Why sloped armour does not guarantee a ricochet

Could a 145 mm plate sloped 27 degrees from horizontal deflect an incoming battleship shell?

Possibly, but only if the shell's actual impact angle favors deflection; line-of-sight thickness and plate slope cannot guarantee a ricochet.

The submitted design specifies a plate 145 mm thick, inclined 27 degrees from horizontal, and more than 300 mm along the line of sight. Those figures do not determine the result by themselves. A shell arriving close to normal to the plate is unlikely to ricochet, while an oblique arrival may deflect after the outer hull or deck has already nudged its path. Ricochet is therefore a function of geometry and chance rather than an automatic benefit of sloping armour.