Hello, although I have been visiting your forums for a couple of years, I am new to posting on forums so I apologize ahead of time if I make mistakes.
This post is intended to possibly clarify and answer some questions about why the SeaHurricane was not adopted before the war. It may be a bit long-winded for forum purposes - if so please advise me.
First I would like to stress that the need for a second seat in any carrier based aircraft was not outmoded as such when the US entered WW2 and for the same reasons was not outmoded at the end of the war. The reason for my stating this is as follows:
Prior to WW2 the USN generally prohibited single-seat aircraft from flying over-the-horizon unless they were heading toward large land masses (in daylight), or unless they were accompanied by one or more two-seaters with better navigation capability (in daylight). This was due to numerous serious failures in navigation that sometimes endangered entire squadrons of aircraft, and did result in the loss of a significant number of smaller flights of 1-4 aircraft. Night flying was generally avoided also (other than a minimum amount for training purposes to familiarize the pilots with the dangerous aspects of night operations in case of unplanned situations) due to the recognition of the inherent dangers in terms of landing accidents and failures in navigation. When the USN entered WW2 the official doctrine stressed avoidance of night operations for these same reasons.
The RN/FAA on the other hand had adopted a policy/goal in 1935 of becoming fully night operations capable. This was primarily in order to accomplish the FAA's missions of striking ship targets in harbor, and other shore and near-shore targets, while being relatively immune to enemy retaliation. In order to be able to accomplish the mission the crews needed to be able to navigate at least most of the flight without landmarks while in the dark. This required above average skill in navigation, as well as in night time launch and recovery operations. The RN/FAA found that the navigation skill required was not difficult to instill in the pilot, but that the pilot had a difficult time navigating and piloting at the same. This was partly due to the navigation equipment available at the time and partly due to the difficulty of multi-tasking. With the advent of more capable radios and navigation equipment in the mid-1930s, reliable relatively long range navigation at night became viable, but only if combined with a second crewman. The multi-seat Swordfish with its (for the time) advanced 3-in-1 radio set was the first operational carrier borne aircraft considered suitable for long range strike and reconnaissance by day or night.
By the beginning of WW2 the RN/FAA had the problems mostly worked out and the remaining finer points were hammered out in time for the strike on Taranto and the action vs the Bismarck. With the advent of shipborne and airborne surface search radar the mission to strike ships at sea at long range and at night had become a practical mission.
By contrast, the USN did not focus on improving their night operations during the early war, and were not as concerned with more advanced navigation skills (what might be called blind flying techniques today) as the RN/FAA. This is not to say that the USN was not aware of the benefits of having multi-crew aircraft, it was more a matter of the carrier doctrine adopted, and then prioritizing the type of aircraft put into service, and the crew training within the time available. For the seizing of local air superiority during a strike at the ranges originally imagined, single seat fighter aircraft were considered the best answer, the second crewman requirement imposing unacceptable performance penalties. And if a type of training did not directly go to accomplishing doctrine it was given secondary importance or set aside altogether. Overall this attitude served the USN well in the PTO, with the following exception: during the war 16% of the losses of USN carrier aircraft (that launched) were lost due to navigation error (often referred as 'fuel starvation' in the reports), regardless of whether they were assigned to CAP, strike, or reconnaissance, regardless of whether they were launched in clear skies, partly cloudy, or full cloud cover, and regardless of limited visibility (ie hazy) or perfectly clear skies. (The previous statement is based on a USN post-war analysis of USN&USMC aircraft losses in the PTO. I have not been able to find any kind of numerical breakdown of what % of the losses was during what type of mission or of what type of aircraft. The only qualifiers in the report were that while a small but possibly significant % of the losses could be due to mechanical failure, or pilot error - with a very small % possibly due to enemy action - the only available evidence pointed to the vast majority of these losses being due to navigational problems. It was also stated that the 16% value remained more-or-less constant throughout the war. The report stated the stark contrast to the RN/FAA experience, with less than 2% losses due to navigation failures under similar conditions.)
