We have succeeded in synthesizing vertically aligned doubled-walled carbon nanotube (DWNT) forests with heights of up to 2.2 mm by water-assisted chemical vapour deposition (CVD). We achieved 85% selectivity of DWNTs through a semi-empirical analysis of the relationships between the tube type and mean diameter and between the mean diameter and the film thickness of sputtered Fe, which was used here as a catalyst. Accordingly, catalysts were engineered for optimum DWNT selectivity by precisely controlling the Fe film thickness. The high efficiency of water-assisted CVD enabled the synthesis of nearly catalyst-free DWNT forests with a carbon purity of 99.95%, which could be templated into organized structures from lithographically patterned catalyst islands.