A thick-walled pipe is bent about a horizontal axis by a couple $\mathbf{M}$. The pipe may be designed with or without four fins.
(a) Using an allowable stress of $20 \mathrm{ksi}$, determine the largest couple that may be applied if the pipe is designed with four fins as shown.
$(b)$ Solve part $a$, assuming that the pipe is designed with no fins.