1. Cruisers? Very difficult to make into any real aircraft carrier, simply not big enough. Several battleships/battlecruisers (especially battlecruisers, because of their lighter/faster builds) were built into very successful aircraft carriers after the 1922 treaties. The USS Lexington, for example, was laid down as a battlecruiser, as was the Japanese carrier Akagi, and the IJN Kaga was laid down as a Tosa-class battleship in 1920. Going even eariler, the HMS Furious and Glorious were laid down as battlecruisers. There was another type of conversion, the hybrid carrier, where half of the ship was a flight deck while the other half (usually the front) still had turrets. The Furious was originally a hybrid carrier before its complete conversion, and late in WW2 the Japanese converted the Ise class battleships to hybrid carriers, replacing the rear turrets with hangars and flight decks.

2. Different types of damage, as you can imagine. Heavily armored targets, such as battleships, tended to be less vulnerable to bombs than to torpedoes. It was torpedoes that crippled and sank the Yamato, Prince of Wales, and Repulse, with bombs having a secondary effect. Against more vulnerable targets, such as carriers, bombs could be devastating and easier to deliver. All four of the Japanese carriers sunk at Midway were with bombs, and with the weaker armor of the carriers this damage led to secondary explosions, burning fuel stores, holed flight decks, etc.

3. It mostly depended on the type of aircraft used, and if any were multi-role. US Navy squadrons were split into different types, VF- for fighter, VS- for scout, and VT- for torpedo, and typically carriers carried at least one of each. The roles could overlap however. Fighters, especially late in the Pacific war, could carry air-ground ordinance like rockets and bombs. The scout squadrons were actually mostly dive-bombers, and the torpedo squadron aircraft such as the Devastator and Avenger could also carry different types of air-ground ordinance like bombs and rockets. The Japanese navy had a similar squadron system.

4. Scouts would typically be catapult-launched float planes from battleships and cruisers with the fleet, as well as attack/bomber aircraft from the carriers themselves. For example, during Midway the Japanese fleet used F1M floatplanes from cruisers like the Chikuma, as well as B5N attack aircraft from the carriers to a lesser degree.

5. A great deal of thought was put into search patterns, trying to provide maximum coverage. I can't comment too much about different navies, however, but I can point you to a book called Shattered Sword, by Jonathan Parshall and Anthony Tully. I don't have it with me now and can't give you a page number, but there is a whole chapter about the Japanese search patterns during the battle, complete with diagrams and plots, as well as the impact the search patterns had on the battle itself.

6. Absolutely. Daily searches, constant fighter patrols, and weather flights were all routine whenever the carrier was at sea. Scouts, like before, would be long-range aircraft like the floatplanes and bombers, while fighters would handle the standing defense patrols.

I would highly recommend you read Shattered Sword, not just for question 5, as it covers pretty much every question you asked in a very readable style.

Oh mein gott, you want the world!

Let's try and start from the top:

1.Cruiser Conversions

There were two types of cruiser conversions: as an archetype for this, we'll use the US Independence class of CVL (Light carrier) and the Japanese battlecruiser conversion of Akagi.

The Independence class were a series of cruisers that were converted to light carriers due to a more immediate need for carriers. As such, they were based off of completed ships, which would remove much of the superstructure, guns, and so forth, leaving the hull as a basis. On top of this hull was added hangar bays and a flight deck. The ships were sometimes too small, so they added blisters to the ships to make the hull larger. This resulted in a rather bulky, small carrier that was able to keep up with the high speeds of purpose built carriers (due to the cruiser chassis).

On the other hand, you also had battlecruiser conversions. The Akagi was laid down as a battlecruiser along with her sister Amagi, but due to the Washington Naval Treaty she was, as several people have mentioned, converted into a carrier. Because her hull had already been mostly built, but not the superstructure or other parts emblemic of a surface combat ship,, the transition was relatively smooth, adding hangar bays and three flight decks. However, she was a bit more sluggish than the purpose built carriers Shoukaku and Zuikaku later on, as a result.

