"主桅支索" can be translated as "mainmast shroud" in English.
In nautical terminology, a "shroud" is a rope or cable that supports a mast from the side, and "mainmast" refers to the primary or tallest mast on a ship. Therefore, "mainmast shroud" accurately describes the "主桅支索" (zhǔ wéi zhī suǒ), which is a cable or rope supporting the mainmast from its sides.