A class hulls for amas

Discussion in 'Multihulls' started by tamas, Mar 18, 2013.

  1. tamas
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    tamas Junior Member

    I may not be in the correct forum but I am considering building a 20ft tri and I was thinking of using an A class cat for the amas and rigging. Has anyone had any experience with this. Any problems? I have heard A class hulls do not have enough buoyancy. A friend build a tri many years ago using a light weight sharpy as the main hull. I dont remember if it was successful. I want a day sailor - racer. Any comments please.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    A Class

    Are you going to use the curved foils that come with most A Class cats these days? The lift from those foils can make up for low buoyancy amas in some cases. See the "Small Trimarans" thread for Ted Warren's Ultimate 20 design specs. He offers such foils as an option but the boat sails well without them. His ama's are similar to A Class hulls in some respects.
     
  3. luckystrike
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    luckystrike Power Kraut

    Hi Tamas,

    Do you have an A-Cat to take the hulls from? What generation of A-Cat or type?

    If you don't expect to be faster than the A-Cat alone it will work for sure.

    The question is what do you expect from this boat. If it should be able to sail on the amas alone you might have to low volume but if you want to sail on the bottom of the mainhull it could work. Do you want to sail single handed or with two aboard?

    And what do you want for the mainhull? A quick built with a simple hullform or a sophisticated round one?

    Best Regards, Michel
     
  4. catsketcher
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    catsketcher Senior Member

    Too light

    Gday Tamas

    There were a few of these running around Australia as we have excellent donor main hulls - the Australian surf boat. A friend built one using the rig and hulls off a Hobie 18. It was fun to sail. I also went aboard another one cruising the Whitsundays with this type of set up.

    The problem with an A class is that they are built very lightly. This means the hulls will not like being pushed down under the water by the huge increase in stability of the larger boat. An A class hull only has to carry about 150 kilos max weight. Your new boat will be much heavier than this and will push the hulls under water. They aren't designed for this. The rig will be much worse. Rigs are designed to stability. The mast will want to fall down and the sail will start pulling apart. This is what happened when I put a 16ft skiff rig (much tougher than an A class rig - 3 guys - 2 on trap) on my little 19ft trailer sailer cat. The mast wobbles and the mainsail delaminated. In the end I put a Hobie 18 main on that had been on a Trailertri.

    There was a boat with a surfboat main hull I heard about that used A class floats and broke a lot. I would go stronger.

    As the Hobie is such a tough old thing it will take what you want to do with it. Find one that has broken beams or no tramp so it is cheap. If you are in Oz look out for an old surfboat. I got one for free for my mate.

    cheers

    Phil
     
  5. Corley
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    Corley epoxy coated

    Better to build a new boat if you want to race. The conversions in my view are too problematic to offer true race boat performance without excessive tweaking and they always seem to end up with nearly a new boat. Look at the Paul Bieker Trinado for example started out with custom main hull and tornado hulls as floats which had insufficient buoyancy, they built new floats so where is the saving? Every conversion I can think of they have had problems with the low buoyancy nature of cat hulls as floats. In most cases they have split the cat hulls down the middle and added volume and increased the laminate which is in my view a similar amount of work to building a new hull. The exception I suppose is Richard Woods Strike trimaran series which seem to do quite well using cat hulls as floats but there is no 20' Strike at the moment.

    The other option I can think of is use a tornado hull for the main hull and build custom floats. Really trimaran floats are easy and much less work in my view than building a new main hull. You only need one mold for them and don't get too hung up about building two the surface areas are not that great. Beams are pretty easy you could use an aluminium beam with or without waterstays on a boat this size or a prefabricated filament wound carbon beam from a company like CST Composites.
     
  6. gonzo
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    gonzo Senior Member

    Tremolinos were built with Hobie Cat hulls and they seemed to do OK.
     
  7. jamez
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    jamez Senior Member

    Yes they did in 1979??? but they do better with the 'half moon ' floats. And the ply version has bigger floats than hobie 16. Further, the newer round bilge version has much larger floats again.

    There was an A class based tri discussed on Multihull_boatbuilder a few years back. Seemed to work well but had a small main hull, which seems to be what you are wanting.
     