2.Torpedo bombers vs. Dive bombers

I'm going to start mostly focusing on Japan as that's what I know more about. The Japanese mainly used the type 99 "Val" and the Type 97 "Kate" as their main dive bomber and torpedo bomber, respectively, in WWII. Throughout the course of the war, the "Kate" was replaced by the "Suisei," or Judy, and later in the war they introduced the "Tenzan" (Jill) and "Ryusei" (Grace) torpedo bombers. While several different models of the Suisei existed, including at least one for reconnaissance purposes, most of the planes were purpose built, i.e. a plane built for a specific type was used for a specific role. For instance, the A6M5 "Zero" was predominantly a fighter craft, although there were some submodels used for other purposes, such as the Type 62 Zero, which was designed for use as a carrier-borne fighter bomber. Another interesting tidbit was that the Ryusei was the only Japanese torpedo bomber to have forward-firing cannon, as it was not intended for these aircraft to get into a dogfight scenario, as opposed to the Suisei dive-bombers, which did have forward firing cannon. Japanese carriers also had designated scout squadrons. Later in the war, they developed the C6N "Saiun" as a designated scoutplane, although it was primarily used on land due to a lack of operational flightdecks.

Dive bombers, while as mentioned were the main cause of the sinking of the Japanese carriers, were very strong against the Japanese fleet. This was due primarily to the fact that the Japanese aircraft carriers were unarmored, and that surface-to-air fire was very inaccurate, especially against a diving plane. The British carriers-for instance Illustrious-had armored flight decks. The idea was that a dive bomber releases its bomb at a certain angle, and that by armoring the flight deck, the bomb would bounce off the armor rather than penetrate the deck, and thus roll or fall off the carrier without doing damage. In fact, the Japanese later created a carrier with this in mind, Taiho.

Torpedo bombers, while significantly more effective against armored ships, suffered from having to attack at a low altitude and a near 180* angle. This made them hideously vulnerable to flak as well as to fighters from a carrier air patrol, as these fighters would be able to dive on them, using the sun to help obscure their presence from an enemy pilot until it was too late.

3.Scout planes

As mentioned the Japanese typically used a combination of a perimeter of floatplanes launched from battleships and heavy cruisers, as well as special reconnaissance and land-based reconnaissance aircraft.

Most of the other topics have been pretty well covered so I'll leave it at that for now.

Weirdly, I just submitted this post over in /r/WarshipPorn. I'll copy and paste it here to amplify a bit of what /u/MeneMeneTekelUpharsi had to say. I also second his/her recommendation of Shattered Sword to understand Japanese carrier doctrine.

Well, the early US carrier fleet was limited in tonnage by the Washington Naval Treaty. When it was put in force, Lexington and Saratoga had been laid down as battle cruisers, and the treaty allowed them to be completed as aircraft carriers. They were fairly large carriers for the time (36,000 tons nominal displacement). At around the same time, the US built the Ranger as a purpose-built aircraft carrier; it was quite a bit smaller, at only 14,000 tons, and slower (29 knots, compared with 33 knots) than the Lexington class.

US experience with those carriers showed that the larger, faster carriers would be quite a bit more flexible in a war situation than carriers the size of Ranger (which in the event spent most of WWII in the Atlantic). Both classes could carry a similar plane load, but the Lexington class could keep up with US cruisers, and developed more relative speed for launching heavier aircraft.

So, the next four carriers the US built under the Washington treaty were the Yorktown class (Yorktown, Enterprise and Hornet), and the Wasp which was a smaller one-off class. The Yorktown class of carriers were 20,000 tons and developed 32.5 knots at top speed, with a carrying capacity of around 90 aircraft. Yorktown and Enterprise were the first two built, while Wasp was a scaled-down and relatively unsuccessful version of the same class (about 50 feet shorter and 7,000 tons lighter, and 3 knots slower). After the Japanese abrogated the Washington Naval Treaty in 1936, the Hornet was built to the original Yorktown design while the US started planning the Essex class.

So the Essex class carriers were planned to be 27,000 tons, but as built they totaled about 30,000 tons nominal displacement, a top speed of 32.7 knots and a carrying capacity of 90-100 aircraft. The US ordered the first three in mid-1940, from the Newport News shipyard, and another eight in September 1940, and three more on 15 Dec. 1941. All of them were built on the East Coast, at Newport News, Fore River and the Brooklyn Navy Yard.

