My question primarily relates to an object planing across a fluid surface. First, does the Venturi effect only apply to enclosed systems such as pipes? My true motives are to understand the physical forces acting on a surfboard as it travels down and across a wave. I’m currently in the process of analyzing how water flows across the bottom of a surfboard. This has turned out to be quite complex because water rarely flows directly from nose to tail on the surfboard. A brief introduction to surfboard design may be needed. Generally, boards are designed with some sort of concave on the bottom. The concave is wide where the water contacts the board and gradually narrows until it exits the tail. This gradual restriction is guided by the rails. Some boards use a single to double concave out the back. The boards also have a convex shape because of the rocker. My question is, can the small amount of channeling invoke the venturi effect? And will the Coanda effect occur similar to a spoon; essentially will the water follow the length of rail out the back of the board?