How can this little single chine plywood row/sail skiff be improved?

Discussion in 'Boat Design' started by BayBoater, May 31, 2018.

  1. BayBoater
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    BayBoater Junior Member

    I uploaded a linesplan and plywood panel layout for a small origami skiff into the Boat Design Gallery.

    This boat is intended to be built from 4mm meranti plywood. It is just shy of 10' long and 4'5" wide. The waterline beam will be approximately 3'3" with one person and around 1.5" more with two. The two waterlines represent the displacement with one person and with two. I made a paper model of a slightly shorter version and the shape came together well, so I don't want to mess with the shape of the bow.

    It is meant to fit into the pan of a full size but short bed pickup without overhanging the back too much, but I want a bit more boat than those 7'10" tenders that can be built from a single length of plywood. The main part of each side will be from a single sheet, but with an extra bit joined onto the ends.

    I haven't sailed before, and was wondering how much sail area it can take.

    Are there any subtle tweaks that can be made to the shape that won't result in needing another plywood panel for the hull? Should the stern be wider or narrower for good sailing?

    I've drawn a slightly warped bottom with the v flattening out towards the stern because I think the slight twist would stiffen the panels but would a more constant deadrise be better?

    Should the rocker be changed, the flare, or the relative widths of the bottom and side panels?

    I don't intend to ever use an outboard on it, just rowing and sailing, would like it to plane under sail if possible.
     
  2. Richard Woods
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    Richard Woods Woods Designs

    Pictures don't tell the whole story. We would need to see your hydrostatics before making useful comments. And also the rig/deck layout/buoyancy placement etc

    It seems the transom shape doesn't match the hull? And you will find it hard to force plywood to double curve so be prepared for some effort when folding it

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. gonzo
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    gonzo Senior Member

    The transom has a pronounced curve, but the sections forward of it are almost flat. I think these designs are often an interesting exercise, but don't generate an optimal hull. If performance is important, then that should be driving the design, whether it can be made of two panels or not.
     
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  4. TANSL
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    TANSL Senior Member

    Totally agree. It is seen at first glance that these shapes, especially thinking about the construction with plywood, should be modified. As for the transom, it can be totally flat, there is nothing in the pictures that indicates otherwise.
     
  5. BayBoater
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    BayBoater Junior Member

    The transom is flat in the profile view, but curved on the edges, so that the hull panels would have to be bent to conform but if they won't do the bend or if it looks hideous I'll just keep the transom edges angular. I just thought the transom looks better curved and that rolling the panels at the stern where the hull panels are nearly straight it might be possible and will help with stiffening the hull.

    This boat is supposed to be first and foremost a quick and dirty job that works reasonably well, not a thing of ultimate beauty or performance. Something I can get built quickly and use this summer while I'm working on more complex boats. I'm pretty sure the 4mm plywood will to the slight compounding at the bow. I've done similar in 4 mm ply before, but each of those had two chines not just one, so for one chine the stresses at the end of the gap will be a bit higher.

    I mocked it up the boat with two people in Freeship and uploaded it into the "projects in progress" gallery but I cannot see or embed the picture yet, so I'm guessing it is not yet moderator approved. The seats go under the people's butts (this is for rowing) and the flotation (blocks laminated from rigid foam) will go mostly under the seats and from there into the ends. I'll definitely do a test capsize to make sure that the flotation is enough, balanced end to end, and that I have something I can attach to the gunwale to help me reboard. There will not be a deck because then the bow person would knock their back on it when they are rowing. There won't be a fixed rowing seat in the middle either, just a movable one or a box, so it won't get in the way for sailing.

    I was planning on using a balanced lugsail, starting small with a 30 square foot polytarp sail till I get the hang of it, but I'm thinking the boat will probably need more for decent sailing. Winds tend to be on the strong side around here and if they are light, I'd rather row than ghost along under sail. The mast will go into the seat that the bow person is sitting on, probably an 8' or 9' mast to start. At first I will try a clamp-on leeboard arrangement; because the top hull panel has so much flare it will need a wedge pad underneath to hold it at the right angle to the water . If that doesn't work I'll build a proper daggerboard and case over the winter.

