Help! An evaluation of a small boat hydrostatics?

Discussion in 'Boat Design' started by kavos, Nov 21, 2014.

  1. kavos
    Joined: Nov 2014
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    Location: Greece

    kavos carpenter, shipwright

    Last summer I counted the lines of a beautiful traditional small boat in the bay of Sithonia Halkidiki Greece. Then I modified these lines, I cut the length almost 3 ft and I added a different transom. Finally i discovered that it seems very much with an auklet dinghy :). Freeship program gave me the following values. Can someone give me an opinion about its stability and performance? Thanks...!
    Danae4_Linesplan.jpg
    Danae Dinghy Hydrostatics:
    Design length : 2.200 m
    Length over all : 2.200 m
    Design beam : 1.200 m
    Beam over all : 1.209 m
    Design draft : 0.135 m
    Midship location : 1.100 m
    Water density : 1.025 t/m^3
    Appendage coefficient : 1.0000
    Volume properties:
    Displaced volume : 0.123 m^3
    Displacement : 0.126 tonnes
    Total length of submerged body : 2.034 m
    Total beam of submerged body : 1.073 m
    Block coefficient : 0.4168
    Prismatic coefficient : 0.5131
    Vert. prismatic coefficient : 0.6183
    Wetted surface area : 1.605 m^2
    Longitudinal center of buoyancy : 1.036 m
    Longitudinal center of buoyancy : -3.311 %
    Tranverse center of buoyancy : 0.000 m
    Vertical center of buoyancy : 0.084 m
    Midship properties:
    Midship section area : 0.118 m^2
    Midship coefficient : 0.8123
    Waterplane properties:
    Length on waterline : 2.034 m
    Beam on waterline : 1.073 m
    Waterplane area : 1.471 m^2
    Waterplane coefficient : 0.6741
    Waterplane center of floatation : 0.982 m
    Y coordinate of DWL area CoG : 0.000 m
    Half entrance angle of DWL : 23.567 degr
    Transverse moment of inertia : 0.099 m^4
    Longitudinal moment of inertia : 0.306 m^4
    Initial stability:
    Vertical of transverse metacenter : 0.893 m
    Tranverse metacentric radius : 0.809 m
    Longitudinal transverse metacenter : 2.572 m
    Longitudinal metacentric radius : 2.488 m
    Lateral plane:
    Lateral area : 0.199 m^2
    Longitudinal center of effort : 1.177 m
    Vertical center of effort : 0.079 m
    Hull characteristics above waterline:
    Lateral wind area : 0.515 m^2
    Z coordinate of wind area CoG above DWL : 0.154 m
    Distance from bow to wind area CoG : 0.694 m
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Those properties are based on the declared midship at 1.1 meters. To get the "correct" properties, do the following -

    Locate the actual midship location -
    1. Save a copy under a new name
    2. Trashcan existing stations and select new stations at an increment that will give you about 50 of them.
    3. Look at the displacement curve and find the station of max displacement.
    Now go back to your original file and set this location as your midship location in the Project>Project settings>main dimensions menu.
    Now you can run Hydro calcs.

    An annoyance, but you have to do this for every case of trim and load you look at. I keep a separate FreeShip file for each different displacement.
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    The drawing shows a hull not well shaped for much. It'll row, though not well, it'll sail though not well and it'll motor at slow speeds, but don't ask much of it.

    Software is a wonderful tool, but it can't tell you how well suited the shapes selected, will do for the goals you might have. It'll just draw the shapes to whatever the input is, without regard to appropriateness to the design's requirements.

    In order to design a boat, you have to know what you'll expect from it and with this in hand, you can develop shapes as best suited to these goals as practical.
     
  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Yup, she's a tub.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The "stability calculations" can all be rendered academic by some clumsy, junk-food loving bozo in that little boat. :D
     
  6. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Sorry about that Davos, but it does seem a bit tubby. Oh, and welcome to the forum. We are not all naysayers, but we are likely to call them like we see them. That is to the advantage of people who seek comments from crusty old veterans.
     
