33 CFR § 117.593 - Chelsea River.
(a) All drawbridges across Chelsea River shall open on signal. The opening signal for each drawbridge is two prolonged blasts followed by two short blasts and one prolonged blast. The acknowledging signal is three prolonged blasts when the draw can be opened immediately and is two prolonged blasts when the draw cannot be open or is open and must be closed.
(b) The draw of the Chelsea Street Bridge, mile 1.3, at Chelsea, shall open as follows:
(2) The 139 foot opening will be signified by a range light display with one solid green light and one flashing green light and the full 175 foot opening will be signified with two solid green range lights.