This is book 14 of the "Rick Brant" series.

This note contains spoilers for the book.

A Rick Brant science adventure, published as by John Blaine.

Apparently, a batch of Malay pirates, operating off a small island on boats. Also terrorizing villages and so forth.

And kidnapping Spindrift scientists who happened to have hired the boy they wanted to abduct. So we have sailing, and mountain climbing, and some more marksmanship.