An experimental investigation has been performed to study the forced convection and pressure-drop characteristics fully-developed steady turbulent flows in air-cooled, horizontal, equilateral-triangular ducts. These ducts were constructed of duralumin, each of the same axial length and hydraulic diameter. The inner surfaces of each duct were either plane or machined with uniformly-spaced parallel, identical V-grooves. For each tested duct, these grooves had a depth of 1 mm, but were all of the same apex angle, theta, where 0 less than or equal to theta < 150 degrees. The uniform separation between the centre lines of two adjacent V-grooves was kept constant at 34 mm. The measurements were performed with the hydraulic diameter-based Reynolds number for the steady-state air flow, ranging from 2800 to 9500. The entire inner wall of the duct was heated uniformly, while its outer surface was thermally insulated. The forced convection was enhanced by the presence of the V-grooves; the greatest forced-convection augmentation being achieved with V-grooves of apex angle theta approximate to 17.5 degrees. Non-dimensional expressions for the determination of the steady-state heat transfer coefficient and the Darcy friction-factor for the equilateral triangular ducts, which were fabricated with uniformly-spaced identical V-grooves in their inner surfaces, have been deduced.