Trimaran amas using extruded polystyrene with epoxy

Discussion in 'Multihulls' started by lane, Mar 8, 2023.

  1. lane
    Joined: Dec 2005
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    lane Junior Member

    I need to build 8 ft long amas to stabilize my existing American Sailing Dink. From the factory, it had a daggerboard, rudder and 50 sq ft lateen rig . The Dink is very tippy when sailing. It weighs 80 lb and has a total capicity of 400lb. Constructed of fiberglass with inner and outer shells.

    For light weight ama hulls, I want to use 1/2" or 1 " extruded polystyrene closed cell rigid board (XPS) in 4 x 8 sheets (Dow or other brand from a big box store). XPS will be used for all exterior hull surfaces and the all interior support structures.
    All XPS will be covered with epoxy/fiberglass cloth on both the inside and outside. Epoxy/fiberglass cloth covered plywood will only be used in stress areas, such as where each ama joins the akas and each mast base. I will retire the factory lateen sail rig. Instead, I will place a freestanding fifty sq ft windsurfer rig, in each ama, to give a biplane/parallel rig.

    Question: For this trimaran, how do I determine how much volume each ama will need? I am concerned with each windsurfer rig submerging its ama. Currently, I have designed each ama to be 8 ft long, 8 to 12 inches wide and 12 inches tall.

    I have searched for amas using this build method, but have found nothing. Not wanting to reinvent the wheel, has anyone seen amas built in this manner in the past?

    Thank you in advance,
    Thomas in Texas.
     
  2. revintage
    Joined: Nov 2016
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    revintage Senior Member

    Hey Thomas, don’t overdo it. Buy an old windsurfing short board and split it in half, should not cost more than 50 bucks. Epoxy the cut areas. Then go for 2” alu tubing as beam, max 8 ft long. Do not move the rig to the amas if you want to stay out of trouble. If you want more sail area, get a slightly larger sail/rig in the Dink.
     
    Last edited: Mar 8, 2023
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  3. rwatson
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    rwatson Senior Member

    Its not the Ama construction that is the problem, its how you arbitrarily decided on the displacement (volume - weight) of the outriggers (8 ft long, 8 to 12 inches wide and 12 inches tall.) , then asked how much volume you would need in the Amas.
    You're getting the cart before the horse, well and truly.

    You could get into some shallow water, load the dinghy with a bag of bricks the same as your weight on the dinghy side, put a pole across the thwarts sticking out on the same side, the same distance as the Amas will be, then use some scales to see how much weight has to be supported.

    Triple that weight, ( allowing for half submerging, plus unexpected gusts) THEN decide on the dimensions of the Amas, to support (displace) the target weight.

    Amas.png
     
  4. lane
    Joined: Dec 2005
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    lane Junior Member

    Rwatson,
    Thank you for your response. I only gave ama dimensions as a guideline of what I was considering for dimensions, not the final dimensions. I am working to determine the needed volume in each ama and will let that dictate the dimensions of each ama.

    Revintage,
    Many Thanks, Math is not my strong suite... Please assist, as my weight is 100 kg, center hull is 35 kg, beam of boat is 8 ft, length of boat is 8 ft, length of ama is 8 ft.
    I want to learn to use the equation above... What is the ama volume in the equation above? I need it in "Boat Design Math for Dummies"...
    Thank you,
    Thomas in Texas
     
  5. Rumars
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    Rumars Senior Member

  6. rwatson
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    rwatson Senior Member

    Which is why I suggested a simple method

    THIS is the sort of work you have to do for a professional result

    RightMom.png

    3. Project blog - Small Trimaran Design Process https://sites.google.com/a/tammingas.nl/trimaran-design/project-updates

    But, with your idea of rigging actually ON the Amas, you have to expect someone to eventually go out and stand on them.

    Using civilised terminology, you want the Ama to be 2.4 metres long.
    Triple your weight, plus say 50 kilos for sail rig and hull weight, so
    350 kilos displacement.
    Say you want a minimum width of .2 metres for safe walking,
    and an arbitrary .3 metres deep

    That's only ~150 kilos IF its a square box.
    Not even close.
    You could use this spreadsheet as a starter. Add a ratio to allow for an irregular hull shape.
     

    Attached Files:

    Last edited: Mar 9, 2023
  7. lane
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    lane Junior Member

    Rwatson,
    I researched the trimaran design project and it was quite in depth, well beyond my mathematical abilities.

    Did your calculation envision me standing on the Ama while sailing? I will remain in the vaca to sail. Each freestanding windsurf rig is set in the mast base while the boat is on the shore. The control line for each boom is run aft to and extension on the ama stern before going to the cockpit for trimming each sail.
    I utilized the Displacement Spreadsheet and added a ration for an irregular hull shape.

    Displacment Calculator for a Solid
    Length 2.4 metres
    Width 0.3 metres
    Depth 0.3
    Hull Shape Ratio 0.75
    Total 0.162 Cubic Meters
    Assume 1000 Kilos per cubic metre
    Displacement 162 Kilos
    Target Displacement 45
    Difference 117 ( minus = deficit)
    Adjusted above to .75 for hull shape

    Please advise,
    Thomas in Texas
     
  8. rwatson
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    rwatson Senior Member

    I did actually say "you have to expect someone to eventually go out and stand on them."

