What sound signal indicates a sailing vessel is underway in the fog and how frequently?

Rules of the Road

Rule 35

Sound Signals in Restricted Visibility

(C) A vessel not under command, a vessel restricted in her ability to maneuver, a vessel constrained by her draft, a sailing vessel, a vessel engaged in fishing, and a vessel engaged in towing or pushing another vessel shall..........sound at intervals of not more than two minutes, three blast in succession, namely one prolonged followed by two short blasts.