Very small hulls

Discussion in 'Boat Design' started by Saqa, Jul 8, 2021.

  1. Saqa
    Joined: Oct 2013
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    Saqa Senior Member

    Regarding FRP composite sandwich

    When designing very small hulls to use as models or proof of concepts, how is the laminate schedule addressed? In other words, how do you scale things like glass cloth, or is that not a factor?

    Can tissue paper be used with epoxy for the skins? Is there another factor that is not immediately obvious to me?

    Thanks for any advice
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    doesn't matter what your scale model is made from, just so long as it is within the weight range required
     
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  3. kapnD
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    kapnD Senior Member

    Foam is a widely used material for plugs, it’s light, easy to shape, and can be readily weighted to simulate.
    A coat of epoxy will probably do for outer skin, I wouldn’t even try to duplicate a scale laminate.
     
  4. Will Gilmore
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    Will Gilmore Senior Member

    Keep in mind that the medium (water/air) doesn't scale.
    A scaled model's performance is therefore always an approximation.

    Understanding that fluids seem more viscous to smaller objects can improve how you expect a model to react to things like: planing speed, ballast vs heeling force, the scale and reaction to waves as well the effects of skin fiction on force.

    Getting so precise as to scale the hull material may actual misinform the concept without keeping these things in mind.
     
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  5. Kayakmarathon
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    Kayakmarathon Junior Member

    If you are building this model for tank testing; then you could have it 3D printed, take a mold off the plastic, and then make a fiberglass shell from the mold.

    If you are building this model for proof of concept to look at; then 2 oz fiberglass over balsa strips will work very well.
     
  6. mitchgrunes
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    mitchgrunes Junior Member

    Sorry to hijack the thread a bit.

    I'm way out of my areas of training and experience, but, as a curiosity, what does a scale model help you test?

    Fluid dynamics isn't the only thing that won't scale.

    E.g., it doesn't seem like material strength would scale right either. E.g., if I just figured things out right, a 1/10 scale laminate would have about 1/100 the tensile strength (1/10 the thickness of material, 1/10 the line length across which tension is pulled), but only 1/1000 the weight - and some types of stresses will scale with weight.

    Again, a lot of laminate structures create stiffness by balancing tension in one material (e.g., fiberglass) against compression in another (e.g., resin, foam, etc.), that is more or less surrounded by the first material. But I guess the amount of compression would scale with the applied tension divided by the size (in a 1st order linear approximation) - so stiffness wouldn't scale right. For that matter, it would be hard to get the adhesive resin to have properties that scale right - e.g., the depth of the surface layer that adheres to the composite fabric.

    Again, in terms of vibrations and sound, the speed of sound would not scale.

    I vaguely recall a prehistoric era physics book mentioned trying to compensate for scaling factors in wind tunnels (a very different case than water) by using a fluid with different viscosity, but I guess you can only go so far that way. For that matter, while to some extent water and many other fluids can be approximated as incompressible fluids, they really aren't, and that deviation would be hard to compensate for. Also, water has a lot of surface tension (self-adhesion), and a fair amount of adhesion to many other surfaces. Trying to find a fluid which would imitate that at scale would be pretty hard.

    Surface water wave shape definitely doesn't scale, so I'm not sure how well a scale model tells you how the boat will interact with weather conditions. For a sail boat, I'm not sure you can scale the way wind interacts with the surface either.

    Anyway, given all the things that don't scale, what do you, and others, usefully get out of scale models?

    P.S. Let me guess:

    A scale model is a way to test static balance and degree of immersion - though mathematical models can do the same thing, and there are even a few computer models available for free. Of course, if you scale things much too small, then as with the meniscus on a test tube, you need to consider how water adhesion causes the water to climb a little higher than the main surface next to the object (if it is hydrophyllic), or to go a little lower (if it is hydrophobic), and surface tension effects would change other things too.

    Of course, if you are marketing the final project to a customer who cares about appearance (including, perhaps, yourself), a scale model may be a useful selling point. You don't always need to get the physics right to sell an idea.

    I know of people who build scale models. E.g., I knew of someone who built scale submarine models for the U.S. navy for many years. So there must be something useful to be learned from scale models. I just don't know what.
     
  7. fallguy
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    fallguy Senior Member

    Fiberglass scales poorly as do the mediums.

