Going back to my slow roots in figuring out how to make things work as I plan my next BAT project... If you have had some difficulties, here is what I found to work for me.
IJN I-400 Submarine & Aichi M6A1 Seiran
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Historically (even if '46-ish) it could carry 3x Seirans, but in latest BAT, they dont work well, the second plane falls in water, and 3rd, 4rth fly. AI when it is in the player plane is buggy. I tried changing Carrier.ini options but does not seem to work. The concept is that the I-400 is a carrier. The takeoff is hard, but the landing works well, it lands on the side of the Sub. Make sure you use the TAKEOFF and LANDING target options for carriers.
For pure AI Seiran (only one airplane):
* Static surfaced I400: 40% fuel and any load with Bumpy takeoff (floats go into water), use less fuel for smoother takeoff
* Moving surfaced I400 at max 44Kmh speed: 40% fuel and any load
For Player Plane (only one airplane) using Autopilot to takeoff:
* Static surfaced I400: 25% fuel with no load. You will most likely lose your floats, so you can crash land. Using any load will have bad consequences.
* Moving surfaced I400 at max 44Kmh speed: Default load, at 50% or less fuel; do not use any other load.
* BEST RESULTS, DO NOT USE AUTOPILOT for TAKEOFF in PLAYER PLANE:
-- Fuel at 40% for any load
-- Use only one plane
-- Unfold wings (Ctr-W)
-- Flaps on Takeoff (V, V)
-- Open Cockpit (Ctr-C)
-- Throttle on overcharge (All the way forward)
-- Wait a few seconds
-- Release Chocks (Alt-C)
-- Maintain control of plane and ease on elevators
-- You may dip a bit and even touch water, that is why it is important to maintain a slight positive angle of attack
-- If even cretins like me can do this, so can you
Oh, not sure if it is known but the Kawanishi E15K can also take off from the I-400 with any load at 40% fuel. I tried other seaplanes but they all crashed/exploded on takeoff.