Ship design and procurement can often feel like a constant losing race against the continuously improving technology of the time, especially when your enemies are already ahead of you. New technologies are often completed far faster than the time it takes for ships to build, increasing the importance of timing your ship construction. Thus, as a player with special knowledge of upcoming future technologies, you can better optimize your ships to be constructed immediately after important technologies are completed and before long stagnant periods in technological advancement.

Each section of this guide will represent a time (year) when significant technological advances are expected and ships that should be designed. If you are not at the cutting edge, there will be a delay. I'm also using the Japanese ship appearance for examples throughout the guide because that's the country I enjoy playing the most, although these principles apply to most countries and playstyles. Keep in mind that this guide is not comprehensive; these designs should also only be treated as basic guidelines, as oftentimes you will be building interim ship designs, restricted by treaties, or interrupted by wars.

Of course, this type of gameplay will detract from the immersive fun of playing the role of an early 20th century navy and it certainly isn't necessary to enjoy or do well in the game.

Skip to the end of the guide to see my basic design principles. I've compiled many of my preferences into a single section to avoid repetition in the guide and moved it to the end so people can get to the ship design part faster.

At the start of the game, you can choose to either continue or scrap the random ships which are under construction. Either way, you'll still likely find that your starting fleet is lackluster and is in desperate need of reinforcement with decent ships.
My preferred ship to construct is a simple protected cruiser with the maximum quantity of 6" guns (14). Two 2-gun turrets and 10 in casemates. I'll also often bring 5 torpedoes; my understanding is that many dislike torpedoes, but I find these very useful for melting early battleships. I find 3" turret/belt with a 1" deck is plenty for this time period, and I can usually fit this onto a 6kt hull. If you feel like your cruisers are dying too easily, try building them with a 2" deck.

Throughout the game I find 6kt to be the best balance between build time (22 months) and armament. Anything more and the build time increases significantly and anything less will significantly reduce firepower without any significant improvements in build time.
If you unlock upgraded 6" guns early on, it is definitely worth refitting. Early oil engines (or coal turbines) in 1905 are also worth their own refit. Beyond that, I refit these in 1926 to serve as HAA escorts until the end of their service.

For colonial service, I like to build corvettes to be exactly the size needed to fill each sea zone. For example, a 6000t requirement can be filled with two 2400t or three 1800t colonial service corvettes. Usually I'll make these into minesweepers, as they don't seem to do a very good job at ASW when simply sitting in active mode in a sea zone (needs to be set to TP for ASW).

For trade protection, I build the smallest corvette I can with the maximum speed, range, and ASW. The reason I build the smallest ships I can is so they can fill "TP" requirements during wartime while keeping losses low when they get vaporized by raiders.

For raiding, I build light cruisers made to be as fast as possible with extreme or long range. The only armament you need is a single gun. Operating range determines the ability for the ship to sink enemy merchants, and the speed improves the rate of favorable outcomes against intercepting ships. The benefits also apply to intercepting enemy raiders. These principles apply for the entire game.

To be quite frank, I rarely build purpose-built raiders; I find them to be a waste of resources when I could be building regular cruisers or literally any other ship instead. That said, the ~7 years between your first ships and Krupp armor is a good opportunity to build some of these.

In 1898, there are significant improvements to ship design that warrant new ships to be constructed. Most significantly, armor on new ships is significantly improved (Krupp). The technology for the rangefinder and a reduced ROF penalty for 2-gun turrets (latter 2 can also be added to your older ships with a refit) is also unlocked around this time. -1 quality heavy guns are also possible, but not necessary.
After those research hurdles is usually when I build a medium/heavy-gun cruiser, depending on what guns I have available. If you have -1 quality 11 or 12" guns, your main battery should be 4 guns on two turrets (fore and aft). Then, I max out the quantity of secondary guns with your best medium gun. If you don't have any -1 quality heavy guns and need ships ASAP, I recommend a cruiser with the maximum amount of medium guns (14) as the main battery (seen below).

Personally, I like to keep the displacement under 10kt. Not only will you still be limited by shipyard size at this point in the game, this also keeps your build time at 24 months.

