Plywood/foam equivalency?

Discussion in 'Materials' started by SpiritWolf15x, Jan 11, 2018.

  1. TANSL
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    TANSL Senior Member

    In the early stages of design, when you have not yet calculated the thickness of the shell, you need to do hydrostatic calculations, power forecasts, stability, ... and, in this case, they are made inside the shell. Neither are the appendices taken into account. In the last calculations of the project yes that all this can be taken into account.
     
  2. Steve W
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    Steve W Senior Member

    I guess I do things differently since, as a boat builder i pretty much know how I'm going to build before i put pencil to vellum so i have never done hydrostatics to the inside. However since the boat in question is an existing design with established hydrostatics this is what matters. Assuming there is a designed displacement figure in the published specs it will be calculated from the outside of the skin, yes? So, if the design calls for a 1/4" skin thickness in plywood and in converting to foam you end up with a 12mm skin, you should reduce the station frames by the amount needed to end up at the same outside dimension as with the plywood skin. You don't have to of course but you should. If for example the foam build looks like it will end up heavier the extra displacement could be helpful.
     
  3. TANSL
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    TANSL Senior Member

    We agree, if you have a hull already known or work with data extracted from a female mold, you should calculate with the outside of the skin. When you are creating the body lines plan of a ship that is not yet defined, the most practical and correct is to calculate with the inside of the skin. But each one is very free to do what he wants, if he does it correctly and, above all, in the way that is most comfortable for him.
     
  4. PAR
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    PAR Yacht Designer/Builder

    It wasn't very long ago where all hydrostatic work was performed on the hull exterior. This was a simple practical consideration, rather than having two sets of lines or more importantly station mold dimensions. All my early work is this way, though in recent years as I continue to upgrade these plans, I'll draw up station molds to the inside of the planking, making molds or pulling templates easier. This is really just a convenience issue for the builder, so they don't have to subtract the planking thickness. Of course if you're making changes, you'll have to accommodate yourself appropriately.
     
  5. TANSL
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    TANSL Senior Member

    Basically I agree with you but I, for example, in more than 45 years, I have always calculated the hydrostatics with the values on the inside. I suppose, therefore, that it is wrong to generalize and that everything depends on the type of ship, material and method of construction, or custom of the designer.
     
  6. PAR
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    PAR Yacht Designer/Builder

    So you drew up your lines to the inside of the planking?
     
  7. DCockey
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    DCockey Senior Member

    Designing the hull to the inside of the planking/plating, and compensating for the hull thickness in the hydrostatic calculations was a common practice for vessels of metal plate on frame construction and larger vessels of wood plank on frame construction. The frames and molds could be built directly to the design shape without needing to compensate for the planking/plating thickness. The initial design of the hull shape for small boats was frequently done to outside of the hull surface, and then the inside surface / frame and mold shapes obtained from the outside surface. Going from one surface to the other can be a slow and laborious process if done "exactly".

    The advent of 3D modeling in computer software has radically changed the time and effort to generate the outside surface from the inside surface, or the inside surface from the outside surface, is a simple and quick operation. If either the inside or outside surface exists as 3D math date then the corresponding surface can be obtained quickly using suitable software, usually be executing a few commands.

    Whether the inside or outside surface is considered the "proper" one for design purposes depends on the background and experience of the individual.
     
  8. Steve W
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    Steve W Senior Member

    I can honestly say I have never heard of designing to the inside until today. I have always designed to the outside and deducted for skin thickness and any other construction related deductions on the loft floor.
     
  9. TANSL
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    TANSL Senior Member

    I totally agree with the last DCockey post. I just want to clarify that, when the plate thicknesses are very different in the different areas of the hull, making the outer surface of the hull needs more than a few software commands.
    Yes of course. In all metal boats, for example, you have to do it that way. When you have to calibrate structural tanks, empty spaces, cargo holds, ... it is very convenient to have the lines defined inside the hull. Only in the case of a ship that is going to be built by an existing female mold, I draw the body lines on the outside of the shell.
     
  10. Steve W
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    Steve W Senior Member

    I can totally see why you would need to calculate the volumes of the inside of a cargo vessel, i have just personally never given it a thought because like most people who inhabit this site, my interests lie primarily with recreational craft. I'm curious as to why you believe that you would need to calculate to the inside of the skin of "all metal boats". Iv'e never designed a metal boat so please understand I'm not disputing this but trying to understand the reasoning behind this. As an example if I were to design a hypothetical 35ft aluminum sailing catamaran what would be the reason for calculating to the inside of the skin? I would still have to do my hydrostatics to the outside but i just can't see a reason to also do the inside. Maybe its because I'm a guy who still designs the old way and it seems like a lot of extra unnecessary work whereas if I were designing with a graphics program on a computer it would be no big deal.
     
  11. TANSL
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    TANSL Senior Member

    Each one can do the calculations as he sees fit as long as he takes into account all the deductions or additions that should be taken into account. For metal boats, since I have use of reason, the calculations are made inside the shell. It is not about old fashiong or modern computer method, this is how it has always been done.
    In addition to hydrostatics, in a large ship you have to do many other calculations and, for most of them, it is more comfortable to work as I indicate. It is very important to know the gross and net volume of each cargo hold or each tank. And a large ship can carry many independent spaces whose volume needs to be known. When the development of the shell plates is done, it is always done working on the inner lines. Finally, I can not think of more reasons for the moment but the reality is that metal boats work as I say.
     
  12. Steve W
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    Steve W Senior Member

    Thanks Tansl, I do understand that it is necessary to know volumes in holds etc on cargo carrying vessels,hence the need to calculate from the inside but I am still having a hard time seeing any reason why anyone designing a smaller recreational craft, even in metal would work from the inside, do designers of recreational metal boats really do this? Certainly I can see deducting shell thickness from the body plan for the sake of making it easier for the builder to build his temporary and permanent frames but that's about it. The only reason I mention the old way of designing vs computer aided is that even in the old days you really needed to fully loft a boat to fair the lines at full scale before making deductions and making your mold frames wheras with computer aided design as Dcockey mentioned you can make the deductions at the design stage very easily so skip the full size lofting.
     
  13. TANSL
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    TANSL Senior Member

    Think that, in addition to the calculations, you must do many other things in a metallic boat, even if it is small. Try to define the parts, floors, bottom girders, etc. that are going to be cut by numerical control, when you have defined the lines by the outer face of the shell.
    But, as previously mentioned, everyone must work in the way that is most comfortable for him, among other things, to achieve the highest possible accuracy.
     
  14. DCockey
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    DCockey Senior Member

    Once the inner surface was divided into parts corresponding to the different plate thicknesses, only a few commands would be needed to obtain the outside surface for each plate thickness.
     

  15. TANSL
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    TANSL Senior Member

    Great, but I do not know what it's for. I'm not saying it's useless, simply that I can not think of what that might be useful for.
     
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