Strength,

$S = bd^2$

Where:

$b = D \cos \theta$

$d = D \sin \theta$

$S = D^3 \cos \theta \sin^2 \theta$

$S = D^3 \cos \theta (1 - \cos^2 \theta)$

$S = D^3 (\cos \theta - \cos^3 \theta)$

$\dfrac{dS}{d\theta} = D^3 (-\sin \theta + 3\cos^2 \theta \sin \theta) = 0$

$-1 + 3\cos^2 \theta = 0$

$\cos^2 \theta = \frac{1}{3}$

$\cos \theta = \frac{1}{\sqrt{3}}$

$b = D \cos \theta = \frac{1}{\sqrt{3}}D$

$d = D \sin \theta = \dfrac{1}{\sqrt{3}} \sqrt{2} \, D$

$\text{depth } = \sqrt{2} \times \text{ breadth}$ *answer*