    As for hydrostatics there is this (I run Freeship in metric but usually think of length and beam in feet. The stations in the linesplan are in feet (0.305m):

    Filename : single chine skiff.fbm

    Length over all : 2.983 m
    Beam over all : 1.363 m
    Design draft : 0.163 m
    Midship location : -1.600 m
    Water density : 1.025 t/m^3
    Appendage coefficient : 1.0000
    Volume properties:
    Displaced volume : 0.166 m^3
    Displacement : 0.171 tonnes
    Total length of submerged body : 2.634 m
    Total beam of submerged body : 1.048 m
    Block coefficient : 0.3700
    Prismatic coefficient : 0.5866
    Vert. prismatic coefficient : 0.4862
    Wetted surface area : 2.473 m^2
    Longitudinal center of buoyancy : -1.608 m
    Longitudinal center of buoyancy : -3.702 %
    Transverse center of buoyancy : 0.000 m
    Vertical center of buoyancy : 0.113 m
    Midship properties:
    Midship section area : 0.108 m^2
    Midship coefficient : 0.6308
    Waterplane properties:
    Length on waterline : 2.634 m
    Beam on waterline : 1.048 m
    Waterplane area : 2.100 m^2
    Waterplane coefficient : 0.7610
    Waterplane center of floatation : -1.700 m
    Y coordinate of DWL area CoG : 0.000 m
    Half entrance angle of DWL : 28.151 degr
    Transverse moment of inertia : 0.153 m^4
    Longitudinal moment of inertia : 0.815 m^4
    Initial stability:
    Vertical of transverse metacenter : 1.034 m
    Transverse metacentric radius : 0.921 m
    Longitudinal transverse metacenter : 5.013 m
    Longitudinal metacentric radius : 4.900 m
    Lateral plane:
    Lateral area : 0.323 m^2
    Longitudinal center of effort : -1.422 m
    Vertical center of effort : 0.095 m
    Hull characteristics above waterline:
    Lateral wind area : 1.511 m^2
    Z coordinate of wind area CoG : 0.450 m
    X coordinate of wind area CoG : -1.515 m
    Distance from wind area CoG to DWL : 0.287 m
    Distance from bow (FP) to wind area CoG : 4.128 m
    Minimal board height over DWL : 0.293 m
    Minimal board height over DWL : 9.815 % Lmax
    Stability characteristics:
    Test stability coefficient : 2.250 if >= 0,8 then OK




    The following layer properties are calculated for both sides of the ship:
    +-------------------------+---------+-----------+----------+---------+---------+---------+
    | Layer | Area | Thickness | Weight | COG X | COG Y | COG Z |
    | | m^2 | mm | tonnes | m | m | m |
    +-------------------------+---------+-----------+----------+---------+---------+---------+
    | bottom | 2.275 | 10.000 | 0.014 | -1.578 | 0.000 | 0.093 |
    | transom | 0.411 | 20.000 | 0.005 | -2.904 | 0.000 | 0.379 |
    | Sheerstrake | 2.300 | 10.000 | 0.014 | -1.449 | 0.000 | 0.339 |
    | stem_keel | 0.131 | 9.000 | 0.001 | -2.324 | 0.000 | 0.044 |
    | gunwale | 0.333 | 20.000 | 0.004 | -1.390 | 0.000 | 0.495 |
    | person head | 0.132 | 40.000 | 0.005 | -2.523 | 0.000 | 1.108 |
    | person torso | 0.886 | 38.000 | 0.034 | -2.525 | 0.000 | 0.623 |
    | person feet | 0.083 | 38.000 | 0.003 | -1.707 | 0.000 | 0.173 |
    | person upper legs | 0.364 | 36.000 | 0.013 | -2.270 | 0.000 | 0.350 |
    | person lower legs | 0.207 | 38.000 | 0.008 | -1.946 | 0.000 | 0.245 |
    | person lower legs | 0.207 | 38.000 | 0.009 | -1.299 | 0.000 | 0.250 |
    | person torso | 0.884 | 38.000 | 0.037 | -0.720 | 0.000 | 0.628 |
    | person feet | 0.083 | 38.000 | 0.003 | -1.538 | 0.000 | 0.178 |
    | person upper legs | 0.367 | 36.000 | 0.015 | -0.975 | 0.000 | 0.355 |
    | person head | 0.132 | 40.000 | 0.006 | -0.722 | 0.000 | 1.113 |
    +-------------------------+---------+-----------+----------+---------+---------+---------+
    Total 8.794 0.170 -1.612 0.000 0.480


