Small trimarans under 20'

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

  1. redreuben
    Joined: Jan 2009
    Posts: 1,999
    Likes: 223, Points: 63, Legacy Rep: 349
    Location: South Lake Western Australia

    redreuben redreuben

    float height

    JPYD.
    Just on the float loading, it's going to depend very much on your expected use of the boat, how much time will it spend in lightship mode? If you want to cruise and camp overnight then have the floats lower, they will carry more payload and you won't have the annoying tilt and slap from one float to the other while overnighting or even anchored for lunch. If you want to go fast then lift the floats up a bit to get clearance from wave peaks when hard on the wind. Compare the Buccaneer 24 (high) with say the Horstman 24(low).
    Further the higher floats will create less drag whilst going about if that is an issue for you.
     

    Attached Files:

  2. JPYD
    Joined: Jul 2005
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: France

    JPYD Junior Member

    thanks for the answers so far
    sail area is that of an F18: main 17m2 jib 4.15m2 gennaker 21m2, unless I can find a bigger jib for cheap

    intended use is just messing around with the kids on board, sailing in protected waters (the golfe du morbihan in France, check it out, best place in the world)
    occasionally take some tents and have a sleepover on an island
    no swell, but the summer sea breeze can lift some chop, I'd say 50 to 75cm at worst
    when we eat or sleep on board the boat will be beached, not on anchor so I am not worried about the boat slapping from one ama to the other
    most of the sailing will be done with me + 2 kids on board, some racing with 3 adults on board and some camping with 2 adults + 2 kids + camping gear.

    oh, and I like to sail fast, my other boats are a 5.5m sportsboat and a foiling moth. If I have to chose a compromise between comfort and speed, I will go for speed.

    I work for a carbon mast manufacturer so I will have access to materials and tooling for the beams and mast.
     
  3. patzefran
    Joined: Feb 2011
    Posts: 376
    Likes: 55, Points: 28, Legacy Rep: 10
    Location: france

    patzefran patzefran

    Hi , TBYD
    I looked at your question and I think there is missing something to give any answer .
    "my first question is regarding the height of the amas relative to the central hull:
    option 1: I design the central hull to be at DWL in lightship condition with the amas just kissing the water
    option 2: I design the central hull to be at DWL in fully loaded condition with the amas just kissing the water
    I have made quick models in maxsurf using both configs and as expected option 1 offers more righting moment, clears the central hull with a smaller heel angle, but has the windward ama closer to the surface for a given heel."


    At what weight loading conditions do you assess the righting moments ?
    If it is at full loaded condition option 1 and 2 will give you the same maximum righting moment (It depends on the half centerlines beam and the total weight, provided the outrigger has enough volume to support this weight), but at lower heel angle for option 1. Obviously the trim roll angle at rest for 1 with full weight is lower than 2.
    If I was to design for pure speed, I would design for reduced weight in racing conditions. But maximum velocity with lifted mainhull depends more on the outrigger design and total weight.
    The main difficulty is you can never achieve your anticipated low weight, wich is the main parameter for speed, together with maximum beam and outrigger volume.
    About the pitch trim when lifting the mainhull, it is tricky and you need a dedicated code. John Suthleworth has writen interesting papers about Trimarans design you may read with profit.
    On my Strike 20, windward outrigger is well above the water at mooring, with or without crew. As it is wide the rolling motion at rest with wind, waves and current is severe !

    Patrick
     
  4. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 349, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    High Performance Trimaran

    It seems to me that any modern trimaran designed with an emphasis on performance would have to consider the use of foils-either foil assist or full flying. I guess it comes down to whether your idea of "performance" is worth the extra cost and effort to use lifting foils.......
     
  5. JPYD
    Joined: Jul 2005
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: France

    JPYD Junior Member

    patzefran:
    I did a stability analysis for both options at full load, and the RM is marginally higher for option 1. I think this is due to the CoG being quite high, 800-900mm above DWL. With the amas higher relative to the main hull, the boat heels more before the ama is efficient, so the high CoG shifts quicker to leeward.

    edit: I did the stability analysis for worst case full load to get the absolute max RM to design the mast and rig.

    DL:
    easy tiger ! foil assist will probably come one day when I get bored but at the moment if I want to go foiling I just hop on the moth. And I want to keep this project simple and be sailing in a year.
     
  6. waynemarlow
    Joined: Nov 2006
    Posts: 435
    Likes: 50, Points: 28, Legacy Rep: 134
    Location: UK

    waynemarlow Senior Member

    JPYD,have you looked at the TC601 thread http://www.boatdesign.net/forums/multihulls/tc601-trimaran-design-tim-clissold-53157.html where many of the problems of where to position the floats are discussed. Certainly as I have found out, it is an area of great practical problems and no one way seems to fit all the requirements you are after.

    From the many many iterations Tim and I discussed we never really fully resolved the many issues fully but instead have taken a " as good as we can get " option.

    The biggest decision you must take is what the boat is for. If its for just messing about with the kids on board then that's a whole different boat to one wanting to race with the main hull out of the water.

    Lessons I have learned is that a boat of the length you are planning will be tough to get down to the weight you are looking at. We achieved 115kgs all up for the main hull but at a cost of using a lot of carbon. I think if we went to fully bagged infusion and with some mods to the way we built the TC601 you would be around the 105kgs bracket.

