Quote Originally Posted by factotum View Post
Of course, if you create a ship which is essentially unsinkable using your available weapons technology, and you can't develop any way of defeating them, you'll probably find other ways around the problem--e.g. you'll build your vessels to be small and fast so they can evade the lumbering "turtle" ships rather than try to fight them, and all battles between big ships would devolve into boarding actions.
Yeah but by 1800 ships could massively damage cities using rockets and cannons, and by 1900 the British could level a city. Air-cannons might be less penetrative then gunpowder but they also can fire explosive or incendiary rounds with less problems, so a massive armored ship just sailing into harbor and destroying your docks would be a real threat.

On that note; in RL Ironclads were cannon proof until a decade into smokeless cannons. Cannons that could penetrate their armor had a tendency to explode (the US Secretary of War was killed in a testfire for instance), and they were built to ram each other until the late 1890s because of this.