Complex sentence: Leaving home at the age of 15, he joined the barque "Rothesay Bay" as an appretice which operated in the Tasman Sea trading between Australia and New Zealand.
Split into simpler sentences: Leaving home at the age of 15, he joined the barque "Rothesay Bay" as an apprentice. The "Rothesay Bay" operated in the Tasman Sea trading between Australia and New Zealand.