For armor, I wouldn't add more than a 3" turret and 2" belt/deck. While these ships could be fitted with more armor (with greater displacement), these cruisers are good candidates for CVL conversion later in the game to provide much-needed patrols, and any belt armor will simply become wasted space after conversion.

If able, the target top speed should be 24 knots to make carrier conversion cheaper.
I generally only ever refit these for turbine engines in 1904/5 or a significant gun upgrade until conversion in 1918.

A 25kt AMC that carries 500 mines (and 2 aircraft, once they are researched). You can build two of them during peacetime. Because they only take 4 months to build, I generally don't bother building them until war is imminent. Technically, the displacement limit is 26kt, but that makes the ship unsuitable for carrier conversion in 1921, as the gun requirement for ships greater than 25kt (8" guns) cannot be placed on unarmored ships. Additionally, it is important that the ship speed is at least 24 knots to avoid the spot penalty. Additional ships can be 26kt, as you only need to convert one ship to bypass the CV gun requirement.

If you can, wait until turbine engines in 1904, 1905, or 1920 before building this to get the best engine on your carriers.

It's simply a 900t torpedo-carrier. I max out the number of centerline torpedo launchers and tubes with a single gun (optional). While you could build destroyers earlier, I find the low displacement and poor torpedo technology to be not worthwhile.
These can also be refit to double the torpedo loadout after 1912.

There are many major technologies unlocked between 1905 and 1909 that warrants a new ship design (consider setting Damage Control, Turrets, and Ship Design to high priority during this time). Battleships become very valuable to build once you can build 4 centerline turrets after 1907 (or 6 in 1908). However, I find that TPS 2 (1909) is valuable enough that I always delay procurement until then. If you start building these a little later, you might be able to catch Director FC during construction in 1912.
For me, they usually end up being BCs because I love making speedy glass cannons. This also improves their usefulness when they're converted to carriers or escort ships. Note that turret and CT armor can be removed, but deck/belt armor cannot.

If you want to add secondary guns to ward off destroyers (and HAA later in the game), I recommend using the tertiary battery instead (by placing the minimum number of secondary guns). This is because tertiary guns cannot be hit (and cause damage).

This is the earliest "useful" battleship in my personal opinion.
A refit in 1920 is important if it does not yet have oil turbine engines; it's also a good opportunity to include gun replacements and LAA. In 1934, I convert my smaller guns into DP guns. Then, in 1945, I refit them once more for radar, MAA, turbo engines, and many other improvements.

If you're feeling a little extra, you can refit them in 1955 to become full AA ships by replacing the main guns with more HAA and MSAMs.

I generally do not build armored battleships, as I find them to be a waste. However, that is not to say they are not useful in the short-term. Here are my design priorities for a "true" battleship:
6 centerline turrets (A, B, V, W, X, Y) with the maximum number of your best guns (ideally 16")
3.5" D, 4" TT, 3" DE
Narrow Belt, Inclined Belt
Maximum allowable T armor (whatever you want, really)
B/BE with x0.9 T armor (e.g. 10" turret, 9" belt), but can be reduced to allow for better T armor.
Turret armor is the most important, as non-penetrating turret hits cause no damage (unlike superstructure/deck hits). Increasing the number of turrets also increases the chance they are hit (regardless of turret size), so I generally try to have 4 turrets at minimum on my battleships, ideally 5 or 6. About 20% of turret hits are TT (despite being so light and small), so it is always worth having 3-4" TT. ~3" deck armor is enough to stop penetrations, but more importantly, it is critical for stopping "splinter" damage (~3.5" against 15" battleships, if you're feeling extra). DE is also important, as it protects the superstructure against about 40% of direct hits and splinter damage, but it is a slightly lower priority. B and BE are essentially the same; you should armor these the same amount to prevent hull damage. I always use a narrow belt, as the penalty is small (+2% hull hit chance). BU is similar to DE and protects the superstructure against 40% of direct hits, but would need to be prohibitively heavy to do so, thus I consider it a waste of displacement.

