Steam power breaks the geography of sail blockade

Which ship-propulsion development had the greatest impact on world history?

Self-propulsion, because it broke dependence on wind patterns that had structured routes, interception, and blockade strategy.

The transition from sail dependence to self-propulsion is identified as the most consequential propulsion change. The Royal Navy could place squadrons where prevailing winds funneled French and Spanish ships, forming the geographical logic behind distant blockade. Trafalgar shows the depth of that system: even a defeated Nelson would leave the combined fleet facing successive Western, Channel, North Sea, and Baltic forces before any invasion attempt.