Amateur trimaran design critique

Discussion in 'Multihulls' started by MrMillard, Dec 1, 2021.

  1. TANSL
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    TANSL Senior Member

    In order to be positive and contribute something to this thread, in addition to giving the list of my titles, I have made a 3D model of your trimaran, based on your drawing, with which I have been able to calculate the volume of the central hull, which is, for a 50 cm draft, 2.2 m3 and that of the lateral hulls, 0.15 m3 each.
    Everything is very approximate because the shapes of the hulls still have to be worked a lot, but these figures will give you an idea of the current situation and where to direct your next steps.

    Edited: to specify the draft at which the indicated figures are obtained and to correct the float displacement.
    Last edited: Dec 9, 2021
    RampantMule and MrMillard like this.
  2. MrMillard
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    MrMillard Trimaran 7

    @TANSL. Thank you for your help and enthusiasm! I have a 3d model though, I just haven't felt comfortable sharing it yet. Maybe I should upload a picture anyway? The numbers I gave in my previous post were taken by measuring the model. Again, thank you for taking your time to help me!
  3. TANSL
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    TANSL Senior Member

    You're welcome, if you think I can help you with some calculation or something else (within my knowledge), you just have to say so.
  4. MrMillard
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    MrMillard Trimaran 7

    Someone mentioned SOR - Statement of requirements. I have tried to make such a document. I had some trouble finding good examples using a simple Google-search, so if you have feedback and/or suggestions for improvement, please share them.

    P.S. The document has been updated as per @gonzo suggestion.

    Attached Files:

    Last edited: Dec 9, 2021
  5. gonzo
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    gonzo Senior Member

    That's pretty good. In the constraints, the easy trailerable should be moved to capabilities, and the cruising/max speeds to constraints. Constraints are a definite number or value that can't be altered. The build time of 2,000 hours is, unfortunately, completely unrealistic. If you are working mostly on your own, and learning as you go, 12,000 may be closer. To give you an idea, we were building a custom Carolina Sportfisherman of 42', in about 10 months with a crew of 10 shipwrights, and a fully equipped shop. Any simple job will be 4 to 6 time longer doing it alone than with two people; sometimes more.
  6. MrMillard
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    MrMillard Trimaran 7

    Updated SOR as per @gonzo suggestion.
  7. MrMillard
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    MrMillard Trimaran 7

    I have made some preliminary drawings of the athwartships bulkheads on the waterline sketch. Trimaran 1 waterline and bulkheads_1.jpg
  8. rberrey
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    rberrey Senior Member

    MrMillard , I understand your beam requirements now , and de-mountable is a plus in my mind . Another question is why not bring your forward top beam ( b,c ) to top of deck elevation , I cant tell if the Alu. beam,s are one full tube,s running from ama to ama ?
  9. MrMillard
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    MrMillard Trimaran 7

    @rberrey. The forward crossbeams are ~5cm/2" beneath the deck line in the drawings to allow for a flat deck in addition to a molded toe-rail. The crossbeams are not continuous, that is why there are 4 holes. The beams slide past each other when the trimaran is folded. The amas are demountable too, but only for storage or shipping.
    Last edited: Dec 10, 2021
  10. MrMillard
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    MrMillard Trimaran 7

    @gonzo, @bajansailor, @Ad Hoc, @TANSL.

    I have been asked several times to make a weight calculation for my trimaran. I have now tried to do that. I have to say though, there are a multitude of issues that plague my calculation, so it should be taken with a huge grain of salt. Here are just the problems I can think of off the top of my head:

    - I have never owned a sailboat. I don't know everything that goes ON it or IN it.
    - I have never built a sailboat. My structural estimates are very rough.
    - I have never compiled a list like this before, ever.
    - A lot of components and equipment are difficult to find accurate weights for.
    - I've tried to overestimate weights whenever there was uncertainty.
    - I may have completely underestimated the amount of material that is required to install some of the things on the list.

    For reference, here are 3 trimarans in approximately the same size and their claimed design weights:

    - Corsair 880. - 1660 kg / 3659 lbs.
    - Kurt Hughes D-32. - 1202 kg / 2650 lbs.
    - Triptyque 28. - 1690 kg / 3726 lbs.

    Remember, my trimaran is 8m/26', shorter than all of the above.

    My calculation ended up at:

    - Plain. - 1463 kg / 3226 lbs.
    - +10% margin. - 1609 kg / 3549 lbs.

    I actually hope that if these plans ever come to fruition that the end result is a lot lighter. Please reply with feedback/improvements/corrections.

    Here are my calculations:

    Attached Files:

  11. Ad Hoc
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    Ad Hoc Naval Architect

    I have no doubt there are issues.
    The fact you're raising them is positive, since you are now aware that weight and the total lightship weight is the key to any successful deign, whether it be a sailing tri or a high speed passenger ferry or a patrol boat etc etc.

    There in lies the difficulty with a new design and with little to no database.
    Get weights that are representative of what you are doing...

    So, what you need to do is create a spread sheet of weight of every item you can think of that will be on the boat. The spread sheet you attached, is a good start, but it needs more detail.
    And liens like Main Hull/Vaka - 300kg. That seems too simplistic and 'exact' to be considered with any confidence.

