The strength jump of 15 to 30 from Cruisers to Ironclads remains a problem.
As has been discussed, historically there was a gradual steady improvement.
Based on land units an increase of 40% to 50% in strength is about right for the game. However, the first sea going modern ship has other advantages over the ironclad, speed and the ability to go in the ocean...
The problem I see is that each ship has to be available for a useful amount of time, without excessive inflation of the
number of ship types.
We have two ancient/classical ships, each with distinct roles: galley/trireme.
We have three medieval ships, each with distinct roles: cog/dromon/caravel.
We have five renaissance ships: galleon/galleass/privateer/frigate/ship of the line.
At this point, it's hard to preserve a distinct role for each ship, and adding further classes arguably becomes superfluous unless we know what to do with them.
We have, if we combine your idea with Xyth's, five industrial ships: clipper/ironclad/"light cruiser"/transport/"heavy cruiser"
This is the first era in which you can have ships upgrading twice in a single era, and I don't think I care for it.
The modern era, of course, has six ships:
submarine/attack sub/carrier/battleship/missile cruiser/destroyer...
But at least two or three of them have new specific roles that are made possible by the technology, and thus they don't infringe on each others' missions so much.
Let me lay out what I think we should have, from the beginning to the end. Only primary prerequisites are listed. If there's any disagreement over specific stats, feel free to bring it up; I'm giving the stats of existing ships from memory which may be wrong.
____________________________
ANCIENT/CLASSICAL
Galley (Seafaring, Str 2, move 2, carry 2, coastal)
Trireme (Shipbuilding, Str 3, move 2, carry 0, coastal)
MEDIEVAL
Cog (Guilds, Str 3, move 3, carry 2 or 3, coastal)
Dromon (???*, Str 4, move 3, carry 0, coastal)
Caravel (Compass, Str ?, move 3, carry special, oceangoing)
*Unknown, suggest Machinery to reflect the kind of torsion artillery and such that would go on a more sophisticated ship like this.
RENAISSANCE
Galleon (Optics, Str 4, move 3 or 4, carry 3, oceangoing)
Galleass (Gunpowder, Str 6, move 3 or 4, carry 0, coastal)
Privateer (Charter, Str 6, move 4 or 5, carry 0, oceangoing, flies jolly roger)
Frigate (Meteorology, Str 8, move 4, carry 0, oceangoing)*
Ship of the Line (Physics, Str 10, move 3, carry 0, oceangoing, can bombard cities*)
*I suggest restricting bombardment to ships of the line, as historically these ships carried much heavier guns than a typical frigate, were better suited for fighting it out with shore batteries, and above all because in game
it creates an incentive to build them, which would otherwise be lacking since frigates are faster and the speed is normally more of an advantage than the firepower. You might, if you wish, allow frigates to bombard.
INDUSTRIAL
Clipper (Refrigeration, Str 6, move 4 or 5, carry 3, oceangoing)
Ironclad (Steam Power, Str 15, move 4, carry 0, oceangoing, can bombard cities)*
Torpedo Boat (Refining, Str 15, move 6, carry 0, coastal, +50% or higher% attack versus cruiser)**
Cruiser (Combustion, Str 30, move 5, carry 0, oceangoing, can bombard cities)
Transport (Combustion, Str 15, move 5, carry 4, oceangoing)
Submarine (Pneumatics, Str 24, move 4, carry missiles, underwater, 50% chance of withdrawal)
[Clippers require no resources. Ironclads, cruisers, transports, and torpedo boats can be built with coal. Cruisers, transports, torpedo boats, and submarines can be built with oil]
*Use existing artwork for this- the one you use in 0.9.4, which has sails. This is an
oceangoing ironclad, and yes it is far stronger than a frigate, but not so much stronger than a ship of the line.
**This will remain reasonably effective after much-stronger cruisers appear, as the ironclad will not. Its main selling points will be speed and low cost, though.
