        hdot    angular velocity of heading in degrees/minute
        v       velocity of ship in knots
    Action space (continuous):
        rudder  angle of rudder
        thrust  propulsion of ship forward
    """

    # some (more or less) physical constants
    dt = 4.        # simulated time (in seconds) per step
    mass = 1000.   # mass of ship in unclear units
    I = 1000.      # rotational inertia of ship in unclear units

    def __init__(self, render=True, ip="127.0.0.1", port="21580", numdir=1):
        # initialize the environment (randomly)