The first of the class, USS Essex, was laid down 28 April 1941, launched 31 July 1942 and commissioned on 31 December 1942, so from keel laying through commissioning was about 20 months. Interestingly, in the US navy, air wings were not connected to carriers (they were and are separate organizational structures), so once the new carriers shook down and were accepted, they could start combat operations quite quickly (with a learning curve, of course).

So, during WWII and through the end of 1945, the US ordered 32 Essex-class carriers, of which 26 were actually laid down and 24 commissioned. Several were canceled before being built, and two were scrapped; one (Oriskany) was rebuilt to be an experimental carrier.

How difficult was it to convert cruisers into carriers (presumably as a result of the 1922 Washington Treaty)? What work would have to be done?

Generally uncompleted battlecruiser hulls (about the same size as a battleship) were used instead of cruisers. Since these were never quite completed as BCs they required most of the same construction as purpose built CVs. Kaga, Akagi, Lexington and Saratoga all fit this profile, as well as some British carriers. These ships required about three years to complete the conversion at a peacetime 1920s pace.

How effective were converted cruiser-carriers compared to carriers built as such from the ground up?

Generally aircraft stowage and elevators were a bit more awkward, but this wasn't a serious problem. The converted BCs were roughly as effective as purpose built CVs, especially considering they were more than a decade older ships.

How effective in attacking ships were torpedo bombers versus conventional bombers using armor piercing bombs?

This question is rather complicated. Basically the answer is that both were fairly effective. Torpedo Bombers were perhaps more effective against heavily armored Battleships, especially if not moving (as at Pearl Harbor and Taranto) but Dive Bombers with anti-ship bombs could be very effective at attacking into the teeth of air defenses as at midway and other carrier battles. Torpedo Bombers are also larger aircraft that reduce the numbers of planes a carrier could field.

What proportion of a carrier's air wing would be fighters versus torpedo bombers versus conventional bombers (and were these last two interchangeable)? Did different nations prefer a different mix? Would the mix be different from carrier to carrier in the same nation's fleet?

This question is also VERY complicated, but the best way to put it is that at the start of the war proportions could be something like 25% fighters, 50% divebombers/scouts, 25% torpedo bombers. Over the course of WWII the proportion of fighters steadily increased. Generally a mix of aircraft was used to give tactical flexibility, in case carriers needed to operate seperately. Specialized carriers began to appear in US forces (i.e. for night operations) as the number of flight decks increased at the end of the war. Also, as Anti-submarine operations became more and more important in the Atlantic, air groups became more specialized to that role.

Which aircraft type would be used to scout for the enemy (fighter, torpedo, bomber, dedicated scouts, other)?

Generally the U.S. used dive bombers and specialized aircraft. The Japanese Naval aviation used cruiser based float planes, especially from Tone and Chikuma which often accompanied the Carrier Fleet (Kido Butai)

If no enemy was suspected to be nearby, would some aircraft always be in the air to scout just as a precautionary measure? What about always-in-the-air fighter screens to protect against enemy bombers (even if no enemy was suspected to be nearby), or would these two roles be filled by the same aircraft at the same time?

Scouting as a precaution was carried out constantly from dawn to dusk. Carriers would also have aloft a small Combat Air Patrol (CAP) of fighters to respond to surprise attacks. Fighters needed their fuel for potential engagements with attackers, so they would not scout. All this activity would increase if the presence of enemy ships or land based air were suspected.

I really just scratched the surface here, and I won't even try to tackle the last two questions. Here are some places you could find more info on much of this.

• http://www.combinedfleet.com/ - great info on the Japanese Navy on this site by well respected authors. Their books on Midway and Surigao Straight are also great. The bibliography page in particular is marvelous.

• I'll also recommend John Lundstrom's "The First Team" for a very wonderfully detailed account of U.S. Navy fighter tactics in the first year of the war.

I don't have nearly as much for you on the Royal Navy's carrier force. Perhaps another user will chime in.