We examine the influence of fin spacing on the over-tube side of a single-row fin-tube heat exchanger through flow visualization and numerical computation. The distance between fins is nondimensionalized by the tube diameter. If this parameter is small, the flow is Hele-Shaw; as it is increased, a horseshoe vortex is formed just upstream of the tube; a separated region is then developed behind the tube; this becomes larger and eventually communicates with the fluid downstream of the heat exchanger. A peak in the Nusselt number occurs at the horseshoe vortex. In the wake region the Nusselt number is very small but increases when there is fluid exchange with that downstream. The ratio of the overall Nusselt number per unit length to the nondimensional pressure drop is found to show a maximum.