$\tau_{max} = \dfrac{16PR}{\pi d^3} \left( 1 + \dfrac{d}{4R} \right)$ → Equation (3-9)
$18\,000 = \dfrac{16P(4)}{\pi (3/4)^3} \left[ 1 + \dfrac{3/4}{4(4)} \right]$
$P = 356.07 \, \text{lb}$
$\delta = \dfrac{64PR^3n}{Gd^4}$
$4 = \dfrac{64(356.07)(4^3)n}{(12 \times 10^6)(3/4)^4}$
$n = 10.41 \, \text{ say 10 turns}$ answer