Small trimarans under 20'

Discussion in 'Multihulls' started by Doug Lord, Jun 24, 2012.

  1. Doug Lord
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    Doug Lord Flight Ready

    ==============
    Something else to consider: if you use a short ama designed as a displacement hull, then it is likely to cause considerable drag as the main hull starts to speed up. But if you design the ama for planing, then the drag will be less as the mainhull accelerates.
    By applying a bit of careful design, you can use smaller amas(with or without foils) successfully which will reduce the all up weight of the boat.
    ---
    Just to reinforce what I'm saying here is Randy Smyth describing his new ama's for 2013:

    My Sizzor Ama's were new this year. They featured flat bottoms and were much wider (14" rather than last year's round bottom 8" wide ama's). My rational was the Ama's on Sizzor don't really touch the water until I'm going over 10 knots since in light air I can basically balance on the main hull. The round bottom shape is low drag at low speed but are high drag at high speed. Flat bottoms are high drag (lots of wetted surface) at low speed but low drag (think water ski) at high speed. So, these new Ama's aren't any different in light air (since they are airborne) but my high end speed is now improved.
    Construction is 1/4" 5 lb density foam built with 2" wide strips over frames (throw away) at 12" spacing. The foam strip edges are routered with a cove on one side and a bead on the other. They are glued with Gorilla glue. This forms the rounded top of the Ama's. The bottom is 1.5" thick of 1/2" x 2 lb density Home Depot blue foam with a 3/16" layer of 5 lb. density foam on the bottom. This foam bottom forms a slanted floor in the ama with the front high and the stern tapered to only about 1/2" of foam. They are glassed with 4 oz. on the inside and 4 oz. and 2 oz. Some carbon is added at the (2) carbon attachment posts. West System Epoxy (you must use epoxy with styrofoam) was use for all laminations.



    Sizzor 2011
    https://www.youtube.com/watch?v=nIDyon2TDag
     
  2. P Flados
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    P Flados Senior Member

    A wider platform significantly increases beam bending loads.

    You need adequate strength for these higher loads.

    Taking care of the structural loads would be easy if it were not for the other objectives. You want a light weight boat that can handle the loads and still have:
    • Low wind drag,
    • Low water drag,
    • Dynamic stability on the water,
    • The ability to tack,
    • The ability to jibe,
    • The ability to manage the shape of the sail.

    It is a real challenge to combine all of the above. To have enough structure to handle 2 hulls in the air in 20 kts true wind speed, it is hard to keep it all light enough for sailing 2 hulls in the air at 7 kts wind.

    You have built a boat and have tasted the satisfaction of sailing your own boat.

    I am not sure you are ready to take this one build and then jump all of the was to a state of the art high performance tri. To get top performance, you probably need more "design tools" (learn how to evaluate hydrodynamics, aerodynamics and structural stuff), construction skills (composites) and resources (money) for at least some carbon (key locations only as a minimum), sails, hardware.

    If you moderate your goals slightly (say something like "better than a Hobie") and you want to build your own boat at a low cost, it looks like it would be hard to beat something like Richard's Strike 15.
    http://www.sailingcatamarans.com/index.php/designs/27-trimarans-under-25/223-strike-15-trimaran

    You would still have some of choices.

    If it is a boat you want to keep long term and or possibly sell so you can upgrade, stick with the plans as close as possible focus on build quality and light weight. You can start with used Hobie 16 sails and the least expensive mast that will work.

    If you want to go for a little more raw performance, you can do a modified build with a "stronger & lighter" focus similar to what a French builder did. This version appears to have light but strong one piece carbon beams and no sit inside box. It can be seen at the bottom of the the Strike 15 web page. Staying pretty close to something proven with only a few straightforward changes is lower in risk but still lets you make it more your own.
     
  3. Richard Woods
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    Richard Woods Woods Designs

    don't forget Randy is one of the worlds top beach cat sailors. And that his previous Scissors broke. So if you are not as skilled as he is, do you want to copy him?

    As I recall Victor C didn't beat "all the C class cats" just the American ones

    A narrower boat will fly a hull earlier. The beam weight becomes massive when you make the boat wider. And where do you launch it?

    Richard Woods
     
  4. Doug Lord
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    Doug Lord Flight Ready

    The problem with narrow tri's is that when(and IF) they fly a hull it's likely to be at a substantial angle of heel. I'm not positive but I don't think Sizzor flew the main hull. But if you want your design to fly the main hull more beam is better, in my opinion.
    Richard, is your Strike 15 designed to fly the main hull?
    Here is a thread on the angle of heel when a tri just flys the main hull: http://www.boatdesign.net/forums/multihulls/trimarans-angle-heel-main-hull-takeoff-39553.html
     
  5. Richard Woods
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    Richard Woods Woods Designs

    Not true if the main hull is very shallow and the outrigger large, it's like a catamaran then, except in very light winds when both outriggers are clear of the water

    RW
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Richard, just out of curiosity ,is the Strike 15 designed to fly the main hull?
     
