learning scantling: hull definition and hydrostatic for small cat, first attempt

Discussion in 'Boat Design' started by magwas, Jan 3, 2010.

  1. magwas
    Joined: Oct 2009
    Posts: 287
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    Location: Hungary

    magwas Senior Member

    (See http://www.boatdesign.net/forums/boat-design/learning-scantling-design-statement-30778.html for context)

    Now, here is the hull definition of the small cat, canoe configuration. If requested I can upload the same files for the catamaran config also, but I have a problem which means I might have to restart design, see Q3.
    The hull itself is a redesign of a carlson .hul file I have found
    somewhere. I added two bulkheads, a keel, a rudder and decking in fore
    and aft. There is a rectangular area in the middle, strengthened with
    a frame. The fore and aft part of the frame are the bulkheads, the
    sides are pieces of plywood. The common deck of the catamaran is
    placed on this rectangular area. There is a mast in the centerline of
    the boat.
    No scantlings have been done yet, elements of shell and rudder have
    been given 4mm nominal thickness, bulkhead, keel, frame and common
    deck have given 20mm. I suspect that additional parts will be
    necessary to achieve structural stability.
    The results of design hidrostatics, hidrostatic analisis, resistance
    calculations and cross curves are attached for both single hull and
    cat configuration.

    I am clueless, don't know where to put the mast.

    Q1: Where should the mast positioned? I have put it in the centerline of the boat, afore a bit. Is there a method to determine ideal position?

    Now let's take a look at hidrostatic results.
    First some links to the underlying concepts:
    Block Coefficient,
    Metacentric height,
    Prismatic Coefficient,
    Frictional resistance,
    Hydrostatics of kayak design (nealy all of the coefficients explained)

    About the canoe configuration:
    I have aimed at 160kg of displacement. It was easy to look up the corresponding draft in the hydrostatic result. At first it was just at the top of the upper strake, so I have scaled up the height (with 1.2 multiplier as I remember) to look better. Because I have built a model, and it seemed to be very stable, I guess it would not make the boat too unstable.
    The transverse metacentric height is 26.8 cm, freeboard is a bit more than 16 cm.
    Now I took a look at cross curves. I could not find much info on cross curves (here are the relevant links: http://www.boatdesign.net/forums/de...regarding-cross-curves-calculation-16705.html, http://www.boatdesign.net/forums/design-software/cross-curves-freeship-29699.html, http://www.boatdesign.net/forums/stability/kayak-stability-23298.html ,Kayak stability at guillemot )
    What I have figured out that whenever KNsin(f) > KG (center of gravity height above keel), there is a positive rightning moment. Other measure used besides KNsin(f) is GZ, which is the horizontal distance of G (Center of Gravity) and B (Center of Buoyancy). Their relationship: GZ=KNsin(f) - KGsin(f)

    Q2: I came to the conclusion that the rightning moment is GZ/2*m*g where m is the displacement weight. So rightning moment is directly proportional to GZ, as the other values are constant. Is it correct?

    Now I could not find KG directly in the hidrostatic results, but GM=G-B, so G=GM-B=0.268-0.132=0.136m, at displacement of 0.16t. So where the graph for 0.16t is below 0.136m, then there is a positive rightning moment.
    Now the graph is below 0.136 below some 33 degrees, which would say that the hull by itself will lay on its side at 33 degrees.

    Q3: I either managed to design a hull which is unstable by itself (which I doubt, because the model of a very similar one has been very stable), or there is something I epicly misunderstood. Maybe KNsin(f) should be lower than G? What is the problem here?

    I stop here for now until this is sorted out.

    Attached Files:

    Last edited: Jan 3, 2010
  2. apex1

    apex1 Guest

    How many attempts will you make, how many threads will you open, how many members will you attract to hammer on you???

    There is NO free ride here! Get it!
  3. magwas
    Joined: Oct 2009
    Posts: 287
    Likes: 8, Points: 18, Legacy Rep: 47
    Location: Hungary

    magwas Senior Member

    I intend to continue my adventure into boat design, and try to make as informative posts as I can. I am the first to admit that I don't have a clue, but those who help me with this adventure have the knowledge, and I am updating my posts with the answers.

    I report any offtopic comments and personal attacks to the moderator.
    This is my last answer to any personal attack. If anyone wants to discuss merits of freedom of information, http://www.boatdesign.net/forums/bo...ement-naval-architects-please-help-30693.html shall be the topic for that, as it is already severely messed up (or open a new at your will).
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