Please Sanity Check My Catamaran Design

Discussion in 'Multihulls' started by Nautically Obsessed, Mar 4, 2018.

  1. Nautically Obsessed
    Joined: Feb 2018
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    Nautically Obsessed Junior Member

    I have my concept sketch together for an 8 ft catamaran dinghy, and I am posting for a sanity check of my design. I plan to power it with a 50 lb thrust electric motor or small outboard. Phase II will be to put a modest sail on it, like a Sunfish sail. I know me and I will eventually venture out into bay waters and wind, so I would like it to know more about the limitations of this design.

    I have no boat design experience, so your feedback would be appreciated. I am leery about the fore position of the deck - will it slap against waves creating a hazard to the structure? Also, the placement of the propeller. If it is mounted at the aft end of the hull, will it pop out of the water in chop?

    I have very specific requirements, and I have considered an inflatable boat, small monohull, etc. One of the drivers of this design is that I will be able to disassemble the boat for storage and transport. Please see the attached pictures.

    If you read this far, you probably want more details:

    The material to use will be 5mm (3/8") plywood, stiched and glued with waterproof glue, and reinforced with fiberglass strips and epoxy resin. There will be bulkheads built in with FRP reinforcement, which will also serve as attachment points for the crossbeams.

    Each hull is 8 ft in length, and is an inverted prism. The triangle base (the surface closest to the sky) is 18", the triangle height at the stern is 19", and the height near the bow (before the curves) is 24". For volume calculations, I used an average 21 1/2" height, and 7 1/2 feet in length, to allow for buoyancy loss at the "pointy" part of the bow. This comes out to about 10 cu. ft. per hull. So each hull displaces 629 lbs (when fully submersed).



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    Attached Files:

  2. rwatson
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    rwatson Senior Member

    Your diagrams are necessarily very simplistic from using Sketchup, but adequate to explain the concept.

    It has precedents, and this little 3 video series might give you some insights for your project

     
  3. Nautically Obsessed
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    Nautically Obsessed Junior Member

    Respectfully, I was hoping to get some specific feedback regarding my design. I've looked at a lot of designs and alternatives. Did I mention I am Nautically Obsessed?

    Another thing is I don't plan to sell my design. If it works, I'll provide it for free.
     
  4. catsketcher
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    catsketcher Senior Member

  5. Nautically Obsessed
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    Nautically Obsessed Junior Member

    Thank you for the book suggestions!
     
  6. tmark
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    tmark Junior Member

    A couple of things to think about ...

    Your center of buoyancy is just aft of center. Your center of load is well aft of that. Ideally, the two should align.

    As currently designed, the boat will trim with the bow well high. She will be difficult to control, may roll excessively, and when sailed will have an unbalanced (if not unusable) helm.

    To mitigate, you'll have to move your load forward. You can see how the boat pictured in rwatson's post positions the load more relative to its center of bouyancy. At 8 feet however, you have a real challenge. Is there a reason why she can't be longer?
     
  7. gonzo
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    gonzo Senior Member

    It will float, but is a bad design. The only thing is that it is easy to draw. Triangular section will increase the draft compared to a circular or shallow vee. Also, 3/8" is about 9.5 mm not 5.
     
  8. Nautically Obsessed
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    Nautically Obsessed Junior Member

     
  9. Nautically Obsessed
    Joined: Feb 2018
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    Nautically Obsessed Junior Member

    I'll research the center of buoyancy issue - thanks. Note that the crew can move forward under power, and I was told that under sail there will be a downward force at the bow - thus the bigger volume up front. BTW, it is tempting to to go 12 or 16 feet. But I know I am going to learn so much from this I can apply it to the next build!

    I think I know where you are coming from. If you have studied hull design, lines, and all that, then this probably doesn't measure up. Nathan Herreshoff would be disappointed with it I'm sure. But float is my main goal right now - I want to be on the water with out much ado!

    Thanks for the replies!
     
  10. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The design is not good, that is clear, but it can be the first step for a design that, I am sure, will make you happy.
    I think @tmark's advice is very relevant. Some deficiencies in your design will be felt as soon as you start navigating, but since the boat is very simple, it will not be difficult to correct them and you will learn a lot from this first design. Since the speed can not be very high, the increase in the resistance due to wave formation due to the low hydrodynamic shapes will not hurt much.
    The boat could be very heavy for its size. It would be convenient for you to do some weight calculations and their distribution as well as estimation of the full load draft and the trim of the boat. If you want help in this regard, you just have to ask for it.
     
  11. gonzo
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    gonzo Senior Member

    Good designs are not harder to build than poor ones.
     
    fallguy likes this.
  12. TANSL
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    TANSL Senior Member

  13. tmark
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    tmark Junior Member

    Well, a canoe shaped like a coffin would be easier to build AND a poor design.
    But in general, yes, your point is valid.
     
  14. rwatson
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    rwatson Senior Member

    Because we have done it !!!

    Its hard to convey this to all the amatuer boat designers we get on this forum, but its true.
     

  15. Nautically Obsessed
    Joined: Feb 2018
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    Nautically Obsessed Junior Member

    Thanks to all who have replied. I posted in order to find out if I was making some kind of fundamental mistake, so I guess its good that only the trim issue has been mentioned. I have limited design ability, tools, facilities and time, so this thing has to be simple.

    I may take you up TANSL, on your offer for help! But first, I'm going back to sketchup to look at a 12 or 16 foot version. That would require each hull be in two bolt-together pieces, which ups the build time, materials etc. I don't want it to break, so I've got to research how those nested dinghys are made. I'll check this forum to see if its been discussed before.
     
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