ignorance is a bliss

Discussion in 'Boat Design' started by breschau, Dec 5, 2010.

  1. breschau
    Joined: Nov 2010
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    breschau Junior Member

    Greetings to all forum residents,
    I am trying to learn abit of boat design to be able to design kayaks for myself. I am not trying to setup a business or design anything for anyone else.

    Some of you may have seen the kayak model I posted recently. I havent recieved negative comments on the hull model, but I think there should be rooom for improvement. Lacking on the theoratcial department I made basic hull models on freeship and run some calculations on mitchlet and freeship. The programs may not be too accurate the models may not be enough to form a decent database but I feel like I hit a wall anyway.

    The power on a kayak is essentially fixed, I can only read the resistance charts up to certain numbers. Displacement is also fixed, I know the weight of what I will carry. I started making major changes on some values while keeping some fixed. Any major paramater I change came at a disproportionate cost. When I saw resistance values of diffrent hull forms with a certain length I felt like I cannot really change anything. For example I tried to increase speed at a fixed Rt, fixed LOA, variable LWL, variable Cp. One hull came out with 5knots at 40N total resistance, the other side of the extreme came out with 5knots at 50N. The cost was about half of the righting moment. When I tried preserve stability I lost most of the rocker curve. I was expecting compromises but not like +%10 for -%25. Its as if the default model is actively resisting any move out of medicore.

    Is this a normal result of having too little to change on this kind of boats. Have I learned something or am I loosing it entirely?

    PS. Above wall of text may sound dumb for proffesionals of the field but I am sincerely trying to learn more and not looking for arguement.
     
  2. peterAustralia
    Joined: Mar 2006
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    peterAustralia Senior Member

    just maybe your over analysing the topic

    perhaps look at existing kayak designs, find one that looks closest to your ideal boat. Modidy it a little bit, then build a full size kayak out of cheap plywood.

    Use it as a 100 pecent scale model. If it works fine then build something the same shape but of good materials and good workmanship

    Even a doos like me can build a boat. I reckon something simple could be knocked up in a weeked. If you google things like 12 hour canoe etc etc. Perhaps buy something second hand on ebay, use it as your control, build one to your own design an see it is better or worse. My first 14ft kayak was only a $100, the paddle was a lot more. For $100 I could not really complain, I used it a lot, learnt a bit, and then sold it.

    I started to learn a lot more when I got away from the computer and started cutting wood.
     
  3. TeddyDiver
    Joined: Dec 2007
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    TeddyDiver Gollywobbler

    About right.. that boat design is mostly compromises. Just try to focus what's the main purpose of your design and try to balance the tradeoffs the best way you can. That's about it.. :)
     
  4. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    What you have discovered is that stablilty is expensive in terms of the power it absorbs. This is true of everything, not just boats. The ideal hull looks alot like a one man scull with about a 10" beam. Once you decide you need more stability than that offers, you can explore different approaches to getting it. The price you've discovered may also help you to understand why we don't get too excited about having a 55% efficient propeller when an 80% is feasable. The hull is likely only 25% as good as it could be in a small craft. One other point about freeship and michlet. Make sure michelet is moving your boat forward through the water and not backwards. The first boat I drew on freeship was copied from a pencil drawing and it was reversed.
     
  6. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    That may be a bit optimistic.

    Cape Falcon Kayaks does a class where you build a skin on frame kayak (arguably the simplest design) and a paddle. The class is 8 hours a day for 5-6 days, and that doesn't count that the mortising on the gunwales is done by the instructor before the class starts.

    A stitch and glue design (which I have never done) may go faster. From what I've read a cedar strip boat will usually take at least 100 hours.

    My own skin on frame boat is now three-quarters of a year along!
     
  7. breschau
    Joined: Nov 2010
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    breschau Junior Member

    @DeCockey,
    Thank you for your pointers for resources. I will check them out.

    @PeterAustraila and Cthippo
    Thank you for the encouregement. You are both right in your own way. The time it takes to build the boat itself is directly dependant on the building method. I am trying to smooth it out as much as I can to reduce the chance of a blunder on the bulding stage. Physical modeling is also on my list.

    @philSweet "stablilty is expensive in terms of the power it absorbs"
    Spot on that is the correct wording that I couldnt make out.
    Incidentally the facing of the model was the first thing that bugged me when I first started with michlet. I think I have it right but I will double check just in case. Also what do you mean by "The hull is likely only 25% as good as it could be in a small craft". Is that %25 compared to the 10" one man scull with barely any draft or %25 to the possible ideal form of 20" 15' kayak?
     
  8. breschau
    Joined: Nov 2010
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    breschau Junior Member

    @TeddyDiver
    Right, everything has a price that makes sense but the bill of the price is akin to a french restaurant bill to me. Quote from Douglas Adams "On a waiter's bill pad, numbers dance. Reality and unreality collide on such a fundamental level that each becomes the other and anything is possible." I must figure it out somehow or I will have to wash the dishes.:D
     

  9. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I was thinking with regards to an unrestrained Godzilla run at a fixed and very light displacement. So yes, the 10" scull. Another point about michlet/Godzilla. If you use the minimum righting moment criteria at near the end of the "in" file, I found that it tries to make the boat deeper by using a reverse curve for the station cross section shape. This is numerically correct but not usually what you are trying to accomplish when fiddling with GM. If you have a base design you like and you want to explore different stabilities, You should fix the F3 variable and then do a series of runs with differing GM. I'm doing this from memory so if it all sounds like gibberish send me a PM and I'll try and be more precise.
     
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