: Yes I suggested too that you need relatively more fiberglass. About the
: weight difference, that depends a lot on what plywood you choose. Hardwood
: is 700 kg/m3, softwood is 500 kg/m3. I built one of hardwood, it was a
: tank but too heavy. Now I use softwood. The test I referred to was
: actually a panel of hardwood, it may have been 5mm not 4. Not sure if a
: 4mm fiberglassed panel would hold my weight. I don't know how much cedar
: weighs.

: Henk

Hi Henk,
Cedar weighs in at about 390kg/m3, I've had some that weighed 350. Given the same dimensional thickness, cedar allows an extra full layer of glass. As the entire hull and deck wouldn't need the extra layer, it allows strategic use for best strength/resilience.

The shape of the hull gives a lot to the overall strength.
If you took a 3mm panel, 150mm wide and glassed it both sides, it wouldn't hold much weight.
If you cut two 75mm panels and glued them at right angles, then glassed them the same, it would carry much more weight.

Hard chine kayaks are polygonal shapes so the bending force applied to any panel would have to distort the adjacent panels in a direction they are stronger.

Water pressure isn't a concern, the overall force is only the kayak and paddler weight divided by the underwater surface area.
Impact tends to be localised compressive forces, the core density determines puncture resistance. Ply is better than cedar here. I try to stay away from rocks.

Ply or cedar core doesn't make any real difference, the vast majority of us never put the kayak or ourselves into conditions that reach the strength of the kayak.

Mike Savage
South West cork