Could battlecruisers fight safely at flank speed?

Did exposure below the armour belt prevent battlecruisers from fighting at flank speed?

No; exploiting the small moving gap would be very difficult, and similar wave effects also affected fast battleships.

Battlecruisers could fight at flank speed despite the possibility that a wave trough exposed hull below the main belt. Renown's illustrated armour layout shows several layers of coverage, with a battleship expected to carry protection farther down. Turning is said to make the exposed gap more pronounced. Hitting that small, moving gap at battle ranges would nevertheless be extremely unlikely, so the vulnerability was an accepted risk. Fast battleships also experienced the wave effect, though deeper lower belts gave them more protection.