MODERN
Destroyer (Electronics, Str 22, move 6 or 7, carry 0, oceangoing, can see subs, +50% versus submarine and attack submarine, chance to intercept aircraft)
Battleship (Automobile, Str 40, move 4, carry 0, oceangoing, can bombard cities
very well)
Carrier (Radar, Str 20, move 5, carry planes)
Attack Submarine (Fission, Str 30, move 6, underwater, 50% chance of withdrawal)
Missile Cruiser (Robotics, Str 40, move 5, carry missiles, can bombard cities
very well)*
Stealth Destroyer (I hate the concept of this ship because it turns out not to work in real life, but I'm not asking people to remove it; as normal)
[All ships except Attack Submarine can be built with oil. All ships except Destroyer and Stealth Destroyer can be built with uranium. Therefore, battleships, carriers, submarines, and missile cruisers can be built with either oil
or uranium]
*I think it might be nice to give the missile cruiser the air-interception and anti-submarine capabilities of the destroyer, so that the destroyer can upgrade into the missile cruiser.
UPGRADE PATHS (paths stop at a branch point):
Galley -> Cog -> Galleon -> Clipper -> Transport
Trireme -> Dromon -> Galleass -> Torpedo Boat -> Destroyer -> ???
Caravel -> ???
Frigate -> Ironclad
Ship of the Line -> Ironclad
Ironclad -> Cruiser -> Missile Cruiser
Battleship -> Missile Cruiser
(Nothing upgrades to Ship of the Line or Battleship; each must be built specially during its era, with the incentive to do so coming from their power to hammer other people's navies to scrap and lay waste to coastal cities more effectively than anything else in their eras)
(Don't know what to do with Caravel upgrades- frigate maybe?)
I am not sure of names, but something like:
Ironclad
Modern Ship #1: ocean going, requires coal, move 5, strength 20 (or maybe 21).
Modern Ship #2 (current cruiser): ocean going, requires oil, move 6, strength 30.
Modern ship #2 is available at a later tech than modern ship #1.
Perhaps can just call them Light Cruiser and Heavy Cruiser, if nobody can come up with better names.
Light cruisers didn't exist as a design concept until around 1910, and the artificial distinction between "light" and "heavy" came about as a by-product of the Washington Naval Treaty, which capped all cruisers at ten thousand tons' weight. At that size, you could build either a well-balanced ship design using six-inch guns, or an overgunned design using eight-inch guns and reduced armor protection. The 6" designs got arbitrarily labeled "light" and the 8" designs "heavy," even though a "light" cruiser was often as good or better as a warship than the "heavy."
If one wanted to one could have two ships between ironclad and cruiser rather than just one. I think the above would do for a quick fix.
By World War I they had battleships called dreadnaughts and then
super-dreadnaughts.
So for future versions there is room to make the set of modern ships more historical, if more detail is thought desirable.
(I am not an expert.)
"Dreadnought" was an informal term for ships with an all big-gun armament. The norm for late-1800s designs was to have big gun turrets near the bow and stern, carrying two or four of the ship's heaviest weapons, then a long box of superstructure in between the turrets which would carry a lot of almost-as-big guns. The theory was that you'd blow the crud out of the enemy's superstructure and smaller ships with masses of relatively low-power gunfire, while punching through their heavy armor with the big guns.
In practice, this didn't work so well- the medium guns weren't powerful enough to cause enough superficial damage to matter. So once it became practical to arm ships with several heavy-gun turrets (think 12" guns and up), everyone in the world started work on classes of ships armed with
no medium weapons at all, nothing bigger than five-inch guns or so to defend against enemy light warships while the big guns were otherwise occupied.
Everyone started on this around 1904 or so, except the French who got unlucky in their design patterns and just happened to already have started work on a bunch of ships using the old pattern. The British rushed one of these new ships and called it
Dreadnought; everyone else's became known as "dreadnoughts" in imitation, and the French got the world's deadliest pre-dreadnought battleships.
If history had gone a little differently and the British hadn't gone far out of their way to rush-build their first all big gun ship, "dreadnoughts" could equally well have been called
"southcarolinas" or
"satsumas." Although names like that might not have caught on.
"Super-dreadnought" was a term which
didn't last; it was a short-lived name for ships built along the same general lines as the first-generation dreadnoughts, but which were further improved in straightforward, linear progression: bigger, faster, heavier-armed, more expensive. Mostly just
bigger; greater size in a warship makes nearly everything else, including speed, easier to achieve.
By 1920 or so, people had mostly stopped calling battleships "dreadnoughts" because there was no longer any real distinction to be made- the "pre-dreadnought" ships were mostly retired or scrapped.