    I get the same answer on most of that, but
    I don't understand how you get a "Target" Displacement of only 45 kilos.
    You have to allows 60 kilos for a tiny sailor, (but 100 is more realistic ), your hull is going to weigh at least 15 kilos, your rig and potential wind effect say another 30 kilos, and if you don't want to submerge the whole Ama every time you stand on it, you need to double that (~105 x 2 kilos) , to say 210 kilos.

    In my experience of building home designed trimarans, this short, stubby little raft will be a pain to sail, and perform like a raft.

    I spent a lot of time building and developing this very home made 18ft Tri, years ago
    Historic Home Made Tri https://www.boatdesign.net/threads/historic-home-made-tri.41690/
     
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  9. alan craig
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    alan craig Senior Member

    You mention free standing windsurf rigs. These are normally supported by the user, they are not designed to be mounted as a cantilever from the base so they might very well fail, if the ama doesn't. Hobie made a sailing hydrofoil with biplane rig years ago, can't remember the name of it right now, it had a strut and wires between the masts to support them.
     
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  10. thepelell
    Joined: Oct 2019
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    Location: italy

    thepelell Junior Member

    lane I built 3 different sets of amas for my paddling/sailing trimaran. I too had to keep max length 2.4m because I didn't want them to interfere with my paddling, as that was the primary purpose of the boat ( main hull was 4.8m).
    The first set I made out of 3mm poplar plywood, and they were basic triangular double ended hulls, sides shethed with 200g glass. They were ok for paddling and for the 80% low windws we get in our local lake, but if the wind picked up , and using a non reefable windsurf rig , it could become quite scary, I went sailing even during winter withour a dry suit, so capsizing wan't an option.
    This is v1
    IMG_5212.JPG

    f67dec75-6a66-4ff6-bbde-50d65ca6ee86.JPG

    During 2 months of covid lockdown I was bored, and having in my garage only xps foam and resin and glass, I kept myself busy building v2 amas out of what I had available (all shops were closed here during those 2 months).
    These are v2, 5cm xps insulation foam, all glassed outside and joins glassed inside. 2m long., 40cm max beam if i remember. Flat bottom.
    IMG_4610.JPG IMG_4636.JPG IMG_4641.JPG IMG_4642.JPG IMG_4708.JPG

    I ended up using them only 1 time, because they were totally rubbish. Heavier than v1 amas ( dont get fooled thinking xps hull is lighter that plywood... it isn't). Flat bottom gave no lateral resistance to the boat ( I did not have leeboard or rudder at the time) , and they punded like hell even on the smallest waves. They were badly engineered, but I really used them just to not go insane during lockdown.

    In the end I made v3 out of 3mm poplar plywood again ( the lightest I can get locally, even if it is not waterproof) . This time I went with max length of a single sheet , 2.4m, 40cm side height, v bottom but with a wider transom in order to gain as much displacemnt as possibile.
    Here is v3.
    IMG_5456.JPG IMG_5470.JPG IMG_5485.JPG IMG_5519.JPG
    This is the simplest/cheapest ama you can build; for my needs these were perfect and have been on the trimaran in the open, uncovered, for the last 2 years, but now i retired the boat becaus of low water levels in the lake , the ramp I could use is no longer functional. I did a buoyancy test standing right on one ( i'm 80 kg) and it still had some reserve.

    Regarding windsurf sails, I tried 3 different sizes , and at the end I bought an unkown dingy sail similar to a laser sail , using the same windsurf carbon mast, and it was way better than previous sails; we have very weak winds and windsurf sails work well only with high winds, they stay too flat .
     
  11. gonzo
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    gonzo Senior Member

    Have you considered building a new, more stable, boat? It would probably be less work and will have more interior volume.
     
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  12. lane
    Joined: Dec 2005
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    lane Junior Member

    Hobie Trifoiler had the rig you described. Each ama will have a very solid two foot tall mast base that should give adequate support to the mast. I have seen this used in small catamarans very successfully.
    Thomas in Texas
     
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  13. lane
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    lane Junior Member

    Rwatson,
    Thank you for your 18 ft tri story above.
    You asked about "Target Displacement", I included only the combined weight of the ama and windsurf rig at 45 kilos.
    Since this is a low performance trimaran, how much of my weight 100 kilos and center hull weight of 35 kilos should be included in the target displacement? I understand that I should add wind effect...
    Thomas in Texas
     
  14. rwatson
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    rwatson Senior Member

    "how much of my weight 100 kilos" - All of it , of course, as you are going to be standing on it
    "center hull weight of 35 kilos s" - none of it, as it will actually be helping support each ama

    Plus
    Rig weight on that 1 Ama- say 15 kilos
    Plus Ama Weight (f'glass, foam, fittings, half of the beam weight) - say 15 kilos

    = say ~ 130 kilos

    Multiply by 2, so that it doesn't fully sink under operation
    so around 260 kilos, without wind effect taken into account

    You do understand, what "Target Displacement" is ?
    It is the amount of water, by weight, that the Ama will displace under most adverse operation.

    It has to be enough to have a safe freeboard, when under maximum load.
    The Ama will sink until is displaces its equivalent weight of water.

    This might help
     

  15. lane
    Joined: Dec 2005
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    lane Junior Member

    Rwatson,
    I now have a clear understanding of the concept of target displacement.
    I am confused by your comment "You are going to be standing on it (the ama).
    As this is a trimaran in still water, I am sitting on the center hull. I would think that only 1/3 of my 100 kilos would be supported by the ama.
    Is my weight being transfered to the ama by the force of wind on the sail?
    Curious,,,,
    Thomas in Texas
     
    Last edited: Mar 11, 2023
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