This work presents an experimental investigation of heat transfer and pressure loss in high–aspect–ratio nominally rectangular channels with a single ribbed wall, representative of wall-integrated turbine blade cooling. In such designs, separate cooling passages lie directly beneath the blade’s pressure and suction surfaces, so only the wall facing the hot gas path requires enhancement. Despite their relevance, systematic data for high–aspect–ratio, single-sided ribbed multi-pass channels at engine-relevant conditions remain limited. A three-pass test section with two 180° turns was tested with air for Reynolds numbers up to 100,000. Three aspect ratios (AR = 3.7, 5.0, 7.5), three rib geometries (V-, W-, and staggered V-ribs), and three guide-vane designs (180°, 135°, and 90° spans) were examined. Surface heat transfer was measured using transient liquid crystal thermography, and pressure drops were recorded in each passage and bend. Increasing aspect ratio leads to a systematic increase in area-averaged heat transfer, accompanied by a corresponding rise in pressure loss, highlighting a clear thermal–hydraulic trade-off. Among the investigated rib configurations, W-ribs generally exhibit higher and more uniform heat-transfer levels, while staggered V-ribs produce lower augmentation with reduced friction penalties; however, differences among rib types remain moderate. Guide-vane design strongly affects bend losses and downstream uniformity, with the 180° vane providing the most favorable balance. Semi-empirical roughness functions for friction and heat transfer are derived and shown to predict the measurements consistently across all tested configurations, with mean absolute percentage errors below ∼ 10% for friction and ∼ 15% for heat transfer, providing a generalized framework for performance prediction in high–aspect–ratio single-wall ribbed channels.