    If you want to build a model, anything from 1.5-6 oz wovens are nice, but overkill. You can also just make an epoxy shell..unless the model is to get wet inside or out, then it needs full encapsulation.

    Lotsa good replies from other members. My scale model got burned after hours of work.
     
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    Just about everything...when done corrcetly.

    It is not the fluid you are scaling, it is the "model" and the parameters at that model scale.

    For example, take speed.
    Lm = length of model
    Ls = length of ship
    V = velocity (s or m)

    Velocity of ship = Vm x sqrt (Ls/Lm)
    Same with Reynolds number and just about anything else.

    So, whether it be kinematic, dynamic, viscosity or pressure etc..it is the model, it is that that is an exact scale of the full size, and all parameters used in the model are a function of that.
    And so long as the scaling is correct, by means of its length, then everything else is scaled correctly.
     
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  9. fallguy
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    fallguy Senior Member

    The OP has made some other posts about xps. If you use xps for a model, it does not scale well to a larger craft because, for example, you would not walk on the model. So modeling has limits. But it is also really fun to do.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    You could make it out of chocolate, so long as it the right dimensions and weight, and it won't melt or otherwise disintegrate, that way you can eat if after the tests are completed.
     
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  11. mitchgrunes
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    mitchgrunes Junior Member

    Thanks!

    Do I correctly understand your assumptions?:
    (1) The relationship of the boat shape and orientation to the water surface is static. E.g., no bouncing, pitching, or rolling.
    (2) If you get the weight, weight distribution, and shape of the boat right, the corresponding parts of the model that are in the water will match those in the full scale boat. E.g., no planing. The boat is assumed to be a pure displacement hull.
    (3) By velocity, I presume you mean "hull speed", and are modelling it as proportional to the square root of the waterline length, based on the assumption that the primary resistance to motion comes from the waves generated by the hull itself. E.g., you ignore skin drag, and externally generated waves and winds.

    In any event, I'm guessing these approximations break down under highly turbulent conditions, or when external waves are higher than the waves generated by the boat, because you can't model that without a detailed dynamic model of the fluid - yes?

    But are these fairly good approximations for most reasonably large boats and ships, most of the time?

    How well do they work for speed boats, which plane and bounce?

    How well do they work for sailboats? E.g., do sails and the wind usually interact in sufficiently simple ways for scale models to be helpful, or do sails move and bounce around too much?
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    1. The model shall flat exactly the same as the ship.
    2. Yes... the weight of the model, at that scale, = the weight of the ship.
    3. No such thing as 'hull speed'. The scale is the scale.. that's it...not sure what else you're referring to in the rest of that?

    As for the rest.. if everything is exactly to scale and modelled correctly, the model shall reflect the ship's behaviour, whether seakeeping, resistance etc..
    That's what models are for and why we use scaling laws from model to ship.
     
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  13. Saqa
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    Saqa Senior Member

    Hi all
    Thanks for taking the time for all the input. Being totally new to RC and model testing, I still am not sure how to proceed

    By Friday, I would like to make a start shopping and build it in the weekend. I have already located local access to motors and props as well as hull material. I think I am onto a feasible craft for my requirements, and I now intend to build the full scale craft in the near future. The craft is a two-seater WIG

    There are 3 static components in the model. The main hull and the two floats. I intend to use paper pre-skinned PVC foam craft boards. to make the mostly flat surface hull threesome. I have seen a lot of vids where the models crash and get damaged. That is why I am enquiring about building it properly in the first place so that I don't have to keep making new components. I.e. good taped joints. I intend to seal the paper skins with epoxy



    Can tissue paper be used with epoxy instead of glass tape for a model?

    Glass will likely be too heavy, as I can't really see ultra-thin double bias options out there

    To answer some questions:
    Model testing is to check the longitudinal and transverse stability, which then will be scaled up to full size. As well as weight placements and my engine mount idea. I feel confident about it performing fairly well as a 'tri' but to be honest really don't know what I am doing at model level

    The components to be tested will be airfoil span
    Engine mount concept


    I do feel that I should get each of those three hulls to be as accurate weight model as possible. By that, I mean the 3 hulls should be a mini copy of the full sized ones. Am I on the right track here?
     
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  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Do you have a drawing of what you are planning to model ?
     

  15. Saqa
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    Saqa Senior Member

    If you want to know the style then think a longer then wide wetta with beams shaped as foils and no tramp
     
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