"He was firing into them, while he shrieked to frighten them away." Suuuure, that's why.

Get out the Grapnel, boys - it's the end of the book, and time to wreck the ship for the insurance money.

Shrieking, screaming - Frank's a bit on edge lately. And I'm not sure how much peril they were in if the floating ice boat might have gone up up up up until it had been repaired, but perhaps the would have been ill-suited to fix it while gasping in the stratosphere.