Why early heat detection offered limited warning at sea

How useful would heat vision have been for naval night fighting?

It could reveal hot external features such as firing gun barrels, but those became conspicuous only after the enemy had already opened fire.

A warship contains hot machinery, but its gun barrels are often the hottest exterior features. Those barrels become easiest to detect after firing has already begun. Thermal observation could therefore help track an active opponent without necessarily solving initial detection in darkness. Its tactical value depended on when a useful heat signature appeared.