According to the Dictionary of American Naval Fighting Ships (at http://www.history.navy.mil/danfs/b7/bobylu-i.htm), the boat's speed was "7 miles per hour", a measurement of speed in statute miles per hour not generally used for boats or ships and perhaps a typographical error for 7 knots. If the speed really is given in statute miles per hour, the equivalent in knots is 6.1.
