Due to a combination of the threat from CAF aircraft, US Navy PT boats stationed at Tulagi, and a cycle of bright moonlight, the Japanese had switched to using submarines to deliver provisions to their forces on Guadalcanal. Beginning on November 16, 1942, and continuing for the next three weeks, 16 submarines made nocturnal deliveries of foodstuffs to the island, with one submarine making the trip each night. Each submarine could deliver 20 to 30 tons of supplies, about one day's worth of food, for the 17th Army, but the difficult task of transporting the supplies by hand through the jungle to the frontline units limited their value to sustain the Japanese troops on Guadalcanal. At the same time, the Japanese tried to establish a chain of three bases in the central Solomons to allow small boats to use them as staging sites for making supply deliveries to Guadalcanal, but damaging Allied airstrikes on the bases forced the abandonment of this plan.