Design for review

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

  1. sharpii2
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    sharpii2 Senior Member

    I never meant to say it was.

    I meant to say I saw that as an acceptable outer extreme.

    A boat like that can have a very sharp bow for its beam. It may not be the fastest boat in all conditions, but in certain ones, such as light winds and glassy swells, it may do better than the competition.
    The limitation is the rather abrupt rise to the surface aft, which can suck up a large stern wave.

    "Flatties", not to be confused with "flat-iron-skiffs", dealt with this issue by having dead rise in the stern sections, while having flat sections in the bow ones.

    This allowed the stern to better shed the water running along it.
     
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  2. PsiPhi
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    PsiPhi Newbie

    Here is the line drawings with the lowest point moved further aft.
    I have also added a skeg, probably the largest possible for the moment, but might shrink it. Was just reviewing Ted Brewers book on Understanding Boat Design, but he doesn't say much on the subject.
    Scarab_2_NewLines.jpg
     
  3. bajansailor
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    bajansailor Marine Surveyor

    I think that your lower chine still needs a bit of work, where it meets the stem - it is a fair curve in the side profile view, but it just does not look right in the plan view (where it has a slight 'hollow') and in the body plan, where it kinks up between the stem and the first frame / section.
    Or am I being unnecessarily pedantic here?
     
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  4. TANSL
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    TANSL Senior Member

    I'm also going to be pedantic. The intersection of two fairly rigid, symmetrical boards, which meet at the stem, cannot be a single curved line unless the boards are severely tortured.
     
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  5. Ad Hoc
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    Ad Hoc Naval Architect

    Nope - it is a good comment.

    upload_2022-1-18_11-22-35.png
     
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  6. PsiPhi
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    PsiPhi Newbie

    No such thing as beint too pedantic, not in a learning situation like this.

    The boards are, currently, somewhat tortured, as per the earlier exports from Delftship.

    I think it was trying to reduce that that introduced the hollow and kink that @bajansailor mentioned. I have tried to smooth those out now.

    Scarab_2_LinesFaired.jpg

    Delftship shows me the stress on the bottom panel, and faired again like this it is around 34%, but when the shape was out it was about 24% - not entirely sure what that means in terms of construction, how big does it have to be to be bad? Delfship also highlights stress on the chine panel, and it is less than 1%
    Stress1.png
     
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  7. wet feet
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    wet feet Senior Member

    Have you tried lowering the intersection of the bottom chine with the stem?I think I'm seeing the same thing as As Hoc and TANSL.I also think it unlikely that a stem with that amount of curvature at around mid-height will be a happy termination of plywood panels and the screenshot seems to show a straight forward end of the panel.Hard to see how it all fits together nicely.
     
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  8. messabout
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    messabout Senior Member

    Will Gimore mentioned that the forefoot needs to be at or above the surface in disturbed water. If the forefoot is buried even a small amount, in rough water, on a course other than directly downwind, the boat will hunt, sometimes dangerously. The skipper of a small dink can usually compensate for the problem by shifting weight aft. Too far aft and the transom drags, which is slow. Put in enough rocker to anticipate the varying conditions and the anticipated load.
     
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  9. PsiPhi
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    PsiPhi Newbie

    Had a go at setting a plumb bow, I like how it looks but according to the Delftship figures the stress on the play is way worse, like four times higher bowcompare.png
     
  10. wet feet
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    wet feet Senior Member

    Have you tried lowering the bottom chine at the stem to the waterline to see if the stress reduces?The other point I will reiterate is that the raked stem with a curved profile isn't a logical termination of the panels of a chine hull.
     
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  11. Will Gilmore
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    Will Gilmore Senior Member

    What shape does the bow take if the plywood panels curve naturally through each other and their intersections are used to define the bow?
     
  12. sharpii2
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    sharpii2 Senior Member

    That is not what I have found. The only way I know to get a straight, raked stem is to go to multi-conic development, which means some twist as well as bend in the panels. The only way I ever got a straight stem was to have the side panels dead vertical. It took me a long time to discover, that with flared sections, I end up with a curved stem. It took me even longer to find out why this is so. What I found was that, if I kept the ruling lines parallel to one another, the bottom edge of the panel has the same curve as the top. But if the sides are flared, the bottom edge is going to meet with the centerline before the top edge is. When that happens, it ends up with a portion of the curve of the top edge.

    OK. Then it ends up ending short of where the top edge ends, so that produces a raked stem.

    The problem is that where it ends is only true for the bottom edge. Further up from the bottom edge, this new portion of the panel has a larger portion of the original curve. This means that it is going to intersect with the centerline further forward than the bottom edge. But, because it is a segment of a curve and not one of a triangle, Its intersection is not going to line up with both the bottom and top intersections. So, to connect these three points, I'm going to have to draw a curved line. So, if my frame segments are straight, my frame sections are virtical, and my side panels are flared, I'm going to end up with a curved stem. If I want a straight stem and vertical frame sections, the frame segments are going to have to be curved. Predicting these curves (a different one for each frame at that segment) is going to be a bit of a pain. To be honest, I don't know how to do it. So I stick with what I call cylindrical development, which means all the ruling lines, in the panel curve, are parallel to one another. This way the only curves I have to worry about are at the stem and the transom.

    My advice to PsiPhi is to make a scale cardboard half model of his hull design. This way he can see for himself how hard or easy it is to fit the panels together. He can make various versions of it until he makes one that is closest to the shape he wants. I don't think the software was ever designed to do this.
     
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  13. TANSL
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    TANSL Senior Member

    The software is prepared to do precisely that, to create a virtual 3D model with which to analyze the problems that may arise in the construction and, what is more important and practical, quickly modify the initial model to check the improvements. or create several similar models and choose the best one, with a speed and at a price that the physical model does not allow us.
    In my opinion, a physical model, to scale, can never faithfully reflect the effort that must be made to shape a panel, especially if it is made with different materials (different stiffness) than those of the real ship.
    I do not defend any software and I am an admirer of the 1/10 scale 3D models that were built in shipyards to correctly define the bow and stern areas, but fortunately that system is no longer maintained in any shipyard. They have gone on to make virtual models, I suppose because the software allows to make them with great precision.
     
  14. Will Gilmore
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    Will Gilmore Senior Member

    If you draw in the stem width and end the panels on cheeks of the stem instead of at the point of the bow, the software may like it better. You will get an inch or so less torture by allowing for the shaping of the stem to the panels.
     

  15. wet feet
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    wet feet Senior Member

    A lot depends on where the apex of the cone is located and some hull forms have many cones,which will allow for slightly more complex shapes.I would still maintain that the variations of curved stem that have been shown will require some oddness,either in the introduction of curvature at the ends of the panels or a flat of varying width on the face of the stem.A point which I think post #44 is trying to nudge the OP to address by recommending the addition of some width to the stem.It used to be quite common for the designer to terminate the forward end of waterlines on the centreline for lofting and to add details of the profile of the stem and it's blend details for the actual construction.
     
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