0) Go into Creative Mode
1) Create a core, jump in - observe mass = 0.1
2) Select a Black Standard Hull (mass = 0.15)
3) In Advanced Build Mode, set X=11, Y=10, Z=10 (note, this reproduces doing a single block at a time - this is just faster)
4) Place block of Black Standard Hull (0.15 mass * 11*10*10 = 165 mass)
5) Observer mass = 165, not 165.1 (hull + core)