Title: Mission of gravity
Description: From back cover Del Rey paperback January 1978:

CAPTAIN COURAGEOUS

For a profit -- and adventure -- Barlennan would sail the Bree thousands of miles across uncharted waters, into regions where gravity itself played strange tricks.  He would dare the perils of strange tribes and strange creatures -- even dicker with those strange aliens from beyond the skies, though the concept of another world was unknown to the inhabitants of Mesklin.

But in spite of the incredible technology of the strangers and without regard for their enourmous size, Barlennan had the notion of turning the deal to an unsuspected advantage for himself... all in all a considerable enterprise for a being very much resembling a fifteen-inch caterpillar!
Subjects: Science fiction, American Science fiction, Fiction, science fiction, general