Design for review

Discussion in 'Boat Design' started by PsiPhi, Jan 12, 2022.

  1. PsiPhi
    Joined: May 2007
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    Location: Queensland

    PsiPhi Newbie

    Thanks guys.
    @wet feet Yes, windage could be an issue, we usually have a good breeze here, and yes Delftship does show how developable it is, and that area at the bow is a little harsh.
    @mc_rash You are right, the transom shouldn't drag like that, thanks.
    I will take some time over the weekend to try and rework that feedback into the design. I think I have been basing too much on old school design techniques, ones that are for custom made steamed planks, rather than modern ply.
     
  2. gonzo
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    gonzo Senior Member

    A design like that will generate a lot of waste. If you expand the plates, it will show S curves when they develop. That is typical of parallel sections. It will also probably have a pronounced twist at the stem, that may require laminating thinner plywood. Many people fall into the trap of designing their own boat. Ask yourself honestly, if the goal is to learn design or have a boat that satisfy your requirements and handles properly. If the latter is more important, a professional design is a better option. There are plenty of free plans from successful boats. Old school design techniques are fine. Computers often give you drawings that look great, but do it too fast. The time spend with pencil and lot of erasers ;) makes you pay attention to the design. Pencil shavings also smell better than any keyboard and mouse ever will. Good luck, whichever choice you make.
     
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  3. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Thanks for the feedback @gonzo I will not deny that the design process is at least as much fun as the finished product, for me. I also agree that pencil and paper are the preferred method, but I am starting with a computer program because it knows more than me about the subject, and makes small tweaks easier to try, and discard.
    I actually bought myself a new drawing board a couple of days ago, the idea being to see if the cad program makes it look viable, then transfer it to paper.
    Also, Delftship develops the panels for me, something I'm not ready to do by hand yet.
     
  4. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    I made a start on the mods, reduced the topsides and moved the transom a little higher.
    Something about the line of the keel looks "wrong" to me, but I don't know why, I have no expertise to base that on, going to go and look at the lines of some other dinghy's again.
    The stress is quite evident in the topside panel and the bottom, but not quite sure yet how to resolve it.
    Scarab_2_Hull.jpg
    Scarab_2_Lines.jpg
     
  5. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Did a couple more tweaks, to the stem area this time. Doesn't look much but it has improved the panel stress a little.
    Pulled outward a little on these two points.
    Scarab_2_HullStress.jpg
    According to Delftship this has reduced the stress on the bottom panel from 33% to 24% and on the topsides panel from 1.8% to 1.4% - not entirely sure what those values represent in the real world, but it is movement in the right direction.
     

    Attached Files:

  6. gonzo
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    gonzo Senior Member

    The rocker should have the deepest part about 2/3 aft of the bow. With a design like this, you would have to concentrate the weight on the forward part of the boat to be trimmed. Ask yourself why is the shape the way you are drawing it. The hull has to serve a purpose. You define the purpose, and the shape defines itself.
     
  7. Will Gilmore
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    Will Gilmore Senior Member

    Good point. Estimate the load distribution with rigging and crew.
    maxresdefault.jpg
    These types of boats tend to be a little bow heavy because the forward position of the mast with all its sails and spars and stays are concentrated at the base of the stem through the forward twart. With two people in 13 feet, one will be sitting pretty close to midships. With a smaller displacement profile forward due to the 'V' bow, that rocker looks about right to me. Of course, the software will be able to calculate that and you'll know for certain.
     
  8. messabout
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    messabout Senior Member

    If you meant to design a planing hull, the forward max rocker would be appropriate. For a displacement hull, which this faux round bottomed boat is, move the lowest part of the keel aft to at least the 2.000 mark of your drawing.

    Try not to get too much enthralled with the round bottom concept, especially when the bottom is composed of several flat panels. It is not really round. Do a bit of math and figure out how much wetted surface you are designing. For displacement types, wet surface is an important consideration. Multi panel designs like this have too many tendencies to generate early turbulence at the panel junctures. A very near equal or better design can generate a wet surface minimum with a trapeze bottom configuration. Also a lot easier and more economical to build. a typical trapeze bottom has notably less wet surface than a vee bottom and much less than a completely flat bottom. Perhaps even less than the configuration that you have drawn.

    If you are committed to the traditional "look" then proceed as you have begun.
     
  9. TANSL
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    TANSL Senior Member

    @PsiPhi. try to do something similar to this, the panels in the bow will be less tortured
    Snap1.jpg
     
  10. sharpii2
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    sharpii2 Senior Member

    2/3 aft seems a bit far. Do you mean 2/3 aft the stemhead? If so, that seems a bit more reasonable. For my money, the deepest section should be 2/5 to 3/5 of the waterline, aft the bow at the waterline.
    More, for better wave parting ability; less, for better downwind behavior. And I think sharper bows tend to want their sail Center of Areas (CA's) further aft.
     
  11. gonzo
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    gonzo Senior Member

    Well, 2/3 and 3/5 are not far apart.
     
  12. Will Gilmore
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    Will Gilmore Senior Member

    International 12
    international_12_dinghy_drawing.jpg
    Handy Andy by Atkin & Co.
    HandyAndy-2.gif Windmill
    windmill_drawing.jpg
    I don't think 2/3 or 3/5 aft is always appropriate. You want correct trim according to area of flotation/ balanced weight. When the wind starts driving a dinghy by that high, forward center of effort, you want your bow to stay above the water.
     
  13. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Once again, thanks for all the great info and help, will review the layout again this week.
    A couple of points.
    • no, it won't be a planing hull
    • yes, main job for the bow will be to cut through the current
    • planning on a lug sail forward, small motor aft, will try and work out the weight for balance purposes
    • image trimming with two by sitting further aft with crew midship, and single handed by sitting further forward
    • Delftship should be able to give me wetted area figures
    • Haven't added it yet, but assuming a skeg, which will add to WA as well as sail balance
     
  14. PsiPhi
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    PsiPhi Newbie

    I guess the next thing is to work out what type of wood to make it out of, so I can calculate weight of the boat and the rig
     

  15. gonzo
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    gonzo Senior Member

    The current does not affect the behavior of the boat in the water. It only affects the speed and course made good. If you are sailing in a shallow estuary, there will often be a short steep chop. A blunt bow will likely pound. However, in small boats the trim is easily changed by the position of the crew. A moderate change can adjust the behavior of the boat in different conditions.
     
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