epoxy over PVC foam

[signum]

I wish to know if in case of using PVC core foam (as Airex let's say )for a boat hull, if I use epoxy resin for laminate layers it's necessary to alternate CSM Chopped strand mat of E-glass with woven roving E-glass, again CSM then woven roving depending of the calculated thickness, or it is enough to apply just woven roving fiber glass in a few layers laminated with epoxy because it's well known that epoxy has a greater strength and mechanical proporties than polyester resin ?
Can somebody help me?
I saw in the book "The Elements of Boat Strength" by Dave Gerr a statement concerning this matter, that is not necessary to use CSM when we use epoxy,just woven rovings uni or bi-directional.
Thank You.
Signum

[John ilett]

Layers of fglass cloth alone with epoxy on PVC is fine.

Generally chop strand mat (CSM) can not be used with epoxys. You would find that it does not wet through very well. There is a binder within the CSM holding all the fibres together. The binder is designed to disolve with the solvents in resins like polyester and viynelester. Then the CSM wets through well and becomes more flexible.

[signum]

Thank you for your advice .
In this case of using epoxy just with cloth fiberglass over PVC cores, the way of calculating thickness for inner fiberglass skin and the outer skin for hull in a sandwich composite with PVC, can I use the same scantling rules from Dave Gerr book "The Element of Boat Strength" ?
I mean a laminated fiberglass with cloth and epoxy should be as thick as one laminated by CSM alternated with cloth and polyester resin or I could reduce the thickness of laminated for epoxy being known that this resin is much more strength?
I have also the rules from ABS for building and classifing Plastic Reinforced Vessel. Can I guide myself by this rules ?
Any advice appreciated.

Signum

[Karsten]

Rules that only specify a laminate thickness are pretty much useless. They have to specify a fibre volume fraction or fibre weight fraction as well. Otherwise you can have 90% resin and 10% fibre and meet the thickness requirement with far too few layers.

If it's a sailing yacht I would use the ABS Offshore Racing Yacht Guide and if it's a motoryacht the DnV High Speed Light Craft rules. Both rules specify a minimum fibre weight. ABS only for the hull outer skin but DnV has basically all parts covered and if you meet the fibre weight requirement it is very likely that you will also meet the strength requirements if the panel size is reasonable. With DnV you only have to work out the required core thickness / core weight and the job is basically done.