Mashup Trimaran (continued)

Discussion in 'Multihulls' started by Bigfork, Feb 17, 2021.

  1. Bigfork
    Joined: Dec 2009
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    Bigfork Junior Member

    Greetings folks,
    well I started this a year ago and have slowly but surely made progress. It was my "covid garage project". The thread was housed under "boat building" and is now towards the bottom labeled "Hobie 18 tri" . I've bumped it to this thread to see if I can source more interest and thus, glean more from you great people (I hope that's ok with the powers that be :))

    There's a picture of all three hulls in the shed: Hobie 18 main hull on the right, Solcat 18 floats to the left. Main hull 80% done, Port and Star floats in various stages.

    Prelim thoughts:
    All three hulls are approx 18' long. I'm aiming for a 12' center of float to center of float. That will give me approx 13'+ overall beam (outside to outside) at widest point. I could go for more beam but I know the loads exponentially increase with beam (not enough beam? convince me otherwise). I envision skimming the main hull when loaded up in 10kn. I don't want to fly the main hull too often. Rig will be a H18 or perhaps larger (tornado??). Overall Dihedral has been factored in and is very hard to explain, but the floats will be higher up relative to main hull.

    Fore box beam will be curved slightly after the folding/hinge point. There's another picture that shows my first go at layout for the boxbeam as it meets the float, the green lines are solid blocking points for attachment. There's an 8* cant of mounting surface to cant floats outward (hoping float is plumb when boat is healed...all educated guesswork:))

    Box Beam Thoughts and Questions:
    Fore bb is illustrated in the picture below. 4"x5" in dimension. There will be solid vertical blocking in the hinge and mast locations for compression and fixatives.

    1. Do I coat the inside surfaces with epoxy during the assembly? Do I sheet them as well?? maybe sheet just the inside surface of the 3/16 ply??

    2. Should I add a internal 'web' as shown below in pink? (if so... on 12 or 16" centers?)

    3. What weave and weight of glass should I wrap the external surface of the box beam with?

    4. Does the profile design even look valid? I figure the up-down direction needs to be stronger than the side-to-side, hence the direction of the vertical grain oak and pine.

    I've been oogling various box beam designs (Water's W17 among others) to glean build types. Weight is going to be my main issue..I know. If anything, I have the tendency to overbuild, over-engineer so I need to be thoughtful of excess. I'm aiming for a float test by fall.

    If anything looks wonky, say so! Most of this build has been based on intuition and visual research online. I know enough to get in over my head.... he he.

    thanks folks!



    Green lines below are epoxy joints. All measurements are Imperial (feet/inches). InkedInkedbox beam profile 2_LI.jpg Box Beam Layout w proposed bolt locations.jpg 3 hulls jigged up.JPG
    InkedInkedbox beam profile 2_LI.jpg Box Beam Layout w proposed bolt locations.jpg 3 hulls jigged up.JPG
     
  2. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    That is a really poor cross beam construction.
    Put your vertical grain oak across the top and bottom, rotating the grain 90 degrees.
    Put plywood on the sides, cutting it so that the grain goes +/- 45 degrees from the length. Use 2 layers so that you have an equal number of layers in both directions.
    Use glass to add back the strength lost from the joints, put it on the outside.
    You can put glass on the inside if you want to improve the water proofing by adding thickness of epoxy.
    As best you can, add a fillet on the inside so you improve strength from the shear webs (plywood) to the caps ( oak).

    You ought to build a test piece and try to break it with loads you think are realistic. I would guess your sizing is much too large.
    That is just a guess, not an analysis of any kind.

    That's why I think you should test.
     
  3. Bigfork
    Joined: Dec 2009
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    Bigfork Junior Member

    Thanks Upchurchmr!

    learn as I go :)
    I get the "put your vertical grain oak across the top and bottom" instead of the sides. That would also help if I need to step on the beam or mount hardware to it (track for jib).
    Epoxy coating the inside before closing will help with resisting moisture.

    Any thoughts on weight and weave of glass to wrap the outside with?

    A test beam would help...it would have to be long enough to accomplish any sort real-world test... maybe 6' (?).

    Size of box beam (4x5 as drawn) is one of those variables I have trouble understanding. Because of that, I lean towards the overbuilt than underbuilt. But that's gonna cost in weight as I'm sure there are other elements I've already overbuilt. My hunch was that I could go smaller and perhaps I will. A test beam would be wisest :)

    Thanks for the advice! And to anyone else who wants to chime in, I hold no one responsible for there gracious thoughts. If it folds in half at speed, it's my arse and only my arse to blame :). I'm just noodling to get other opinions and to glean knowledge from those who know more than I.

