Building a Full-Foiling Fiberglass Beach Catamaran

To scale up or down you have to know that that weight/displ varies as the cube of length, SA as the square of length , RM as the 4th power of length.
So for weight: 50 cubed is 125000, 18 cubed is 5832 and 5832/125000 is .0467.
Since an AC 50 weighs about 6380 lb with six crew, 6380 X .o467=297.9lb for a scaled down 18 footer including crew. Subtracting 16olb crew(from first post) leaves 137.9lb.

 

Are there any build threads or blogs on building fiberglass sailing dinghies?

 

That leaves a boat of 62kg, which is unrealistic for a 5.5m foiling catamaran. I don't think you should include the crew in the scaled weight like that, removing them from the weight first (say 500kg) gives:

2,400kg * 5.5m^3 / 15m^3 = 118kg​

which is a much more achievable weight considering the A Class are 75 to 80kg and Nacra 17 is 141kg. I'm also not sure about that weight for an AC50, I've only seen a number for the hull from Wikipedia. Whether that includes the rig and all the hydraulic stuff is unknown.

 

You have to use the design sailing weight ,which includes the crew.