    Another thought process is that wanting the boat to be practical as a day sailor means almost certainly it will need to be a folder, that puts a whole new dimension to it as you go through the permutations of what and how you want it to fold.

    Another thought process is do you want to have some sort cabin to allow a small loo, the Pulse 600 fails abysmally in this respect and you just won't get the ladies and kids on board on a regular basis unless you do.

    Another issue, straight beams or rounded beams giving wave slap protection.

    Another issue at anchor if the boat doesn't sit flat it will roll and rock horribly but the dihedral will allow for the main hull to fly whilst underway.

    And those are just a tiny few conundrums
     
  7. JPYD
    Joined: Jul 2005
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: France

    JPYD Junior Member

    hi Wayne
    I had read your thread a few weeks ago, seems like I need to have another look then...

    folding: no. the boat will be assembled at the beginning of the season, will be on a mooring as long as the weather is nice, and disassembled at the end. not really a daysailor as you mean it.

    intended use: ok maybe I worded it wrong. let's say this is a racing boat that I will take the kids out on, with reduced sail to keep things mellow.

    interior and loo: not necessary for me and my family, this is really going to be a beach tri with space to store stuff, but no interior so to speak. I have brought up my kids to be able to pee and poop in just about any circumstance, a bucket is luxury for them ! and I am lucky enough to have a wife with the same quality !

    straight beams: yes, I don't want to compromise speed/comfort, but I have to compromise speed/cost. I have access to tooling for straight tubes, so be it.

    anchor rock and roll: how effective would a couple of 20l jerrycans lashed to one ama be to prevent the boat from rolling ?

    OK I'm off to read your thread again, thanks for your interest !
     
  8. Corley
    Joined: Oct 2009
    Posts: 3,781
    Likes: 196, Points: 63, Legacy Rep: 826
    Location: Melbourne, Australia

    Corley epoxy coated

    Some people with a large trimaran stick a RIB under the float to stop flip flopping. On a small trimaran you could probably just push an inflatable pool toy under there, pushing the boom out to one side with a weight on it and lashing it can work too.
     
  9. Tiny Turnip
    Joined: Mar 2008
    Posts: 865
    Likes: 274, Points: 63, Legacy Rep: 743
    Location: Huddersfield, UK

    Tiny Turnip Senior Member

  10. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 349, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    adjustable amas

    Thanks, TT-that's pretty cool!
     
  11. waynemarlow
    Joined: Nov 2006
    Posts: 435
    Likes: 50, Points: 28, Legacy Rep: 134
    Location: UK

    waynemarlow Senior Member

    Certainly on the land trials I have done with the TC601 with floats in place and fully attached to the beams, and finally found the time to fully work out the folding and beams to a point where I have locked in the design ( getting close ish to testing it on water ) it was quite apparent that one could alter the most important height of the Ama's entry into the water, that of the foward one third, you can simply alter the diagonal stay ( most weight saving way ) by just a few millimetres and allowing a bit of flex in the mounts to allow the very front of the ama to twist up or down to the height you want.

    The other break through I had was to say OK straight beams are out, how do I get some way of bending 100mm Ali tube to allow for some beam height. That then lead to a lot of enquires to tube benders who to a man said they couldn't bend that diameter. But I hadn't asked if I they could "roll" the same degree of bend, to a man they all said yes no problem and for a relatively modest cost. That roll then allows an easy way of getting beam height. If you want to adjust the height you simply start long and slowly cut the tube length to get the required height. If you want more dihedral cut the length and you increase the height at the same time, yes you loose a bit of total width but in the grand scheme of things, it's minimal.

    I spoke to Tim at length about this dihedral thingee and he said the 8.5 metre class Tri's in NZ had tried so many iterations but all boats acknowledge that sitting at an angle with a blow up tube under one Ama to stop rocking was a pain as it meant you are constantly sitting, eating, playing, talking at an angle. All those trying it all eventually rebuilt their boats to just having the Ama's touching the water, loaded, seemed to be the best option.
     
  12. dubbgs
    Joined: Sep 2016
    Posts: 4
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Alabama Gulf Coast

    dubbgs New Member

    What about using a ballast tank to bring the boat down a little while at anchor. place it high enough that you must pump to fill and pull a plug or turn a valve to empty quickly.
     
  13. dubbgs
    Joined: Sep 2016
    Posts: 4
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: Alabama Gulf Coast

    dubbgs New Member

    If you had a tank in each ama you they could also be used to aid in righting the boat if a capsize occured.
     
  14. Skyak
    Joined: Jul 2012
    Posts: 1,462
    Likes: 145, Points: 63, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    It would be far less work and mess to fill floats with air rather than water and if there were plates on the bottom of those floats to provide significant damping I think the boat motion would be acceptable. If you don't want to carry plates you could make two sea anchor type cones, or better yet sort of an umbrella arrangement.
     

  15. waynemarlow
    Joined: Nov 2006
    Posts: 435
    Likes: 50, Points: 28, Legacy Rep: 134
    Location: UK

    waynemarlow Senior Member

    Be careful with ballasting recreational craft, it's a great idea in racing craft but the additional RM of the water filled Ama's can put large loadings on the structure which then means a heavier build, which then means more cost due to the use of exotic materials to keep the overall weight down, it's one of those spirals that you really need to consider as for every 100 litres of water ballast it will only lower the boat by a few millimetres.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.