Answer:
Ned's plan once the Nautilus had entered the Mediterranean was to use the life boat to escape.
His plan was that they would escape when the Captain was in bed.
Ned was to be at the central companionway with Conseil while he told Aronnax to wait in the library until he receives a signal from Ned.
Ned had taken a monkey wrench with which he was going to unscrew the nuts bolting the life boat, which he knew how to operate, to the Nautilus. The action was meant to propel them to reach the surface of the water fast without alerting anyone
Step-by-step explanation:
In Twenty Thousand Leagues Under the Sea Ned explained how they were to escape from the submarine in the lines:
"In that case, I should seek to make myself master of the pinnace. I know how it is worked. We must get inside, and the bolts once drawn, we shall come to the surface of the water, without even the pilot, who is in the bows, perceiving our flight."
However his plan to escape the Nautilus while in the Mediterranean was made impossible, as it went entirely too fast the entire time it was there.