A subscale boundary-layer boiling (SBLB) test facility was developed with the aid of a scaling analysis to simulate the phenomena of pool boiling and critical heat flux (CHF) on the external bottom surface of a heated hemispheric vessel. Saturated and subcooled boiling experiments were performed in the SBLB facility to measure the spatial variation of the CHF and observe the underlying mechanisms, including the vapor dynamics and the resulting buoyancy-driven two-phase boundary-layer flow along the downward-facing hemispheric heating surface. Based on the experimental evidence and an advanced hydrodynamic CHF model, a scaling law was established for estimating the local CHF on the vessel outer surface. The scaling law, which compared favorably with the available CHF data obtained for various vessel sizes, was shown to be useful in predicting the local CHF limits on large commercial-size vessels. Additional work, however, is needed to determine the effect of thermal insulation.