Complex sentence: The "Islander" was the 34-foot yawl that Harry Pidgeon sailed around the world single-handed in 1921-1925, becoming the second person to do so after Joshua Slocum.
Split into simpler sentences: The "Islander" was the 34-foot yawl with which Harry Pidgeon sailed around the world single-handedly from 1921 to 1925. Pidgeon thus became the second person to do so after Joshua Slocum.