AoN (1912) significantly improves the survivability of your ships by redirecting superstructure hits to the TT and hull. This makes DE armor significantly less important (disproportionately to total hit reduction), so I often do not use DE armor if AoN is unlocked. BE also becomes less important, but I'll usually keep it the same. The main downside of AoN is a ~20% increase in unprotected hull hits which is a non-issue for ships with large displacement.

Theoretically, you could make a design with 4 additional wing turrets (for a total of 10 turrets, possibly increasing the turret hit chance to up to 50%) on an exceptionally large ship, but i have not tested such a design.

Of course, once aircraft start showing up after ~1920, these designs immediately become obsolete. Putting aircraft on battleships also makes them much more vulnerable due to fires, so I never put aircraft on battleships. If you insist on building more, you'll want to sacrifice belt and turret armor for a large tertiary battery of 5" HAA on newer ships. For refits, consider removing a centerline turret to free up displacement for AA.

1100t destroyers and 2-tube launchers are researched in 1912. I never build 900t torpedo ships after this point.
1500t destroyers and 3-tube launchers are researched in 1914. I never build 1100t torpedo ships after this point. It's worth building a lot of these because this will be your largest DD hull until 1923.

At around ~1914, you may have researched AoN, which significantly reduces superstructure hit chance and reduces armor weight (at the cost of more belt penetrations, like narrow belt). If you get this tech (60% chance), it's worth using. By then, you will have also researched 4-gun turrets. Although this is a somewhat incremental improvement on the 3-gun turrets in the 1909 design, it warrants considering a slightly delayed procurement for this new design if displacement allows.
However, if you don't believe new ships are not immediately needed, there are some important technologies that unlock in the following years which makes delaying procurement worthwhile:
Gun elevation (1916), 4" DP (1917), Unit Machinery (1917), HAA (1917) and Improved Director (1918).
All four of these can technically be added in a refit as well, but they would likely have to be started immediately after construction is completed if the ship is built on a 1914 design.

You might notice this is the last battleship design in this guide. This is because large aircraft carriers (CV) become available after 1926. I find that any resources that could be put towards building more modern battleships are better used for carriers, especially when you already have a decent number of battleships. Normally, I wouldn't even build these battleships, as my money mostly goes towards CVL conversions.
This ship can follow the refit schedule for the other battleships (for the sake of simplicity), although I would skip the 1920 refit simply because it isn't worth it (in my opinion). If you like 4" HAA, you get twin mounts in 1926 which are worth refitting for.

I turn all of my CAs into CVLs after this point. Usually I get them to fit ~20 planes with ~10 spot. I primarily use them for scouting, especially after purpose-built carriers join the fleet.
These can continue to be useful into the jet age carrying light jet fighters for CAP. However, they may become too expensive to maintain, at which point it would be advisable to scrap them (perhaps immediately after a war).
Generally, these are perfectly capable until the end of the game. However, refits can be worthwhile for deck catapults in 1937 and turbo diesels and radar in 1944. If you want to save money, skip the catapults until 1944.

Immediately after large carrier conversions are unlocked, I will often turn one of my 25kt merchant ships (see above) into a sacrificial carrier to eat the 8-gun requirement on a cheap conversion. This is much cheaper than building a dedicated carrier, especially without a machinery rebuild.
I can usually fit 48 (four 12-plane squadrons) with a spot value of 24.

If you plan on fighting a war in short time, you can also convert both into carriers (with guns on only one of them) and start construction on two more AMCs to be converted into carriers. This will give you a pretty big advantage by having large carriers for cheap, many years before everyone else. I like to fill them with torpedo bombers to annihilate undefended enemy fleets.
Follow the refit schedule of CVLs

Same as the previous ones, except it's 2kt and carries more torpedoes.

12 6" guns in these triple turrets is a big upgrade to the 9-gun broadsides of the protected cruiser. At this point I only bother adding the obligatory 1" narrow belt and 2"deck/turret (essentially a protected cruiser lol). Use AoN if you have it.
These serve to replace the protected cruisers from 1890 once they are converted to AA ships.
The 1945 refit for improved radar and AA, as well as a 6" autoloader, is crucial for this ship and should be considered a main priority.
After 1955, consider turning these into AA platforms, similar to your old battleships, as destroyers replace the role of gunboats.

