I think the calculations are reasonable, if perhaps a little optimistic. OTL, once they cleared the area, the fleet cruised at 10kts and it is fairly consistent with doctrine that IJN in 1941 would plan on maintaining a 25-30% fuel reserve (like everybody else). Stability becomes a real issue at lower fuel levels. As a result, from a planning perspective the 24-day cruising range (at 12-14kts, available after re-fueling on 6 December) really starts at something like 18 days. As a result, they would believe (again maintaining "normal reserves") that they would compete their Day 1 operations with 13-14 "cruising days" remaining (not 20).
KB's cruising radius varied from ship to ship, as low as 5,000nm for destroyers (18 cruising days) to 10,000nm for three of the carriers and the battleships, (34 cruising days). The lighter carriers were about 26 cruising days, (Soryu and Hiryu burned all their remaining barrelled oil during the run to Pearl Harbor. Akagi, the third carrier with extra storage, will have done so as well. On the 24 hour run in, 3 carriers (Akagi, Soryu, Hiryu) and 2 heavy cruisers (Tone, Chikuma) had ranges issue. The other five capital ships, (Kaga, Shokaku, Zuikaku, Hiei, Kirishima) did not. Of the shorter legged ships, the 2nd division carried 1,400 tons of barrelled oil and the Akagi 1450 and the cruisers 580 tons each. This was sufficient, in all cases, to cover the consumption for the run into Hawaii. (For example, on the link use Saratoga CV3 for Akagi. At 24kt she burns 1100 tons, which is 300 tons less than Akagi's barrelled reserve).
Another planning factor is that nobody has a clean bottom for long, so consumption planning is biased toward higher reserve levels. Naval officers would generally consider anything less than 30% as literally "running on fumes". There is a real, practical reason why the tanks are sounded on a regular basis (each ship burns at a different rate for a variety of reasons causing actual consumption to frequently exceed planned consumption). The escorts are always a limiting factor due to their "shorter legs".
Fouling was a non-factor AFAIK - all the ships had been recently prepped for the operation and it was cold waters. Escorts were to be refuelled from capital ships if necessary, and the capital ships had been trained for the task.
All that said, I agree that he can run for the Marshall Islands with the fuel he has on board (which was his contingency plan IIRC), but believe he is taking a significant risk (more than a "bit riskier") by staying until the 9th. Even if he can in fact squeeze 14 days of "cruising" fuel out of what he has in his tanks, I do not believe Nagumo's staff will think he has that much time/range in the face of operational requirements and reserves.
Certainly should be fine for ops to the end of the 8th. A withdrawal to the Marshalls after that assumes no extended assault, but this was also an option - Nagumo would have to rendezvous with the tanker train and spend a day or two refuelling, then go back in.
As a result, IMHO he does not stay longer than required to launch a third strike (assuming he has enough aircraft to make that feasible), if he decides to launch a third strike at all. IMHO, a lot depends on when KB comes into contact with Halsey.
Nagumo's plan of attack suggests he had no intention of conducting a 3rd raid regardless of the results. Had he been interested in doing so, he would have ordered an adequate scouting plan for the morning of the battle.
I do not believe he has a tanker group in position to actually help during a run to the Marshalls.
The two major options for post battle were a withdrawal to the north, in which case the tanker train would be in position well to the north of Hawaii and not set up for a Marshalls withdrawal, or to pass Oahu to the west while continuing the assault, in which case the tankers would come down for a refuelling, depart to the north, and KB would pass down the west side of Oahu.
The first replenishment group must have limited supply (I have never seen any information on remaining supply after 6 December),
That's easy - KB had storage of about 45,000 tons and a range of something about 7,000nm. Getting to Hawaii was half of 45,000 tons, or about 23,000 tons. The eight tankers of the two groups had 80,000 tons storage so should have had about 57,000 tons remaining at the point Nagumo detached for the run south.
and the tankers are limited to 10kts.
16-19.5kt on the tankers selected for the mission.
No way they can catch Nagumo running southwest at 14-24kts (from their position northeast of Midway).
At 16kt the tankers could move 384 nm per day. I doubt Nagumo would want to bring them through the line between Midway and Hawaii unescorted by carriers though.
The second replenishment group has not yet left the home islands (IIRC) and has the same speed limitations.
No and no. The second group departed with the rest of the force on November 26th, and this group had speeds of 19.5kt for all three, (Toho, Toei, Nippon Maru).
