Question

A log 2 m in diameter and 12 m long is towed at a constant speed of 3 m/s in fresh water at 20oC. What is the drag force, in kN, on this log if it is towed lengthwise (water facing log end), what is the required towing power in kW, and what would the drag force be in kN if the log was towed widthwise (water facing log length) ?

Answer 1

It has D=2m, L=12m and Velocity V=3m/s.

When log is towed in length wise,

so cross-dectional area which faces drag is rectangular so drag coefficent will be Cd=0.82

So,Drag Force= 0.5*Cd*density*Area*square of the velocity

=0.5*0.82*1000*12*2*3*3

=88.56KN

Towing Power Required is =F*V=88.56*3=265.68kW

For case-2:

if the log was towed widthwise, then Cd=0.47

Drag Force=0.5*Cd*density*Area*square of velocity

=0.5*0.47*1000*(22/7)*2*2*3*3

=26.588KN

Towing Power Required is =F*V=26.588*3=79.764kW