    All the best, Ben

    (Tiki 30 'Pilgrim' - Lanzarote de Canarias)
     
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