: What are the comparative strengths of 3mm vs 4mm for these sized boats? I
: have/use both 4 and 6 ounce cloth. Will I have trouble/weakness using the
: 3mm plywood?

My S&G has a 4 mm hull and 3mm deck, with the hull glassed inside and out with 4 oz glass and deck glassed outside only with 4 oz glass. During the "final" sanding of the deck, the kayak took a number of journeys off the sawhorses to the floor - a 30" drop to a concrete floor. The boat was uninjured, but it left a scuff mark on the floor. When sanding the hull, the deck was resting on top of foam saddles on the sawhorses. The boat took another trip - from the saddles to the sawhorses - a 3" drop, and it cracked the deck! Fortunately, the boat wasn't finished yet, and the crack was near the rear hatch, so it was an easy repair.

3mm will work, but you will have to compensate with glass. I would recommend 6 oz, or a double layer of 4 oz, both inside and outside the hull, and 4 oz inside and out for the deck.

Good luck.

Ken