    So, with the more detailed spread sheet, place a column on the far side.
    In this column you indicate whether the weight shown is
    i) an estimated weight
    ii) calculated weight
    iii) suppliers weight
    iv) as weighed weight
    v) unknown and a pure guess

    Once you finish you detailed weight estimate you can run your eye down the side and see pretty quickly how much confidence you have in the final weight estimate.
    As such you can apply an appropriate weight margin as a percentage of the whole weight summation. I would suggest to start with at least +10%. But that would depend upon how much of a percentage of your weight breakdown is filled with iii) or iv). The %'age of margin you add is a reflection of how much real hard data you have and thus your confidence level.
    Then over the build time, you constantly update the weight by changing the notation from v) to i) to iii) to iv)...and as such, you can begin the process of reducing your %'age design margin.

    Remembering boats never get lighter, they always grow in weight!

    The basic of a weight break down are into 8 categories:

    1 Structure
    2 Mechanical
    3 Electrical
    4 Comms. and Control
    5 Aux system
    6 Outfitting
    7 Pyrotechnics
    8 Fluids

    Then each main category, with breakdown into ever small units.
    Here is Group 1 example:

    or a Group 4:

    or Group 6:

    Finally finishing with your summary:

    Makes sense?
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  12. TANSL
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    TANSL Senior Member

    Nor should we forget the transversal position of the CoG, if one does not want to get unpleasant surprises.
    The most difficult and laborious item to determine is the weight of the structure. For this reason, it is convenient to break down the items that compose it much more or, as modern designers do, once the scantlings have been calculated, make a 3D model with the entire structure with which to obtain a weight and of it and its CoG with greater precision. Being the relatively small boat, creating such a model is not too tedious and it will also help you in many other ways.
  13. Corley
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    Corley epoxy coated

    I'd like to understand your thinking in regards to the floats, just a few observations. The rocker line isn't too bad but not a fast shape the unusual vertically consistent beam will cause a lot of wetted surface in use as they will immerse a lot more than a more rounded shape. Consider a trimaran has to lean on its float to sail or excessive heel will spill a lot of wind. The optimum heel might be something like 15 degrees so you don't want slack bilges in the floats and buoyancy needs to pick up quickly. Also consider that the floats vertical at rest will travel through the water skewed over at an angle when heeled most modern trimarans have their hulls canted out to address this. Narrow float hull waterplanes aren't a bad thing in light conditions but slow when more of the righting moment is used and greater immersion occurs.
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  14. MrMillard
    Joined: Dec 2021
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    MrMillard Trimaran 7

    Hello again fellow boat design enthusiasts!

    I have not written anything in this thread over the holidays and the entirety of January, but now I'm back with a vengeance! Sorry about that, and thank you to @Ad Hoc and @TANSL for the excellent examples and advice. Thank you to @Corley too, but your questions may not be relevant anymore, or you will get satisfactory information from this post.

    First and foremost! I have altered the concept (pray that I do not alter it any further). I have some new drawings I'll be sharing and I have made a basic model that I have taken screenshots of that I will upload.

    Some of the things that have changed:
    1. The design now has a 8,5m/28 feet hull length.
    2. The integrated bowsprit has shrunk to 0,5m/1'8" and there is now an extendable bowsprit/light wind sail holder.
    3. The beam is 7m/23feet.
    4. The mast is no longer freestanding, but will preferably be without spreaders and no backstay.
    5. There is now one double cabin/berth, 2 single cabins/berths, a dinette/settee that can be converted to a snug double and a watch station/quarter berth that is a bit narrower. So theoretically 7 sleeping passengers, but that will feel crowded.
    6. The main hull now has basically 2 sections instead of 3, one below the waterline and one big flared section for accommodations.
    7. The main hull is now 225cm/7'5" wide.
    8. The amas must now be demounted BOTH when using a trailer and shipping with a container (now a 40 foot HC (high cube).
    9. The highest waterline is now at a draft of 45cm/18" and the main hull displaces 2500 liters/2,5 metric tonnes/88,3 cubic feet at that level.
    10. The ama shape is simplified and they displace about 3000 liters/3 metric tonnes/106 cubic feet.
    11. The amas are tilted 10 degrees inwards.
    12. The main hull has a L/B ratio of 10 at the waterline, and the amas' L/B is 18.
    13. The shape of the cabin now more closely resembles a deck saloon style sailboat.
    14. The shower and head are now separate compartments and can be accessed at any time and their use does now interfere with moving about the trimaran.
    15. Everything is in a single cabin, no separate entrance rear cabin.
    16. A huge technical area without dividers is located under the cockpit. I still think electric outboard is the way to go, but you could easily fit an inboard diesel here.
    17. The entire hull is taller, enabling a raised floor. The entire raised cabin has room for tanks and stowage beneath the floorboards.
    18. The combined navigation station, watch station and quarter berth works like this: a MFD for plotting and other equipment are mounted on a swivel on the back of the cabin wall for navigation whiles sitting in the seat (a foot well can be opened up by removing a cushion+board). When navigating/sailing from the cockpit, you can swivel out the nav. equipment into the cockpit through the door opening. When not in use for these purposes, just put the board and cushion back and it becomes a narrow quarter berth, excellent for solo sailing/watch keeping as you can nap and then look up at the nav data.
    There may be more things to point out, but I can't think of them right now. I haven't updated the SOR or the weight calculation, which I will, but I am still confident that a total buoyant force of 3 metric tonnes should be more than sufficient.
    Last edited: Jan 27, 2022

  15. peterAustralia
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    peterAustralia Senior Member


    call me a cynical so and so,, my main issue with this design is that in any seaway the waves are going to hit the underside of that chine step creating pounding and a lot of extra drag. My 2 cents worth, go with a professional set of plans. Nice to try, but I dont think it is very good
    bajansailor, gonzo and redreuben like this.
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