Catamaran scantlings

There are a lot of places where they build and repair boats in your neck of the woods. I worked for quite some time in Totnes and know the area well. Most shipwrights are friendly people and will answer questions; over a beer or two.

 

That's a good Idea Bideford Quay is the nearest place to me thanks mik.

 

mikthe stik - the panel thickness depends on the panel size. If you use small panels then a thinner skin is OK. Large panels require thicker skins. So you need to look at how you subdivide your hull vs a formula just based on displacement. Lloyds have sections for timber so have a look at those. Peter S

 

I tried to check youngs modulus for plywood and found 1.08. Applying earlier formula to Richard Woods Romany it could be made with 7.57mm solid GRP. With a modulus of 1.08 8.05mm ply would be equivalent. Romany is built with 9mm ply covered with 300gr of GRP. There is therefore no doubt as to the strength of this boat. Using further formula from John Teals book, the moment of inertia for 7.57mm of solid GRP =.0047. To make an equivalent GRP foam sandwich with a 7.57mm core requires 14oz GRP inside and out. I skipped the calculations which some may think boring. Plywood is far stiffer stronger SO 8mm Ply covered on both sides with 14oz GRP would be incredibly strong if a bit heavier than a foam sandwich. Gerr recommends a 43/57 split or inside 12 outside 16ozs Would that be as strong as a 50/50 split. This mod would turn an already sturdy boat into something which would better stand the frozen north.

 

What are the units of your modulus figures? plus can yopu also spec the glass in g/m2 for us metric people. Peter

 

I had difficulty in getting metric figures I could make sence of, my figures are in 10^6lbf/in^2. Normally I can handle metric ok. 7.57 * (1.3/1.08)^0.333 = 8.05mm. 14 oz/yd^2 = 482gr/M^2. inside and out. A 14oz skin is 0.57mm thick. mik

 

there are several plywood structural codes that specify its stiffness. AS1720 for instance specifies the short duration modulus for F17 plywood as 14000MPa, and for F34 ply as 21500Mpa. So your 1.08e6psi is about 7000MPa which is a bit low for most plywoods. Plus you need to take account of the ply direction and its construction if you are doing a detailed analysis. What is your cloth spec of 482gsm? weave , CSM or other? multaxial? cheers Peter S

 

Cloth would be 10oz plain weave plus 4oz plain weave in E glass. mik

 

John Teal says 12mm ply in his example was 1.8 or 11667Mpa. I guess you could specify good quallity Marine ply and it would be good enough.mik.

 

Plain weave glass is not stiff or strong vs a multiaxial cloth as ther PW fibres are crimped (bent). So if you used a 600gsm quadaxial (where the fibres are straight) it would have less resin in it, be twice as stiff and be twice as strong as the PW. If you used a 600gsm triaxial in the correct direction it would even be stiffer/stronger again. Peter

 

I don't know much about Glass cloth I thought plain weave E glass was bi-axial and the stuff Gerr intended for use in " the nature of boats" mik

 

There's a lot to learn about composites. Most fabric suppliers have lots of info about this area. http://www.gurit.com/product-brochures.aspx cheers peter