  8. tamas
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    tamas Junior Member

    I forgot to mention, I can buy the trailer, f/glass hulls, 9m mast (rigged), boom (rigged) with some fittings all in good condition for $150. No sails. I know the price does not matter if it does not work but it seems I am getting it all for about the price of the mast on ebay.

    It is probably an older model, I did not check the boat number, if it has one at all. So it wont have curved foils, to old. I dont expect it to be as fast as an A class but I would like to compete with say a Ross 780 on CBH of about .790 or a ostac tramp. We have a few boats about that speed in our club. I just sold a 24ft TS and my wife hated sailing on it's side and wont come out any more. My other option is buying a Tri but there are few for sale similar to an ostac. I see a few Haines but they are too slow from what I can gather. The small main hull may be worth looking into.

    Many thanks for your help.
     
  9. luckystrike
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    luckystrike Power Kraut

    Would be easier if you have some photos if you don't know the type. There are dozens of different A-Class types out there over the decades.

    But for 150$ you cannot make mistakes. The trailer, mast, mainsheet and traveller alone is more than worth it, if the parts are ok. If the mast is made from aluminium it will probably be stable enough for a 20' tri.
     
  10. luckystrike
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    luckystrike Power Kraut

    Hi Tamas,

    just a idea how a possible mainhull could look. Please don't ask why the cockpit sole is incomplete, I have no idea!

    This is a first concept, a real design would require the exact measures from the A-Cat.

    The hull has a displacement of 320kg max, sailing would be like the Weta-Tri mainly on the mainhull with the low volume A-Cat hulls as amas. If the A-Cat hulls are no good for any reason it's no problem to design a pair of amas for this hull. The attachments show a reverse bow, a vertical stem is possible to match the look of a older fashioned A-Cat. The cockpit and the seats on the wings are for comfort only, the hull could be built without them and sitting on the trampoline. This would save 1/2 sheet of ply and around 7 - 10 kg wheight.

    A quick material estimate calls for 5-6 sheets of 6mm okume marine plywood, 10kg Epoxy and some light softwood (Fir, Kiri (Paulownia, Western Red Cedar or similar) for stringers and stiffeners.

    Husky6 Arbeitsmodell.jpg Husky6 Arbeitsmodell_Linesplan.jpg

    Any comments or ideas?

    Best Regards, Michel
     
  11. Doug Lord
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    Doug Lord Flight Ready

    -----------------------
    Michel, nice shape-I like it! About what do you think this main hull would weigh?
     
  12. John Perry
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    John Perry Senior Member

    Just a small point, but filament winding is not a good process for making structural members that are to be subjected to bending and or axial loads. It is good for making pipes to carry fluid at high pressure.
     
  13. Corley
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    Corley epoxy coated

  14. John Perry
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    John Perry Senior Member

    To achieve optimal properties for bending strength you do need a fair proportion of the fibres to run paralel to the axis of the tube, or very close to it. Looking at the CST website, I see that they do achieve this, so their tubes should be fine for bending, but I am not sure wether you can really describe the process they use for laying the axial fibres as filament winding. They dont say exactly how they lay longitudinal fibres onto their tubes but it would seem likely that its done with a tow laying head that runs back and forth along the length of a non-rotating mandrel. This may be a matter of semantics, but is it 'winding' if the mandrel is not turning?


    The point is that if you buy filiament wound tubes without checking what the fibre orientations are, you may well get tubes made on traditional filiament winding machines that are unable to lay axially oriented fibres. Such tubes will be sub optimal for appications where bending strength and stiffness are important.
     

  15. hump101
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    hump101 Senior Member

    We've made carbon tubes for axial loads by winding a biax cloth onto the mandrel, angled to the bias, so ending up with 50% axial and 50% circumferential fibres. Very good for things like rudder stocks that are loaded both axially and radially. Fibres aren't continuous axially, but if the tube wall is thick relative to the layer thickness, then inter-laminar shear is adequate to transfer axial loads between layers.

    For masts we combined axial UD with biax to bias the axial direction.
     
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