The cost of rotomoulding is a tricky one, most rotomoulded tooling is cast aluminium, traditionally the castings are quite complex and therefore expensive. However rotomoulded tooling can also be fabricated, that means you can make it by welding together sheets of aluminium. This in turn means that if you know the right people you could probably fabricate tooling for any boat that could traditionally be made in marine ply relatively cheaply. Although tortured ply designs would require more skill to fabricate than developable surfaces. I guess this is your field of expertise. Sorry I haven’t mentioned any real numbers. I have a friend who is a rotomoulder so I can get some real numbers for you next week.
The next thing to do is find a rotomoulder as you will then need to add the jig fixtures to your tooling and they may be specific to the rotomoulder.
The next issue is weight, I guess the average hull would be 5mm thick polyethylene which has a density of 1g/cc so an F12 with a surface area of 4m2 would weigh 5x1x4=20kg, When you get into more complex rotomoulding that uses foamed polyethylene cores you may get down to 14kg but this gets more expensive to produce. Because polyethylene is no where near as stiff as GRP foam sandwich you may need to tweak the design a little to add stiffening ribs like the Dart 16 has.