    Cheers and thank you!
     
  4. upchurchmr
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    upchurchmr Senior Member

    If you design the beam to work in wood, then just wrap it in 6oz glass.
    Its mostly to give the surface of the wood some hardness for abrasion resistance, help build up the thickness of epoxy for water resistance, and add a little strength on the sides where there are joints in the plywood.
    I don't think I saw a sketch of how your beams are built along their length. How will you attach to the center hull, are there mechanical joints for folding, etc. That will help choose a test length for the beam.
    4x5 is really big. It depends upon the joints/ geometry and the total weight of the boat.

    I once built beams for a 6' wide catamaran rowboat. My first pair were about 5# each. After trying something smaller in a test I got them down to 1# each. This was nothing like what you are doing, so there is no help about sizing, just shows what testing can do.
    You should have 3 -5X more capacity than you think you need, for safety and because you cannot tell exactly what the strength of a piece of wood is.

    The design for an old Cross 18 trimaran used a 2x4, solid, with metal joints at the center of each arm for folding. With the 4" (3.5 actually) dimension vertical. But the total beam was only about 13'. And the total sailing weight was around 1000#.

    Did you find any dimensions for the W17 tri beams?
     
  5. Bigfork
    Joined: Dec 2009
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    Location: Montana, USA

    Bigfork Junior Member

    Hey gang,
    I've attached profile picture viewed from the front. The pink highlighter is VG blocking locations for fixatives. Earlier pictures show the bulkhead mounting platform on the main hull. It's sort of "T" shaped in order to get wider than the main hull, thus giving a wider bolt-down leverage. There's also doodle of the hinging mechanism on the beam. I'm not set yet on this style of hinge, but leaning that way. The drawing is fairly accurate in overall beam (12' center to center or 13' outside of float to outside of float) and float cant (8*). In comparison to the Cross 18, I think my target weight is going to be around 600lbs, plus or minus.

    Where the box beam attaches to the main hull and where the beams attach to the float, I was thinking of using 1/4" threaded stainless bolts. If the float test goes well and I deem the arrangement of beam, dihedral, float balance, etc, all acceptable, I may epoxy bed the beam-to-float and beam-to-main hull and get rid of the 1/4" fixatives. Essentially hold it together with epoxy instead of bolts (or use both methods??).

    Before I commence with the box beam, It's my last chance to change the overall beam...is 13' too narrow for a 18' vaca and ama? I know the loads go up exponentially when the footprint grows (more loads on the box beams).
    cheers!

    Doodle of box beam, fixatives, etc..jpg
     
  6. trip the light fandango
    Joined: Apr 2018
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    trip the light fandango Senior Member

    The box beam is made out of 3ply I'm guessing. 6 ounce would be good, use 2 layers and increase the beam by 2 foot,,I reckon at least.... right angle corners are awkward to glass and worse to fall on so rounding the corners is well worth the effort. Using quad for the corners adds a little weight but is easier to create chamfer, plus you can bend a curve over the length that will look nice, practical, and is pretty easy to cut in ply, water stays are very common and help carry the loads created by the mast stays.
    If the beams bend ,it affects the trim of the jib[ forestay tension] to some degree .
    The higher the beams are from the water the better, if you can make seats like a Tremolino out of canvass ..hammock like , the boat is way more comfortable.
    Carbon is great at resisting stretch so a line glassed in under the beams is light and strong and not that pricey for what you get... you wet out the carbon in place and then lay the matt[ I think] over. glassing some carbon into your bulkheads adds strength and means you can use lighter thickness, , less timber. The better quality..more expensive matt you use the less chance there is that it will break..the plies need to mainly run parallel, ..all the hints are in oldmulti's thread.. filling bulkhead joints etc.. good luck.
     
    Last edited: Feb 24, 2021
  7. upchurchmr
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    upchurchmr Senior Member

    All that only really works if you can do some engineering.
    Actual calculation.
    Otherwise its just elaboration for the sake of bragging rights.
    And extra cost.

    Except for rounding the corners - that is necessary to put down the cloth. 1/4" minimum.
    And the high amas, with seats - that could be good, but it completely changes the concept of the boat. You would really need a different main hull.
     
  8. Bigfork
    Joined: Dec 2009
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    Bigfork Junior Member

    Thanks guys! just a couple of questions.