    Parameters of ship sinkage:
    Difference of midship draft : 0.000 m
    Midship draft : 0.163 m
    Initial transverse metacentric height ho : 0.554 m
    Initial longitudinal metacentric height Ho : 4.533 m
    Angle of heel Psi : 0.000 degr
    Angle of trim : 0.048 degr


    Sectional areas:

    +-----------+----------+
    | Location | Area |
    | m | m^2 |
    +-----------+----------+
    | -2.828 | 0.014 |
    | -2.440 | 0.051 |
    | -2.135 | 0.084 |
    | -1.830 | 0.108 |
    | -1.525 | 0.109 |
    | -1.220 | 0.089 |
    | -0.915 | 0.061 |
    | -0.610 | 0.032 |
    | -0.305 | 0.004 |
    | -0.194 | 0.000 |
    +-----------+----------+

    NOTE 1: Draft (and all other vertical heights) is measured from point of the hull Z=0.
    NOTE 2: All calculated coefficients based on actual dimensions of submerged body.
    Note 3: The bulb characteristics was calculated right, if F.P. is through point of intersection
    forward line with DWL.
     
  6. gonzo
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    gonzo Senior Member

    I have played with developed shapes a lot, specially in the winter. I find that the easiest way to test the shape is with cardboard and tape. If it lays fairly well, you got a shape that will work. Also, it gives you a real view of what the boat will look like.
     
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  7. BayBoater
    Joined: May 2016
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    BayBoater Junior Member

    The picture with the two people still doesn't show up in the gallery so I attached it here.

    single chine skiff 2 people.jpg

    The cardboard model of a predecessor came out surprisingly well, and establishes that the general idea works. I wouldn't even post about something that doesn't pass that preliminary test. I'm now working on a model 1:8 scale out of 1.5 mm okoume plywood.

    underside smaller.jpg side view.jpg

    I've had some impressive fails with cardboard models, where the sheerpanel came out all wobbly, and am starting to get a sense of how the chine needs to run as to not distort things. Freeship doesn't consider any of it developable except for the transom, maybe steel wouldn't do it but 4mm plywood is pretty willing.
     
  8. TANSL
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    TANSL Senior Member

    If the curve, which appears in the picture with the two people, represents the areas of the cross sections and, if you are going to use it for something, be careful because it is not correct. If not, forget this comment.
     
  9. BayBoater
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    BayBoater Junior Member

    Yes, this is the curve of the cross sectional areas for the designated waterline, and it changes based on whether I check the box that includes the parts (the people) in the hydrostatics. I think here I had it unchecked, so it is for the boat only, and doesn't include the cross sectional area of the people's legs, torso, etc. However, the boat is set at the displacement waterline appropriate for the boat and the two people in it. The only thing I have used it for is to set the midpoint of the boat for doing the Kaper resistance calculations.

    Could you explain a bit what is wrong with it, why I shouldn't use it?

    I'm not sure what this curve should optimally look like for a small sailboat.
     
  10. TANSL
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    TANSL Senior Member

    The bow and stern ends of the curve have no choice but to coincide with the respective bow and stern ends of the waterplane. Something similar to the blue curve in the figure.
    The displacement and longitudinal position of the buoyancy center, which I imagine is what you want to determine, would not be correct with the red curve.
    I have not calculated the area of the frames so my curve can not be correct. I only show where its ends should be.
     