The USN was aware of and alarmed by the losses quite early in the war, but the carrier operational doctrine was working and that was considered paramount, the losses would just have to be accepted.
The Japanese IJN, as mentioned above, also adopted single-seat fighters. They managed the navigation problem through an extremely stringent screening, selection, and training program where navigation was concerned. But even the IJN, with what could be considered far above average flight crews, had problems with losing aircraft due to navigation failures. At the start of the war when the air crews were all 'elite' there were relatively few problems, but as the war progressed the problem increased as more new less well trained air crews entered service. However, even at the start of the war, the IJN squadron and flight leaders were often selected as much for their superior navigation skills as for their leadership qualities.
Just pre-war the RN/FAA carrier doctrine focused on two types of fighters, the single-seat day fighter, and the two-seat long range fighter/reconnaissance aircraft.
The only RN/FAA operational single-seat carrier fighter in 1939 was the SeaGladiator, but it was of limited value due to small fuel load, the previously mentioned navigation problem, and low maximum speed. It had a fuel load marginal for CAP duties and unacceptable for escort or reconnaissance, could not navigate reliably at longer ranges due to the lack of the second crewman, and was considered too slow to intercept the new generation of fast bomber coming into service.
The Hurricane and Spitfire were in land service in limited numbers, and while both types would have solved the speed problem, and could have probably solved the range problem through the use of drop tanks, both were single-seaters which prevented a solution to the navigation problem. Hawker offered a folding wing navalized Hurricane in late-1938 and was turned down by the decision makers for reasons I am not familiar with. Supermarine first offered a folding wing navalized Spitfire in 1941(?) but was turned down due to perceived production issues.
The Fulmar was intended and procured as a temporary solution to meet the need for the two-seat long range fighter/reconnaissance type while the Firefly was developed. It was originally hoped that a speed of just under 300 mph would be achieved, solving the speed problem mentioned above. It had enough fuel to function as CAP, escort, and reconnaissance, and it had the second crewman to help with navigation. As it turned out the actual top speed (247-265 mph) and sustained climb rate (1200-1400 ft/min) were marginal. In spite of its limited performance the Fulmar shot down more enemy aircraft than any other RN/FAA carrier fighter type.
The Firefly was the original intended two-seat ship-board fighter. If it had entered service in 1942 when originally planned it would have met the requirements of speed, range, and navigation. Delays in development of the Griffon engine, due to the rationalization program of 1940, caused a 1-2 year delay in the Firefly's operational status.
I do not remember the exact range requirement of the FAA navigators operational test, but the qualification test required a flight of over 400 nm total, composed of 3 or more legs, over open ocean, arriving at the destination within a 25 nm radius. The 25 nm radius was considered the maximum range for reliable location finding of the carrier using the Type 72 homing beacon.
One more factor I feel should be mentioned is a problem with intercepting incoming aircraft. Pre-war/pre-radar exercises by both the USN and the RN/FAA revealed that even if there were 3 flights of 4 aircraft each (12 total) at different altitudes (ie above, same, and below the altitude of the incoming enemy) and the weather was clear, there was only a 25% chance of intercepting any particular group of aircraft before they released weapons. During the early war, with radar, the chance of intercept with the same CAP rose to only 40%. It was not until the end of the war in the PTO that the majority of incoming formations/individual aircraft could be reliably intercepted, but it was only possible because of greatly improved radar and radio equipment along with much higher numbers of CAP aircraft. Even during the last of the Japanese air operations against the USN fleet, it was not unusual for some formations/individual aircraft first be spotted by lookouts on the ships, not by radar, and hence there was almost no chance of aerial intercept. This problem of successful intercepts was a big part of the RN/FAA decision to not focus on single-seat single-purpose fighters for CAP duties.
[edited for clarity]