Although you will be significantly limited by size at first, I find it worth building one or a few purpose-built carriers to take advantage of the new technology. I like to fill my carriers with torpedo bombers to take advantage of (likely) limited enemy defenses. Whatever your preferred strategy, you probably want to build one ASAP. Keep in mind that you will be relying on escort carriers to provide AA support until your AA cruisers are built after 1926.
Of course, if you skipped building the sacrificial carrier mentioned above, your first carrier will have to be burdened with guns. In such a case, I build a single "throwaway" carrier to meet this demand.
These follow the refit schedule of past carriers.

Once 4" DP and AA directors are unlocked. Immediately I refit all of my 1890 protected cruisers to serve as escort ships for my carriers. I do this by removing all of the casemate guns and adding a large DP secondary battery with four directors. Although the top speed is going to be low, it's plenty for keeping up with a carrier force.
The role of this ship is to simply serve as an HAA platform to accompany carriers and occasionally fight destroyers that get too close. These also serve as important dummy targets for enemy aircraft as it is more likely for them to target the wrong ships if many are sailing near the carrier.

I find that the ideal size for carriers at this time is 30kt. Not only does this meet the requirements for jet conversion with steam catapults in 1952, it also provides plenty of space for 100 aircraft (not jets!), the optimal number of aircraft. However, the size of carriers you can build is limited early on. The size limit of your carriers increases with every Shipboard Aircraft research level. By about ~1930, you may have only unlocked the second size increase (23100 > 25600 > 28000). This provides just enough space to fit 100 planes, but leaves little room for additional equipment and does not allow for jet conversion. If you wait until the third size increase (28000 > 30400) in ~1931, you can build more future-proof carriers.
That said, carriers under 30kt are still perfectly capable of serving the role of escort carriers which provide CAP and free up space on your strike carriers.
These could follow the carrier refit schedule until 1944, but I find it is only necessary to add catapults/lifts to improve sortie rates and delay any more refits until the 1952 jet-conversion. This is because I find the 1944 upgrades not so urgent for such a modern carrier.

Although the 2000t destroyers were large enough to fit 6 quadruple swivel mounts, the extra displacement allows for longer range which improves their participation in battle. Additionally, this makes future conversions to carry SSMs more viable. Also, note that 2500t is the maximum size of a corvette, so any larger hulls will not be able to be converted into missile corvettes in 1969.

You get deck catapults and edge lifts around this time. Refitting your old carriers is critical to improve your carrier effectiveness and delaying construction for these upgrades is worth it. Additionally, if you have it, make sure you equip your carriers with the latest TPS.
Additionally, with an increased carrier size limit, It might be tempting to build new carriers in preparation to be converted to carry jets in 1944, but I find it isn't worth building these so early when you aren't getting attackers or HJFs until 1953, especially when 100 non-jet aircraft can already be fit onto a 30kt ship with plenty of room for equipment.

Quintuple launchers are unlocked this year. This is just about the last torpedo destroyer you will be building. You can get away with not building them, but keeping up your destroyer numbers is important.

In the years prior, you unlock a large number of radar techs. I recommend putting radar research on high priority with everything else on low, simply because the density of techs is so high from 1939 to 1944. This should give all of your ships up to level 4 radar (search and fire control). If you delay until 1945, you can get improved AA on your ships as well, so any ships with lots of AA guns should begin their refits after these upgrades are unlocked after ~1945.

Carrier-borne jets are unlocked in 1944 as well. If you have any carriers over 40kt, it's worth upgrading them, but I usually don't have any. What's more important is replacing your escort carrier fighter squads with LJFs.

Although heavier, I find they are much stronger than torpedoes. This is a good use of your spare income during this interim time until your first new jet carrier in 1950.
If you have extra space, keep some torpedoes and a gun or two, as they are still useful for finishing off enemy ships.

The CV size limit is increased and angled flight decks are unlocked. Although jet-capable carriers are enabled in 1944, I think it's worth waiting until 1950 before building purpose-built jet carriers unless tensions are high. This is because I think jet strike carriers are only worth using after ~1953 when attackers and HJF are unlocked.
Somehow, a 44kt hull (36 mo. construction) can carry 100 jet aircraft.