  7. waynemarlow
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    waynemarlow Senior Member

    The last few pages of posts have always tended to racing craft. You seem to be all saying that with increased RM and long Amas you can increase sail area and then win races. Sorry to disillusion you but it's just not that simple.

    How many of the posters race regularly a really modern beach cat, if they did I'm sure the posted responses would be quite different. These little beasts, particularly those designed since 05 are quite exceptionally refined craft that defy most design logic. Just their short thin rounded hulls should dictate that upper speeds should be limited to normal displacement design rules. Sorry but my 16ft F16 should max out about 15 knots, I regularly see low 20's. Conventional design should dictate 180kgs or so, it's 107kgs, sail area is just 13.5 Sqm with 17.5 spinny, to be getting the power to do 20 knots most conventional design rules would be saying 17.5 main and 21 Sqm Spinny.

    So then we add in normal racing rules and handicaps, where weight, beam and sail area are the deciding equations, Tri's need large beam and sail area, yes first over the water you may win but unless you want to play OD then Trimaran designs will be penalised hugely, yes Randy may beat a Tornado but on handicap he would have to give a lot of time over the water.

    Yes Doug may counter with his usual foil argument but then I can counter with the new cycle of cats such as the Whisper and Nacra 20's that are foil designed.

    So are you getting my drift, beach cats have become increasingly refined and most will defy normal design rules, at under 20ft we are really not going to be able to compete at direct sail area / beam / weight comparisons.

    Will that stop me designing and building a Tri, nah they are the best looking form of sail boat, we just have to look at what they can be best designed to do.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    I don't see why Randy would be penalized in a handicap race when he had two thirds the sail area of a Tornado and less than half the weight-as a singlehander. Would it be because of the weight and beam?
    Are you saying that no trimaran could sail to or better than it's rating?
    I don't know where you get your "conventional design rules"?!
     
  9. waynemarlow
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    waynemarlow Senior Member

    Weight and total beam are major factors, the use of a jib is hugely penalising and i would doubt that scissors ran much less sail area than the Tornado. Go to the SCHRS site and pickup their ratings calculator, it won't quite fit the Tri format but the formulae is designed around total beam / weight / sail area together with board sizes. It has got quite a lot better over the years at trying to predict ratings.

    At 20 ft I think we would be really pushed to get any where near our calculated rating, at 14ft and 40 ft it all changes.
     
  10. Doug Lord
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    Doug Lord Flight Ready

    Thanks for the info.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    Wayne, here are the specs of Sizzor and the Tornado:

     
  12. rcnesneg
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    rcnesneg Senior Member

    But But... USA 17, BMW Oracle's trimaran from the Americas cup flies the main hull and wins races, so our little trimarans should too! :D

    Joking aside, It does look like there are actually two branches of nonfoilers, the powerful and the lightweight.

    My catamaran sailing has been limited to a 1981 Hobie 16, and a 2005 Hobie Fox(F20) All of the sailing on the fox has been light wind, and I have never pushed it, although it can be an absolute beast from what I hear. Hopefully this next season I'll get to sail on it a lot more.

    Doug, How well does sizzors work in rough water?

    After reading through the Strike 15 page with the comparison to an A-cat, I'm surprised to see how well it apparently performs. I think that is what most of us are striving to acheive. What do you guys think is the reason that tri is faster than so many other boats? The main hull looks like a horrible shape, and seems like it would be fairly slow.
     
  13. waynemarlow
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    waynemarlow Senior Member

    Don't get mislead by comparative scaling of weights beam and sail area, any handicapping system has quite complex formulae associated with each factor and will look quite strange as for example the comparison re the tornado and scissors. One only has to look at the A class stats and the Tornado to see an even larger weird look about things.

    You asked about why I used " conventional " in my wording. There has long been tested drag figures v length and aspect ratios of the hull. You could work out pretty precisely the max hull speed by those tested figures. It wasn't until the real push by the likes of the Sydney 18ft skiffs that we started to understand how hulls could exceed those expected figures. Berthwaite has quite a good section in his book about this.
     
  14. Doug Lord
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    Doug Lord Flight Ready

    Wayne, do you know anything about the Texcel rating-is it used in the US?
     

  15. Richard Woods
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    Richard Woods Woods Designs

    It either performs or it doesn't. No "apparently" about it

    Probably it looks "horrible" because you don't know what you are looking at

    I was at a "Get a Multihull" event in Annapolis years ago. We had the smallest boat there. Afterwards someone asked me "why is your boat so fast?" The only answer I could think of was "because I designed it"

    To Wayne. I was going to ask whether you still had your F16, but obviously you do

    I have spent much of the last 10 years in N America where unfortunately there is hardly any small boat racing as we know it in the UK. So the last time I raced a beach cat was a Dart 18, number 7000, some time ago. But I do race dinghies when I get home to the UK, the last time was a RS400 in November

    Richard Woods
     
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