Field measurements of drag force on Sargassum horneri (Turner) C. Agardh towed by a boat and estimation of drag coefficient

Abstract

Sargassum horneri is one of the most important Sargassum species forming floating seaweed rafts. Drag force is the most important factor involved in detaching seaweeds from substrates. We conducted field experiments on drag force on S. horneri thalli > 40 cm by towing them with a boat at steady speeds of 0.5-4.0 m s^-1 in Nabeta Cove, Japan, from in April 2014 and February 2015. The holdfast of thallus was attached to the end of a non-stretching fishing line; the other end was attached to a spring scale running through a steel pipe attached to the side of the boat and on low-friction pulleys fixed at both ends of the pipe. The lower end of the pipe was projected into the sea at a depth of approximately 1 m to keep the thallus submerged. The thallus was towed while force was measured on the spring scale. The relative speed of the boat towing the thallus versus seawater was continuously recorded by a flow meter attached to the end of the steel pipe. The results showed that drag force on the thallus was increased with thallus length and current speed; the drag force was roughly proportional to flow speed^3/2; the drag coefficient, Cd, of S. horneri was expressed with the following equation: Cd = 18.295Re^-0.571 in a range of Reynolds number (Re) between 10⁴ and 10⁶.