Although these things can be pretty useful, they become obsolete after 1960. While they can still function in battle just fine, they are too high-risk for the mediocre benefit they bring, especially with how close the enemy ships can start in the battle generator. The game essentially ignores the strategic and operational levels of military/naval combat for game simplicity reasons. Thus, it is only "viable" to construct tiny destroyers beyond that point.

Steam catapults are researched this year. Although it may be tempting to turn your 30kt carriers into strike carriers after the refit, you're better off keeping these as dedicated CAP ships.

A "modernization" of submarines will make all preexisting ASW ships obsolete in 1955, thus it is critical to rebuild your ASW fleet as soon as possible. I generally scrap all of my old corvettes or convert some of them into minesweepers. Additionally, when designing these ships, ensure that there is extra displacement available for the addition of helicopters in the future (~1958).

Again, remember that ASW is almost completely useless outside of trade protection, so don't bother putting any ASW equipment on your regular ships.

I don't bother building new SSM ships prior to this because they cannot be radar guided. Once blind-fire and double mounts are researched, I build my first missile ships. I place as many forward-facing HSSM mounts as I can on a 3500t hull with a single below-water torpedo tube (for DD designation without using topside space). I generally place wing mounts first because they weigh less than centerline mounts (at the cost of a reduced launch angle). I also do not equip my ships with reloads; I find them slow and a waste of space that I could be using for more tubes. Additionally, if you plan to reload your SSM tubes, always set the number of reloads per tube to 1; the game allows you to edit this value but this will not actually give you more reloads. Remember that these ships also play a crucial role as meat shields in battle, so I find they are not worth protecting with AA at the cost of a reduced loadout of SSMs. Any MSAMs/HAA would likely lack the range to protect your carriers anyway, as they will be moving farther ahead towards the enemy.
If you can, refit these in 1958 with helicopters. as they are important for spotting to extend the range of your SSMs.

Mounting all of these SSMs on the front probably makes these ships rather frontheavy and unstable. Eventually I might get around to making custom graphics for these to make them more realistic, but I can't say I'm terribly motivated when the game is already somewhat unrealistic.

By the 40s, your battleships will be mostly serving as makeshift AA platforms, but are still useful for finishing off enemy ships. However, with the advent of missile destroyers, these are finally fully obsolete. While you could simply scrap them, I like to extend their useful life as AA ships; they have lots of space for SAMs and have enough displacement and armor to tank quite a few missiles in stead of your valuable carriers. If possible, delay these refits until 1959 to get dual-mounted SAMs.

Defending your carriers with these ships also becomes much more valuable after ~1974 once SSMs start ignoring smaller ships like destroyers. In those times, having additional large-displacement ships next to your carriers reduces the chance they hit your carriers.

Pretty much the same as the other one except you get a helicopter. Refit your old ones if you can.

By around 1960, you will have researched some crucial technologies, namely quadruple HSSM launchers (1959) and cruise missiles (1960). The latter allows your ships to share targeting information, making your missile salvos more potent, not to be confused with "Tactical Data Systems" tech (1963) which allows aircraft to do the same. You also get Alu superstructures and gas turbine engines, both of which are must-haves.
As you can see, this 3800t destroyer can fit 20 forward-facing SSMs. Unfortunately, only up to triple mounts are allowed on non-centerline mounts, but they are still lighter than centerline mounts, so they are worth using. Additionally, although helicopters become unnecessary after you build dedicated targeting ships, I still find them moderately useful so I tend to keep them around.

The role of this ship is to scout far ahead of my line ships to provide targeting and bait missiles. These also serve as the regular surface combatant of your fleet after your missiles are expended once you run out of old destroyers to throw into the meat grinder.

A cheap MSAM platform to protect your ships, including your SSM blob.
Probably the lowest priority out of the three destroyers you should start building in 1960, simply because you already have some amount of AA for your carriers.

Generally, I think quadruple HSSMs are better than dual MSSMs. However, if you have any ships where you could only fit double launchers, give them a refit. Do a full refit in 1966 once you get quadruple MSSMs. Keep HSSMs on older/slower ships for extra range.

