Design for review

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

  1. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Sorry about the delays, life keeps interveining and making me do things, like my job :)

    @wet feet Yes, I did try lowering the bottom chine at the stem, but that increased the stress

    @TANSL & @sharpii2 Re the models, you are both correct. The software is designed to tell you everything you need to know, and Delftship is a good product, but no software can cover every eventuality (I know, I am in the software industry), and no software image is the same as holding the real thing in your hand. So currently I am trying to knock it into the best shape I can, then I will make an actual model. I am having difficulty finding a material to make it out of, corrugated cardboard is plentiful but appauling to work with, thin card you can get from the stationary or art supplies is too thin, it will bend with no stress, and thin bendy ply seems to have vanised off the face of the planet.
     
  2. PsiPhi
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    PsiPhi Newbie

    @Will Gilmore I will try adding the width of the stem into the equation next
     
  3. PsiPhi
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    PsiPhi Newbie

    Here are the lines with an allowance for a 2 inch stem/backbone. I simply moved all the centreline points out by an inch. This appears to have slightly increased Delftship's stress rating, the opposite of what I expected.
    I also faired a couple of lines using DelftShip's control curves, this looks like the best shape so far, taking the fairing and the stress into consideration
    Scarab_3_HullLines.jpg
     
  4. bajansailor
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    bajansailor Marine Surveyor

    I still don't like the look of your lower chine where it meets the stem - I think it might look better if you lowered it a bit, perhaps to where the station meets the waterline?
    As there is still a slight 'kink' in the lower chine in the body plan in way of the second frame from the bow.
     
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  5. PsiPhi
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    PsiPhi Newbie

    @bajansailor Thanks for the feedback, I will work with that. You're thinking keeping the same curvature of the stem, and moving the chine lower around that curve, or are you thinking dropping it lower where it is and changing the curve?
    As to the kink in the lower chine, I actually only used the curve control on the side view, not the plan, so I will look at that again.
    Once I have the right shape I will export the offsets and draw it properly on paper too.
     
  6. bajansailor
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    bajansailor Marine Surveyor

    I am thinking keep the side profile of the stem the same, but dropping the end of the lower chine where it meets the stem, such that it meets the stem at the waterline that you have shown.
    And then see how the curves look visually to your eye.
     
  7. PsiPhi
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    PsiPhi Newbie

    @bajansailor Here is the lines reworked as suggested.
    According to Delftship the stress is a little higher, but only marginally.
    I'm not entirely sure how to judge if the lines are good, don't really have the right experience. They look fair, smooth, and they look artistically pleasing, but it for better people than I to judgeif they are seaworthy; that's kind of what I came here for.
    I did a little more fairing with the curve control and think I have a smoother curve now.
    Scarab_3_Lines.jpg
     
  8. PsiPhi
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    PsiPhi Newbie

    I did make a cardboard half hull, but it was somewhat lesser quality than a third graders school project :)
    The cardboard was too thin and the scale too small (I just printed the unfolded panels on A3 paper and cut them out), to make it very accurate - and it was based on a printout from before I made the above amendments.
    If it was anywhere near accurate then the one thing that stood out for me was that where the bottom panel (garboard is it?) met the chine panel at the stem, there was the most severe bend in the hull material. I think that vindicates @bajansailor 's recommendation to lower that point - the next model should tell.
    I have found some 2mm MDF, which is horrible to work with, but if cut with a fresh blade, sealed on the edges with a little Aquadhere wood glue, it might stand up long enough to make a reasonably acceptable model.
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Yes, I think that was due to the 'kink' that you had in the chine previously.
    It is all very well having Delftship calculate stresses, but you can get a reasonably good idea yourself from the rate of change of shape of the sections in the body plan - if the angle of the sections between the keel and the lower chine start changing significantly then there is a lot more 'twist' in the panel, and hence stress.
    If the angle stays fairly constant, then that makes it easier to wrap the panel around the sections.
    Re the next chine above the lower chine, you could also try raising the height of this chine where it meets the stem (perhaps by the same amount as the chine above it), and then see how that looks.
    You could also try making the stem vertical down to this chine, and then have a straight line between it and the lower chine, and see how this looks, as I think it was mentioned previously that you could have problems if the stem is curved between the chines, re how the hull panels meet at the stem.
    But the next model should help to determine if this will be a problem or not.
     
  10. waterbear
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    waterbear Junior Member

    Regarding the weight, Welsford's 13ft houdini is 85kg and somewhat wider than your design. 85kg boat + 2x 85 kg passengers = 255kg which your design displacement, right? Houdini by the way has 9mm ply on the bottom and 6mm sides.

    I think when manufacturers of 13ft boats say "up to 4 passengers" they do not mean 4 90kg passengers.

    houdinidwg.gif
     
  11. PsiPhi
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    PsiPhi Newbie

    @bajansailor I have had a play with the next chine up as suggested. I was dubious because the lines looked good (to my inexperienced eye) but these amendments do look better, and taking Deftships stress indicators into accound, this is a better shape.
    I did try the straight sections you suggested, but just could not abide the look of it.
    Scarab_4_lines.jpg
     
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  12. Will Gilmore
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    Will Gilmore Senior Member

    Is your irregular section spacing intentional? Are there thwarts and other structures dictating frame placement?
     
  13. PsiPhi
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    PsiPhi Newbie

    @Will Gilmore Yes, that is an intentional side affect of my uncertainty with using Delftship.

    I placed the original stations in line with the control points I use to shape the hull, these were where I wanted frames to be, to align with thwarts, mast partner, fore and aft of the centreboard, etc.
    I use the control points to instert lateral planes to represent and export the frame shapes.
    Then I added additional stations in between, just to help visualise the hull shape better.
    I had meant to re-align them to even spacing as I should really only be concerned with the hull shape at this point.

    I was reading that if you export to DFX you can calculate the frames based on the stations, instead of creating plane off the control points.
    Using the control points for this distorts the hull shape as you have to crease the edges of the frame to get a straight edge to them.

    I wonder how other people extract frame shapes?

    Here are stations set at 1 foot intervals, much to the releif of anyone with OCD :)
    Scarab_4_aligned.jpg
     
  14. gonzo
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    gonzo Senior Member

    For a builder, the stations are preferred equally spaced on even numbers. For example, every 2 feet or every meter. Otherwise, is what we call ******* divisions. They are hard to work with and increase the time to set them up. Also, they increase the possibility of errors.
     

  15. PsiPhi
    Joined: May 2007
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    PsiPhi Newbie

    Got some material to make a model, might take a little while (busy at work) but I will be back, with photos this time.
     
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