Question

The fixed keel of the sailboat is about 38 in. deep. The boat moves at a speed of 3 knots (i.e. 5.1 ft/s) in a lake with water temperature of 40°F. The keel can be approximated as a rectangular flat plate with an average length in flow direction l=(34+15)/2 = 24.5 in, and a depth of 38 inches. (Considering a laminar BL to turbulent BL transition happens at Recr= 2x10° in this situation.) 1) (3 pts) What is the Reynolds number Rei for flow over the plate? 2) (3 pts) What is the coefficient of drag for the plate, Cpg? 3) (3 pts) What is the friction drag force on the keel? (notice there are two sides) 4) (1 pt) If the downstream keel flow BL transitions into turbulent BL, what is the effect of this on the overall skin friction drag? 134 in 38 in 15 in

Answer 1

solution : Re = SUL or VL u 29 where v = Kinematic viscosity at 40°F 22 0.0000015486 m L = 24.5 in = 0.6223 m V 2 500 ft/s 1.55 m/s 622-86x103 i Re 1-85X0.6223 0.0000015486 as Re < 2 2x106 for laminar flow The flows is caminar. 1.328 Cof ? [ according to Blasius corelation] Rex Cog 1.682x10-3 Drag force ? Fo I PAV.COF Fo? 2 x 10 1000 x0.622 X-9052* 1.55" x 1-682x10- -3 To = 1.213 N now as the transition from laminan to tubulent takes place. CDE 5 7 X (0.059/e's Re = 2x106 CDE E 4.05 X10-3 » (COP) Caminan </55) febulent.