Because they are still limited by speed, these don't fully replace destroyers. Instead, they serve as very cheap HSSM platforms that can launch with targeting data from your faster ships. If you have space, give them a SSM reload.

This is the end of the tech tree. Although it may seem silly to refit your ships for just a single CIWS improvement, it's critical for winning engagements until 1990. The final "meta" is as follows:

A swarm of guided missile corvettes (MTB squads at critical territories)
A number of fast, protected gunboats to serve as scouts for your line ships
A small number of AA screening ships for your main fleet
Cheap carriers to find the enemy fleet and possibly provide a limited first strike.

This strategy will allow you to win most confrontations, as the AI will still be building cruisers and carriers. Missiles, although limited in number, are very powerful and cannot be effectively stopped when dispersed among numerous ships.

A lot of these designs cater to my personal preferences. I've compiled these things here to avoid repetition in the guide.
I tend to be very conservative with armor, as armor is very heavy and becomes obsolete fairly quickly (without options to refit). I never armor my carriers or destroyers. Turret armor can be useful and can be removed in refits, but I still seldom include it in my designs.
On the topic of protection and damage control, I try to avoid building a secondary battery. If I want to place smaller guns on a ship (for example, HAA), I will place the minimum amount of secondary guns and use the tertiary battery. This is because tertiary guns cannot be hit for turret damage (they can be disabled by hull hits).
I swear by reliable engine designs at the cost of a decreased top speed. They are very useful in battles where I need to chase or flee from enemies. Regarding designs, I generally increase ship speed incrementally (following technological advances) without bothering to "match" speeds with my older ships. There's a point at which the increase in engine weight with increased target speed grows significantly (based on some hidden engine technology value); I never go beyond this value.

Unit machinery is also very useful, not only for the basic damage reduction, but also for the reduced speed penalty with engine damage. This makes chasing or escaping from enemies much easier.
Contrary to popular belief, older ships do not get more expensive to maintain (excluding random events). They can become less worthwhile to maintain relative to newer ships, but when left alone, they will simply be the same old ships as before for the most part. What usually increases maintenance costs is refits. Generally, it's bad practice to constantly refit your ships and it's even worth delaying ship procurement to avoid refits shortly after completion.

The exception to this is fire control. If you unlock new fire control technologies while a ship is under construction, you will be allowed to fit them with no refit penalty. Because of this, you can start building ships shortly before you expect to unlock upgraded fire control, although a more practical consequence of this is that you can simply ignore fire control upgrades when it comes to building ships and not worry about refitting until they're super out-of-date or something else becomes worth upgrading (like engines or guns).

Additionally, I've learned that most players like to constantly scrap old ships. Personally, I'm a bit of a ship-hoarder (perhaps because I mostly play smaller nations). I find that refitting ships does not reduce my readiness, as it can be done fairly quickly, and helps to grow my fleet size. The AI does not know the difference between a 30kt BB from 1898 and 1938 until it's within shooting range, so old ships can be very useful for blockading, supporting invasions, and dissuading AI confrontations.
I believe submarines are usually a total waste of resources. Build 3 (to avoid the research penalty) and never think of them again. They can be very viable if you plan your entire game around submarine use (i.e. building hundreds of submarines and maxing out your prestige to enable unlimited sub warfare doctrine), but otherwise they are not useful. Modest numbers of submarines provide very little VP or tactical value compared to an equivalent quantity of ships and can be countered with a very small number of very cheap ASW corvettes. Despite this, the AI will still make submarines and ASW ships (even if you don't have any submarines), giving you a material advantage.
As much as I would love to build a fleet of heavily defended/armored, missile-equipped, large-displacement ships (like the Kirov), RTW3 severely limits the number of missiles you can put on a ship (I guess because the game was made by Americans and their Iowa modernization was a disaster). This means that not even small missile cruisers are preferable to destroyers. It also doesn't help that you never unlock VLS (once again, probably because Americans love arm launchers); the Kara class (my beloved) from 1978 fits a modest 48 vertical HSSM and 8 forward-facing HSSM. Also see: 1950 Aircraft Carriers