    Trip, when you say "the box beam is made of 3-ply", are you talking about how many layers of plywood I'm using to build the beam?
    And what is "Using quad for the corners"? I know the glass fabric doesn't like 90* corners. I found that out through trial and error. All the ply corners will be rounded with a router.

    I've thought that I might need a water stay. A stainless strap that fixes to the bulkhead and pokes out both sides and then cable to beam right before it attaches to the float.

    Increase the overall beam ya think? :) maybe to 16'.... And yes, hammock style seating to keep weight conscious, and maybe only partial at that. Seating hammock might only go from hull to the hinge point of the beams, leaving open water between the hammock support and the float. My beams will already be higher than the factory version of the H18 or Solcat.

    As far as beefing some stuff up with carbon, what weave and weight would be good for "a line glassed in under the beams" . If I order a couple yards of carbon, I want to be able to use the material in several places for several applications so a weight and weave that can accomplish a variety of tasks.

    Thanks folks! If I can see this through till the end, I think it might have the chance to be pretty slippery and perhaps, go like the clappers! Put a H18 jib up front and maybe a 2' prodder with an asym or code 0 on a furler...(getting ahead of myself...:))
     
  9. upchurchmr
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    upchurchmr Senior Member

    You are going to have this thing so overweight it won't go at all.
    Why not put a cabin on it also?

    If you are going to increase the beam, why are you going to do that?
     
  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    600 lb will need 9 cubic feet of displacement. Did you calculate the submerged volume for the drawing?
     
  11. trip the light fandango
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    trip the light fandango Senior Member

    Well the bragging rights will be pretty lame if someone on the other side of the world's boat just snaps or sits so low in the water it is a lemon.

    I'm responding to Big Forks post in a way that I think should work, am I guessing without calculations to back it up ? .Yes.

    I think you should add waterline to the bow or stern Big Fork as I have suggested before, weight is a party spoiler with this type of craft so extra buoyancy is critical.

    If I could influence people to believe that by removing critical thinking from education means that they are often unable to understand the difference between emotion and logic/reason in a discussion, ala Qanon, now that would be something to brag about.
     
  12. trip the light fandango
    Joined: Apr 2018
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    trip the light fandango Senior Member

    How much flex you have in the beams will tell you whether they're strong enough or if they need more glass/ weight.
    I don't know what you call quad in the US .. tri quad? .. hardwood 3/4 square inch box in 8 foot lengths or rounded on one side to finish a corner..

    I think you will need water stays for both beams.
    Other posters here are concerned with weight and strength, they know more than me, adding to the water line isn't that hard. Then again the longer you spend the more you have invested , raising the stakes. Cutting your centre hull down the middle and glassing in 6 inches of beam would likely also solve the potential issue. Your own weight Big fork .. and what you will be carrying weigh heavily into the calculations.
    More beam overall is more speed when the wind gets over 12 knots because you are less likely to bury a float and the ride is flatter, more leverage /more beam /stay stress, a lot to do with who is on the helm. But there is a safety factor having more beam, provided the loads hold.

    I just got my snatch blocks so I can winch my main hull out of the slipway and home, cheers.
     
  13. upchurchmr
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    upchurchmr Senior Member

    I must be getting really old.
    WTF did the sentence about Qanon mean?

    I thought we were talking about boats, not fanaticism or philosophy.

    Don't do anything massive to the main hull until you know it's necessary.
    This is the sort of thing where making a statement which "could" be true, leads you down a rabbit hole, when you don't have a single fact to suggest it is necessary.

    "more beam overall is more speed" - only if your rig and structure is capable of the increased stress, and if the hull you have (ama) will take the increased load without flipping you into a summersault.
    Nothing is as simple as that - everything on the boat is interconnected.

    You never said how fast you want to go in what wind. Nor how much weight you want to load on the boat.

    My last suggestion. Set the beam at 13' like the Cross 18. Then you know what the beams need to be without guessing.
    Put a similar sail area on it, so you won't overload it.
    Now you have a decent baseline to compare against and a chance it will sail alright assuming the weight is not too much different.
    After you sail the hell out of it, work out the bugs and decide what you really want, then think about increasing beam, increasing sail area, etc.

    Good luck.
    Marc
     
  14. trip the light fandango
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    trip the light fandango Senior Member

    The sentence about Qanon was about something useful to big note on, one aspect of getting older is becoming easily irritated, something we obviously share, ha
    regards stuart
     

  15. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The waterline on the hull needs to be calculated instead of guessed. In fact, calculating a volume is pretty simple and straightforward. This is the kind of problem where critical thinking comes in handy.
     
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