If the boat is quite an ideal construction, with the same acute angle at stern and bow, and minimal displacement, the boat can be unstable. So it is practical to make e.g. a trimaran.
First guess is to calculate the immersed volume from the weight. For instance the motor weight is 120 g, burner 50 g.
The rated weight of the main hull (LWL=40 cm, B=4 cm laminated wood) is about 150 g, outrigger hulls (30x3 cm cardboard) 80 g.
The draft (T) can be about 2 cm.
So the displacement is 400 cm3.
Step 1: Main hull
On the picture it has a windshield too.