How air-independent boilers would reshape early warships

A boiler that did not require combustion air would remove funnels and uptakes, free the armored deck, and tempt designers toward closed-loop propulsion and more deeply submerged hull forms.

Question

How would early twentieth-century warship design change if boilers could heat water without needing combustion air?

Short answer

Funnels and many deck penetrations could disappear, layouts could change radically, and closed-loop submarines would become possible, but battleship-submarine hybrids would still carry severe compromises.

Notes

Removing the need for combustion air means fewer openings through the armored deck, no smoke plume obstructing fire, and much greater freedom to arrange machinery and superstructure. The same system could power fast submarines or encourage partially submerged surface ships. Merging battleship and submarine roles would still be unwise because a hybrid tends to inherit both sets of complications without receiving all of either type's advantages.