Ply thickness

[Trevlyns]

Just mildly curious…
In a deep V catamaran design (Wharram for example) what factors would determine the ply thickness for the hulls? I know for example that the Tiki 21 uses 6mm and larger designs up to 12mm. Any thoughts?

[Eric Sponberg]

The primary determinant is the hydrostatic and hydrodynamic pressure loads on the hull. But what is also important is the size and spacing of the framing on the inside. The farther apart the frames and longitudinals, the thicker the hull skin has to be. So two different hulls could be built for the same loading, one with 12mm ply and wide frame spacing, the other with 6mm ply with closer frame spacing. The thinner hull will be lighter because "plating" of the hull skin comprises about 2/3rds of structural weight. This follows that adage that if you want to make the structure lighter, add more frames because the plating thickness will go down and weight will reduce faster than it will increase due to the extra framing. This reduces the structural arrangement to an optimum somewhere between weight and cost--more frames cost more labor to build--so for every boat there is a kind of ideal solution.

Eric

[fhrussell]

Eric, That theory is what I always thought to be the reality, yet Kurt Hughes has been designing his wingmasts with thicker walls and less 'framing'. How does this play into the logic? Your answer is much appreciated!

[Trevlyns]

Thanks Eric for a detailed (and logical) reply - we can always trust these postings to deliver the goods. I just think it's so good to see proffessional people such as yourself maintaining an interest and sharing information and ideas with the "little guys" Well done!
And fhrussell, I somehow managed to delete all our correspondence on your CSK. I did receive the pictures you sent - thanks! The design I'm considering is a bit more simple construction-wise ie, stitch and glue - but thanks for your valued input as well.

[Eric Sponberg]

fhrussell,

I cannot vouch for what Kurt Hughes might be doing on his wingmast or how much engineering he has put into them to optimize weight. Wingmasts are a slightly different kettle of fish than boat hulls. In a stayed wingmast arrangement such as Kurt might design, the mast is loaded in compression and one has to be careful of local buckling, which is directly related to wall thickness. That is, the thicker the wall, the less likely the wall will buckle.

On a boat hull, to which my comments were pointed, the loading is perpendicular to the plane of the plating. The hull skin is loaded in panel bending, tension one side of the skin, compression the other, each panel being supported by the transverse and longitudinal structure. So to compare masts and boat hulls does not necessarily make sense since they are much different geometrically and carry loads in different directions.

On my free-standing wingmast designs, one side of the wing is in tension, the other in compression, and I also have to deal with buckling issues on the compression side. The mast wall can be too thin, so I design to any overall wingmast section shape and determine the optimum wall thickness, being sure I don't go below the minimum for that size. If I go larger in section size, the wall thickness goes down pretty fast, and interestingly, the larger sections are lighter than an equal strength smaller section. But that buckling problem comes into play pretty quickly, and so that is why the solution quickly finds itself to the right size section for the right amount of strength, stiffness and weight.

Eric