Maybe someone else can? I will post on some other things that work for me though (your mileage may vary).Subs - Subs are an excellent unit for wreaking havoc on all sides. Their one huge advantage is that they don't use extended supplies to move (they only use supplies for more torpedos). For countries with limited supplies, they are a great buy. Moreover, the only answer to subs is to research ASW, something that in and of itself is a waste of research points as it has no other utility. If you are Germany or Japan, this is a huge advantage from the start.
It is also very costly to kill subs usually. A German sub can get to many transport fleets in the Atlantic, for example. The WA cannot kill a u-boot with a single light fleet, they need at least 2 and to be sure, 1 air + 2 light fleet. That is a very expensive proposition for the WA until they research ASW since they don't have enough of either early on and thus have to make extended movement to get the CAG and light fleets into range. There are also only 2 realistic ways of killing subs (light fleet ASW and CAG ASW).
Subs with good evasion and torp attack are also great for killing other kinds of ships besides transports. Heavy fleets especially are vulnerable to subs. Carriers as well but that is a bit more risky. On the other hand, subs are the cheapest and faster combat ship to build, so they are a good attrition unit (4 turns versus 6 for light fleets).
I've found 3 ways to use subs that seem to give good returns:
1. Combined fleet. I do this when I play the WA. I will leave my subs with the main fleet for protection and then run them out in packs to attack transports and also use them in combined fleet actions. Torpedos ignore armor ratings and get you a lot of kills. In this roll, I'm not really using subs to kill transports (I do, but that is not their main mission), I'm using them to kill other fleet ships.
2. Wolf packs. Combine your subs into groups of 2-3 to attack shipping. This is safer than using single ships because your subs will have more survivability. Unless the enemy is keeping up with ASW, he will need larger fleets to take out your subs (will need 2:1 odds). You will sink less tonnage this way though cause you are also using 2-3 subs to take out single transports (unless they are grouping multiple transports in single areas). Your enemy will need to expend a lot of supplies to collect a large enough force to take out your subs.
3. Single subs. This is to harass allied shipping for the most part. You can either go out only far enough so you can return to port the same turn or you can send your subs out on suicide missions to make the enemy come after you, expending lots of supplies to do so. The first method will result in diminishing returns as the enemy will simply start protecting his transports, the second way will be more costly for you, but also for him.
Light Fleets - The major roll of the light fleet is to take out subs. The other major roll of a light fleet is to act as a covering force for your heavy fleets and CV groups. Ships like cruisers and destroyers manuever in front of a fleet (or around the fleet) to prevent enemy fleets from closing too soon. This allows heavy fleets to bring guns to bear in an optimal range as well as for CV to launch their CAGs. This is represented in the game by matching light fleets against each other before heavy or CV fleets enter the equation.
In game terms, what this means is that you want at least 1 light fleet for each CV you want to protect (or Heavy fleet). The major function of light fleets in this roll is PROTECTION. In other words, you don't usually want to send them in on the offense because they are easily damaged. Say you are the WA attacking the Japanese fleet in the middle of the Pacific somewhere. What you would do is to get next to the Jap fleet (or as far away as possible depending on the range of your CAG) and then send in your planes and possibly your heavy fleets. This will guarentee that you bring the maximum firepower to bear at the minimum of risk. But don't send in heavy fleets if there is a possiblity of subs around. If you are worried about fleet composition, then send in the CAG first, then followup with an appropriate force.
Heavy fleets - These represent BB and CA ships for the most part. They are the main firepower of a fleet. They also have a torpedo attack that is useful to increase to turn them into real killing machines. On the other hand, they are very expensive and take a long time to build. Probably not worth it unless you are the WA. The best role for a heavy fleet is to take out light fleets and carriers (and other heavy fleets). They are very vulernable to subs, however. I tend to keep them with my CV fleets for added protection. Historically, BB tended not to be grouped with CV in the same task forces. The USA later in the war DID group BB with CV, but generally as enhanced AA platforms. In other words, up their Air Attack to protect from CAG.
I generally never build new heavy fleets, I tend to group them all into a large task force with either a shore bombard mission or to kill enemy fleets. Heavy fleets are very, very expensive to move more than 1 sea area, so plan at least a year ahead of where you want them to be
Carrier fleets - These are the most powerful units historically (not necessarily in game unless you RESEARCH). It is very, very important that if you want to use CV fleets, never allow them to be damaged or, god forbid, sunk. What that means is either you dump a lot of research into stuff like evasion or you never put them in harm's way if you can help it. I prefer the 2nd strategy. What that means is that I tend to use large fleet groupings with a mix of forces (heavy fleets, light fleets and subs). The subs are used to attack any close enemy fleets if possible. This breaks up those fleets and inflicts some damage. If the close fleets are heavy with light fleets, then I don't do that, but then, I'm not really in danger then either.
Now, what makes a CV important and powerful is the CAG that is on it. If you don't upgrade your CAG, then it isn't worth building CV. Otherwise you have CAG sitting in the production queue all the time and your CV sitting there doing nothing. The most important thing to research on a CAG is evasion. This alone forces the enemy to do a lot of research to be able to hit your CAG. They either have to start researching ship to air or air to air on THIER CAG. Getting to range 4 will allow your CAG to fly 2 double bar sea zones away. That is huge because it leaves your main fleet relatively safe and it is costly to engage in (supplywise). Air-to-air makes sure your CAG survives the initial air-to-air fight to fire at enemy ships. Finally, torp attack is the best bang for the buck for taking out ships. Researching ship-to-ship and ASW is less important, but if you have the points, do ship-to-ship first. That will make your CAG able to sink ships in a single round of combat.
So, now that we have an overview of what ships do and what missions they fulfill, how to best use them and what to research? This is where country-specific strategies come into play. Lets say up front that only the WA can afford a "real" navy comprised of all different ship types. While other countries might be able to squeeze some heavy fleets or CV out, they can't keep up in the research. So what to do as Japan or Germany? The best idea for both of those countries is to focus on a few areas of research and only a few ship types. As noted above, submarines are the best bang for the buck. If you build enough submarines (fewer, upgraded ones are better) then the WA are forced to respond with either dispersed fleets (making them more vulnerable to your big fleets) or a lot more ASW and light fleet research. For new builds, it is probably better to just focus on light fleets that have upgraded torp and evasion. They will get hit a lot but also do some damage on their own.
Another thing to consider as an Axis player is to upgrade your land-based air unit range and torp attacks and then always leave your fleets in range of your air bases and then use your end of turn movement to place those planes in sea areas protecting your fleets. In this way, you are leveraging units you already need and build (land-based air) and also protecting your fleets without a large navy build and research investment. It is very important to keep your main fleets together for support because you cannot compete with the WA on the seas, it is always a defensive strategy.
