33 C.F.R. § 117.661
The draw of the Duluth Ship Canal Aerial bridge, mile 0.25 at Duluth, shall open on signal; except that, from March 16 through December 31, between the hours of 7 a.m. and 9 p.m., seven days a week, the drawbridge shall open on the hour and half-hour for vessels under 300 gross tons, if needed; and the bridge will open on signal for all vessels from 9 p.m. to 7 a.m., seven days a week, and at all times for Federal, state, and local government vessels, vessels in distress, commercial vessels engaged in rescue or emergency salvage operations, commercial-assist towing vessels engaged in towing or port operations, vessels engaged in pilot duties, vessels seeking shelter from severe weather, and all commercial vessels 300 gross tons or greater. From January 1 through March 15, the draw shall open on signal if at least 12 hour notice is given. The opening signal is one prolonged blast, one short blast, one prolonged blast, one short blast. If the drawbridge is disabled, the bridge authorities shall give incoming and outgoing vessels timely and dependable notice, by tug service if necessary, so that the vessels do not attempt to enter the canal.
33 C.F.R. §117.661