Estimating Warspite's speed from a photograph

How fast was Warspite moving in the 1942 Indian Ocean photograph?

Probably about 18 to 20 knots, judging from the bow wave and hull attitude.

The estimate comes from the ship's bow wave and visible hull attitude rather than treating the photograph as a precise instrument. The wave appears too developed for Warspite's roughly 12-knot economical speed but not large enough for her full 23- to 25-knot performance. That suggests about 18 to 20 knots, with 20 as the more likely figure. Naval photographs can support bounded performance estimates when the visual limits are stated clearly.