​Those "equivalences" bear no relationship to what is actually used.​    

Base laminates. 

3mm ply = a small stitch and glue dinghy.       12mm foam with 12 oz glass each side = a 12m harryproa or lightweight cat hull.

4mm ply = a stitch and glue 28' racing cat with stringers and frames to support it or a sheet ply beach cat.       17 oz either side of 12mm core would be silly.  17 oz either side of 19mm core makes sense and would be used on a 12m cruising cat or a C50 cruising harry.

6mm ply = a lightweight 30' cat built with stringers and frames.       750 gsm either side of 12mm does not make sense, normal would be 25mm.  Steinars 20m harry is 800 either side of 30 (thicker for better insulation)

9mm ply = 30-40' lightweight cats. 1150 gsm/22 oz triax = 50' plus monos, with 25mm core.  

12mm ply = 40-50' cats.  I don't know any non condo multis using 1500 gsm triax, or why they would need to. 

Boat hulls are about stiffness, more than strength.    If a hull is stiff enough, it will be strong enough, generally speaking.  Which is why so few boats break from water loads, but all of them deflect to a greater or lesser amount..  

Stiffness is a linear function of material strength (double the material, double the stiffness), but the cube of thickness (double the thickness, 8 times the stiffness).  Hence, core thickness is critical.  As is matching the core properties to the skins.  Consequently, a 10mm thick ply sheet will be nearly the same stiffness as 10mm H80 foam with 12 oz glass each side.  

Strength requirements are usually local (around masts, beams, chainplates, rudders, etc) and are addressed with extra laminate (glass or tow) which is far lighter than ply doublers for equivalent strength and can be laid exactly in the location and alignment required.  

Ply vs foam/glass at the extremes is not about strength or stiffness.  3mm foam with 100 gsm glass each side would be about the same stiffness as 3mm ply, but would have no impact resistance.  25mm ply has similar stiffness to 25mm core with 1,5000 gsm each side, but is far heavier.  

Bjorn,

Thanks for the table.  

Maybe remove the "equivalent" and adjust the core thicknesses for a given weight of glass to those above.     

Foam core usually requires ~200 gsm per side to wet it out regardless of whether it is hand laid, infused or bagged.   

According to the Gougeons, unglassed ply should have 3 coats of epoxy each side at 8 sqm per litre.  ie 120 gsm x 3 x 2 = 760 gsm.  I am pretty sure no one with an unglassed ply boat that lives outside for more than a couple of years old has not wished it was glassed on the outside.    As plywood quality has slipped, this has become more important..  

Triax is not available in less than 800 gsm (400 lengthwise, 200 at + and - 45), so perhaps adjust the laminates accordingly.  I suspect triax was used in the comparison to lower the glass laminates strength in the off axis direction so that this would be the minimum strength to compare with ply.  

If you want to get complicated and accurate, add stringers and frames, coatings and glue for the sheet ply.