Designing a 9 meter sailing catamaran

For this project I started other threads, but it seems better to concentrate the discussion in one thread.
The boat: 9 x 5,8 m, catamaran with spine (middle hull not touching the water), cat A., 1340 kg mldc, SA 50 sqm. One of my objectives was to design it myself to learn about structural boat design. Maybe that can ba off help for other non-pro builders.
Buildblog on facebook: Catamaran Tink https://www.facebook.com/Catamaran-Tink-328374460702512/

 

The first subject is about the bottom core. My supplier wasn't able to get higher density PVC (flexyfoam). So I glued 10 mm 80 kgm2 PVC core to 6 mm marine ply. To check shear strength and modulus of this combination exceeds a 100 kgm2 PVC core I did short beam shear tests. The flexyfoam spec specifies 1,2 MPa (Iso1922) shear strength and 30 MPa shear modulus (ASTM C393) for 80 kgm2 core and 1,6 and 38 MPa resp for 100 kgm2 core.
I compared 2 samples 15 mm core with the combined PVC/wood core: test 3 with the ply at the inside, test 4 with the ply outside. The information I have about the test methods is from internet pictures and articles.
The measured shear strength of the single PVC core is much less than specified: 0,3 in stead of 1,2. Is this because of the test method? I suppose I have to divide the measured shear stress by 2. I've read there is a lot discussion about short beam shear test.
How to calculate shear modulus? G=Fl/Adx?

An easier way would be to say that combined wood/foam is 6 times stronger as the single foam? As the tangent is more than 6 times steeper so will be the shear modulus. Reasonable?

 

Starting from
tau = G * tan(theta)
your formula
G = F * l / (A * delta x)
looks right to me.

But as you will know, F is the force with the line of action shown in the sketch. And the force applied in a short beam shear test acts perpendicular to it.

The site
Short Beam Shear ASTM D 2344 http://www.ptli.com/testlopedia/tests/Short_Beam_shear-D2344.asp
gives this formula:

Shear strength =
0.75 x breaking load
width x thickness

 

Thanks Gunter!

"As you will know" Well... I did not, but understand now.

Have to think of something different.

 

When is your completion date?

 

I'm not a fast fast builder. Spring 2020 would be great.

 

Actually, if this is PVC core, You can get shear strength from:
a) certificate, say GL certificates always show shear strength; this would be my preferred source of information and they use minimum values, not average values.
b) ISO12215 standard, they have typical properties depending on density and type;
c) manufacturer's specification; however I would be conservative about those numbers as they can be too optimistic.
There are many non-branded or new cores on the market, I would not take a risk and use only proven one, with service history.

 

Thanks Alik, I know. I have the manufacturer's specs and ISO 12215. The question is what the specs of a combined foam/plywood core are. But thinking about it I suppose it can be calcuted knowing the specs of each.

 

I am curious why you did not use more glass vs the ply.

The Nature of plywood and water seems an unusual combination with foam; given foams ability to absorb little water vs ply.

What glass is over the ply on the bottom if I missed it?

 

Plywood is thicker as glass and has a shear modulus of average 170 MPa. Shear modulus of foam is 30 MPa. I could do full ply; that is very heavy. But motivation was I couldn't get 100 kg/m2 foam. The ply is on the inside so not very vulnerable for water. Inside is stronger as flexural strength of plywood is also bigger than that of foam. Besides that most of the bottom (in high loaded place) is two 120 mm curve so also a lot form stability. Ofcourse it is marine ply (okoume garantiemultiplex). A lot of boats are built with it.
Glass: that is a little bit open yet.

 

So how do you interface hull bottom and sides? Are the sides down to the bottom and thus enclosing the ply edges?

You must have some glass plan. Does this mean the exterior a/o interior of the hulls are not glassed? I thought you used a waterbag to glass an interior.

Sorry to be a pain. I am still having some discomfort over my glass schedules remaining. He is asking for a lot of 17oz biax and it peels a little easier than I like, but I am not smart enough to argue better scantlings!

 

All connection are convex/concave grooves 7 mm depth. So ply dimishes gradually. I indeed worked with a glass plan. But by changing method (Gerr to ISO12215) I first have to check or it is OK what I have done so far. Therefore I stopped building for the moment and concentrate on material tests and ISO.

I will PM you over your glass schedules

 

Gerr's method is not suitable for such a light sailing catamaran, it will be too heavy
As I understand You are using inside plywood as structural plug? In this case, plywood will work as inside skin of laminate, and shear strength of the panel will be defined by the weakest element - foam or bonding glue.
To calculate such compound panel, You might look into stack laminate analysis in ISO12215-5, it is somewhere in annexes.

 

What are you looking for Pammie? Shear modulus is a slope. The shear modulus is also known as the rigidity. shear modulus = (shearstress)/(shear strain) = (F/A)/(x/y) . This equation is a specific form of Hooke's law of elasticity. This has little to do with your design.

In designing around a sandwich or cored composite (ends fixed, uniformly loaded), you need to find the ultimate shear strength. Max Shear occurs at the middle of the core where it tries to go against each other. If you are using ISO, there is a table there that shows the expected shear strength of the core of your choice. Of course you have to make allowances for the FoS.

The core is sandwiched by the skins. In the outermost skins (top and bottom) is where the greatest stress is and you need to know the compressive/tensile strength of the facings. There are basically two computations, the shear strength of the core and the tensile/compressive strength of the facings when loaded. The third computation is the shear flow. The bond between the core and the bottom of skin.

The shear strength of the core is dependent upon thickness. Making the core thicker will reduce the shear strength required (including the stress on the skins). There is a calculation to find the optimum core thickness. I will check with ISO if it is in the short form or table computations.

 

@Alik: yes I understood Gerr's method would be to heavy so I reduced the laminate. This in proportion to laminate thickness from a study plan for a slightly bigger light boat. Have one hull partly build so want to find out how it comes out of ISO. The ply is not a plug. I glued it together with the foam. See pictures. But what makes a material skin and what makes it core?
I have seen this stack analysis (I believe it's in annex H). But have to read carefully to see the right workflow.

@RX: I've read about shear curves and flow but cannot yet quite follow it. I have all specs of individual materials. But not from the foam/ply combination. Maybe I have to see the ply as skin as Alik says. So for now I will follow ISO and see what comes out.

Also an update from my earlier testoverview. Was in OpenOffice spreadsheet which not everyone can load properly. Not relevant for absolute values but shows difference between foam and floam/ply.