This mod focuses largely on capital ships and the design, upkeep, refitting, and converting of them.
One thing that I always found to be problematic in vanilla (and elsewhere), is that naval construction is largely simplified. Each nation faced it's own unique set of challenges and interests, and produced quite a range of different types of ships and had a great deal of diversity even in ships similar in both time and scope. Many chose different options to meet similar challenges. The attachment system does not completely satisfy on this count.
So I chose to jump on the opportunities presented by the upgrade system introduced in DH 1.03.
Capital ships: Carriers, Battleships, and Battlecruisers are now revamped and given a more realistic usage.
Historically, when building major warships, nations would make then the centers of their fleets, refitting them to incorporate advances in technology and ideas and time went on. Some of those refits were quite extensive and resulted in sometimes quite different capabilities afterward (and sometimes not so much).
Also, nations faced major questions in terms of armament, balancing cost and effectiveness. Those choices are presented here.
In game, you can choose three different main gun sizes for your capital ships. Pocket battleships (an individual type) are given two choices, that were historical for the Germans. Super heavy battleships are given a much more extensive tree, and you can research (in secret weapons) some bigger 20 inch guns, and even 22 inch guns, later, allowing you to build ships with guns of those sizes.
Nations also faced the aircraft age, and the massive changes it brought to naval warfare, with the advent, rise, and dominance of the aircraft carrier. Most carriers early were conversions from other ships, including capital ships. Almost all battleships and battlecruisers are given the option to possibly convert to aircraft carriers. Though, to be sure, they were not as capable as purpose-built carriers, and that is reflected here. Each has a rather unique carrier that it is converted to, reflecting its heritage in its prior form. Some are more successful than others, and are different from each other, and from the general line of purpose-built carriers.
Also, aircraft underwent a great deal of change, from light and short-ranged wooden aircraft that resembled kites, to metal monoplanes that were much heavier and faster, and much more effective. Finally aircraft reached it's pinnacle in jet aircraft, with even greater requirements is size, weight, and fuel, but a level of effectiveness that was even greater.
In game, this means CAGs are separated into 4 main types, early, regular (WW2 type monoplanes), and jet, there's also a later era jet CAG with even heavier and better armed aircraft..
Obviously carrier aircraft changed apace with these succeeding technological advances. Many changes required changes in the ships that would service and fly them. Runways had to be longer, fuel bunkers bigger, and weapons (and plane) storage had to be larger, among other things. So the ships changed over time. Some ships received refits to accommodate these changes, some other older ones were abandoned as too old or impractical to update.
In game, this means you will have to convert a carrier to carry a new type of CAG, you cannot simply interchange them, or upgrade the CAG if the carrier cannot handle the new type.
This quite complicated naval planning, which had to be done years in advance, due to long lead times in construction and research to be effective and get the new concepts in the water. You'll be faced with many different options, and several research paths in capital ship planning and construction.
Sometimes in emergency situations, naval planners came up with stopgap solutions, like a hybrid battleship/carrier. Many older ships can be converted to hybrids, with a unique CAG of their own. If you wish, you can then convert them to full carriers, too. Or you can convert straight to a carrier, if you think you have enough time and production space.
The scenarios for 1914, 1933, and 1936 are included, so that several different periods could be shown while also having time to make some decisions for yourself.
Looking back at the WW1 era, you will see a few interesting innovations, like the "light battlecruisers" of the Courageous class, that had little armor but high speed and big guns, including one with 2 one-gun turrets with 18 (!) inch guns. These were later converted to aircraft carriers, which is also in-game.
To clear one misunderstanding I want to clear up now: you cannot upgrade Schleswig-Holstein to a Bismarck-class battleship. Each ship has refits and conversions, if you wish to take the time to do them. Refits will happen automatically in port, unless you manually stop it from doing so. To convert, you will need to use the drop-down box to decide what to convert to.
Also, included is the possibility of up-converting the guns on your battleship or battlecruiser, it costs time and IC to do so, but it can be cheaper than building a new ship with those guns. Historically the Germans had thought to do this with Scharnhorst and Gneisnau from 11 inch to 15 inch guns, but never got around to it.
Once you do a conversion, though, it is a one-way trip, you cannot go back.
There are many concept ships included that never got off the drawing board from the WW1-era to post WW2. If you make it to the late Cold War-era ships, you will also see vertically-launched missile armed ships, the example of which is the Kirov, these have a long range, almost as long as their era's best carriers, and are intended to be a surface combatant capable of hitting carriers, and will destroy earlier carriers from outside their range.
In order to enable SHBBs, you will need to do the new decision to widen the ports and dockyards of your nation. As the US, you will have to take the decision to widen the Panama Canal before you can starting researching SHBBs, a very expensive proposition.