Molding a stitch and glue built boat

Ok, thank you Wet Feet,
I didn't contact anyone except you for the moment. I'm doing this tomorrow morning. I let you in touch if I find someone that I can afford to do this job.
Thanks again

 

Rumars , 6mm is suitable for a row boat. A smallish row boat. Like a 10' rowboat. I used 7mm okume, with 10 oz glass on the inside and 1708 biax on the outside for a light-weight 16' sailboat for crying out loud. Plus the chines and keel had 1 foot of overlap on the outside and three more layers of 9 ounce tabbing on the inside. It held up pretty good. It could cartwheel up to a beach in the surf without any issues. It took a rear end collision on the intestate that totaled the car and destroyed the boat trailer, but the boat was okay with the transom ending up on the car's steering wheel and the car's engine under my truck bumper. Broke a pair of 3000# tiedown straps by straitening the hooks out and launching them into space. I used 20,000# pad eyes on the bow and stern for dock lines. Girlfriend knocked a phone pole sideways in a parking lot backing up. I dropped it out of a tree onto solid limestone. It survived 20 hurricanes in the Florida Keys. Toughness is not overrated if its going to spend time around me. See any scantling rules for plywood power boats. Gerr's method yields 16mm as a decent bottom thickness for starters on a 24'er with 8.5' beam. It can be modified if it is mostly a core material. Speeds over 25 knots require additional thickness, but probably not enough to bump it to 19 mm.

 

Well, the OP said his boat is 5mm okoume (probably 3-ply, most manufacturers start 5-ply at 6mm), with 12oz biax each side and additional chine reinforcement. Apparently it hasn't disintegrated in sea trials, so he must have done something right.

Sheated ply isn't "tougher" then foam core. On both the exposed surface is fiberglass and resin, so it has the same abrasion resistance. Once that is breached you are down to the woods properties, and okoume isn't particularly hard wearing. Sure it's harder then foam, but foam is usually going to have more glass over it, especially when done in mold with polyester resin.
It's up to the designer how he manages local and global loads, and what safety factors he applies. In a boat that prioritizes weight abrasion and puncture are always going to be a compromise

 

Plywood as a core is tougher for two main resons. First, it has a much higher tensile,compression and shear strenght than foam. Secondly, it has much better puncture resistance than any foam.

 

A core, in a well-designed sandwich panel, does not need to have high tensile or compressive strength values. The core's mission is simply to add a material that is as light as possible, which moves the resistant layers away from the neutral axis of the composite, which provides thickness without increasing weight. The only thing the core needs is to have good resistance to interlaminar shear stress (shear stress in the plane is not so important). If you use plywood as a core, you are making very poor use of the sandwich material. Also, as has already been mentioned before, the resin impregnation of the plywood is much less. With a plywood core, you are placing very resistant material where it is not needed, that is, you add strength, and a lot of weight, where it is not needed.
I think someone is not clear about the technical reasons and advantages of using sandwich laminate.