The rectangular gate shown in section is $14$ ft long $($perpendicular to the paper$)$ and is hinged about its upper edge $B.$ The gate divides a channel leading to a fresh$-$water lake on the left and a salt$-$water tidal basin on the right. Calculate the torque $M$ on the shaft of the gate at $B$ required to prevent the gate from opening when the salt$-$water level drops to $h=4\mathrm{.}0ft.$

Answer: $M=$ kip$-$ft