33 CFR § 117.697 - Hampton River.
The SR1A bridge, mile 0.0 at Hampton, operates as follows:
(a) The draw shall open on signal from April 1 through October 31 for the passage of vessels during daylight hours from three hours before to three hours after each high tide. “Daylight hours” means one-half hour before sunrise to one-half hour after sunset. High tide occurs one-half hour later than the time of high tide for Portland, Maine, as published in the tide tables published by private entities using data provided by the National Ocean Service. At all other times, the draw shall open on signal if at least three hours notice is given.
(b) The owners of the bridge shall provide and keep in good legible condition two board gages painted white with black figures not less than six inches high to indicate the vertical clearance under the closed draw at all stages of the tide. The gages shall be so placed on the bridge that they are plainly visible to operators of vessels approaching the bridge either up or downstream.
(c) Vessels which can pass under the closed draw with a clearance of one foot or more shall not signal for the opening of the draw. In case a vessel gives the prescribed signal and the drawtender is uncertain as to whether the vessel can safely pass, the drawtender shall open the draw. If the drawtender finds that there would have been a clearance of one foot or more had the draw remained closed, the matter shall be reported immediately to the District Commander, giving the name of the vessel, the time of opening the draw, the clearance under the bridge as indicated by the gage at the time of opening the draw, and the approximate vertical clearance required by the vessel.