  7. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Show me a seven foot dink that isn't tubby. I like it. I probably wouldn't try to tow it behind another boat, but other than that, it's fine. I'm not sure the hollow entrance is doing you any good, though. Will you be able to climb back aboard if you fall out? That is one stability concern related to tiny dinks.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The smaller the boat, the more difficult to make it do the things it might need to.

    [​IMG]

    Simply put, you can fudge the lines around a fair bit on larger craft, where their may not be as big and impact, but when every ounce counts, such as the case with tenders and dinks, you'll do need to have a clue about what you're doing.

    What are the goals of this design? Have you a rudimentary SOR?
     
  9. kavos
    Joined: Nov 2014
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    Location: Greece

    kavos carpenter, shipwright

    Hi again guys,
    Thank you all for the information, advice and opinions as for the welcoming in this forum. I’ll follow all your instructions and I really will take in consideration your comments!
    Well, I agree. This boat is like a tub. But as you understand, when you want to build a small 7 foot boat, the shape choices are drastically reduced especially if you want to achieve enough space for two adults. Anyway I hope it won’t float as a bathtub:).
    I think that the boat it looks like this one.
    View attachment 95939
    Well my goal is to build a cheap really small boat for protected harbors, reliable enough to put my children in, light enough to be easily transported, easily moved by oars or small engine and finally to maintain the traditional characteristics of the Greek shipbuilding - at least keeping the lines of a very old and traditional hull type from Crete.
    This boat is basically based on a combination of Mediterranean Sea hull types. Old shipbuilders still make those wooden fishing boats empirically and only using a saw and a lath hammer, using a planning method Greek builders call “by the eye” and it means having no plans but following a pattern having in mind defined only by the length of the boat and the width of the medium bulkhead. These people are remarkable.
    I post a photo and plans from such boats to have a look.
    View attachment 95940
    Gaita-Chanea-1.JPG
    In short, however, tell me if you think it’s ok to build this boat after some refinements or recommend me any “dramatic” improvements you think I definitely have to do. Please reply.

    Thanks in advance!
     
  10. kavos
    Joined: Nov 2014
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    Location: Greece

    kavos carpenter, shipwright

    post again the previous replay missing images
    04.JPG
    Gaeta Chanea.jpg
     
  11. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    Why so short?
     
  12. kavos
    Joined: Nov 2014
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    kavos carpenter, shipwright

    I need a boat so short for many reasons mostly becouse it can be built by only two sheets of plywood and no need expensive registration
     
  13. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You won't be able to build it from plywood sheets, the material will not follow that shape. All you will get trying, is a migraine !
     
  14. daiquiri
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    daiquiri Engineering and Design


  15. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    Kavos, it's an impossible goal.

    If you want build it in plywood with just 2 sheets you can find plenty of plans, some free. So you won't have to struggle with a software as Freeship which is unable to deal with developed surfaces. In fact it's faster and more accurate to use cutter, cardboard and tape. And it's even faster with no possible mistake to get the PDF of plans in Internet. The Optimist or similar dinghies plans are a good choice, specially if you have a children with you. It will do the job very well, it's unsinkable and it's very simple to make. Very cheap if you do not enter in the complications of a sail boat (mast sails, daggerboard, etc...) but just keep it as a row boat. Works very well with a chinese scull "yuloh".

    At this length it's a floating light tub, no way to escape from this shape as you need a "correct" longitudinal repartition of the volumes and stability. Your SOR is similar to the SOR of a small tender for a yacht with oars or scull, or/and a small sail, or a 2HP outboard engine.

    Greek (and other countries) traditional shipbuilding (the WB pic shows a typical working boat called "pinasse or pointu in France, it's a boat used from the Black Sea to Spain, in the 70' you could find hundreds in Sicily) uses shapes totally incompatible with plywood and needs soft lines at the stern, as it was a sail boat, modified for engines around 1920-1930. So you need a minimal length. And it's very tricky to make if you have not experience of traditional naval carpentry, specially the S shaped planks at the bow.
     
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