    Attached Files:

  11. BayBoater
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    BayBoater Junior Member

    I think I get it. The program uses some kind of mathematical smoothing between the stations rather than real data. If I change the station spacing to 5 cm I get something different and now where the sectional area shows the ends closer to where the end of the boat hits the water.

    single chine skiff 2 people more stations.jpg

    Also, I was incorrect in saying that I had unchecked the box for using the people's parts in the hydrostatics. I went in and unchecked and the curve looks much smoother, no longer distorted by the people parts.
     
  12. TANSL
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    TANSL Senior Member

    I do not know if I understand you well, probably not, because hydrostatics should not be affected by two people. Unless you mean that changing the position of people changes trim and, therefore, hydrostatics values.
    Another consideration: if the transom is submerged, the area of the curve at the aft end must be greater than zero.
     
  13. BayBoater
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    BayBoater Junior Member

    The bottom of the transom will be a tiny bit in the water, 2.5 cm. The end of the curve should in theory coincide exactly with the aft face of the transom where it becomes immersed. The areas curve doesn't interpolate well between the stations at the ends of the boat, so the end of the curve is still a little bit off from the end of the waterline, but is now much closer than it was with stations spaced 30 cm.

    I think you do understand me right, because it is a really strange feature of the software (Freeship Plus3.4), maybe you could call it a bug. When you check the "use for hydrostatics" box for the people it really adds the cross sectional area of the below-waterline people parts (the feet and bits of the lower legs) to that of the boat. It treats the parts of the people below the waterline as if they are outside and submersed, even though the people are entirely inside the boat. I had already accounted for their weight by adding draft.

    You can enable "automoving the model to baseline" but I usually don't, I adjust the amount of draft manually till the displacement in the "design hydrostatics" output (see earlier post above) matches what I estimate the total displacement of the boat (with occupants and gear) to be. Then in the layers dialog I give each layer (both the boat parts and people parts) a specific weight and thickness to come up with plausible figures (boat alone 38kg, stern person 61kg, bow person 65kg). I'm hoping to build the boat lighter than that in the rowing version (no spars, sails, foils).

    I don't think the software can automatically trim the boat; I manually moved the people around until the hydrostatics output told me that the boat should float nearly level (Angle of trim : 0.048 degr). I is weird, even if I uncheck the box for using the people for hydrostatics, it still uses their weights in the trim calculation. I can manually adjust the trim level by rotating the boat around the transverse axis.

    I don't know yet if it is possible to heel the boat as a unit in that software. If I try to rotate it around the longitudinal axis it either splits the boat or collapses it because it mirrors everything about the longitudinal plane. I haven't figured out many things about Freeship yet, and especially struggle with interpreting all the outputs. I'm a biologist by training, so I can deal with numbers and graphs to some extent, but I'm not a naval architect or engineer.
     
  14. TANSL
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    TANSL Senior Member

    That is not right. The volume that generates buoyancy is the inmersed volume of the hull. What is inside the hull, is not in contact with water and can not, therefore, generate buoyancy.
    I do not know Freeship but, from what you tell me, there are serious problems in the calculations that it carries out. If you need some calculations of naval architecture, I can do them with pleasure, so that you have some reliable reference data.
     
    Last edited: Jun 5, 2018

  15. BayBoater
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    BayBoater Junior Member

    Thanks so much for that offer, probably not needed for such a small boat, but maybe a bigger one. Because of this problem I usually look at the hydrostatics numbers and sectional areas before I put the layers with people in. I just add the people at the end to figure out where to put the seats so that the boat is in relative trim. I also do it with a single person for the middle thwart. It will all be approximate anyway, just a person being heavier than predicted or shifting back and forth on their seat a few cm can change trim in a small 3m boat.

    Once I tried to add in a surface to approximate the weight of a filleted and taped seam. It was also wholly contained within the boat and it totally screwed up the calculations. I guess the design programs you have to pay for will probably do a better job.
     
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