Draw a line from the port quadrant of the circle to the starboard quadrant. Use this line to trim the forward half of the circle away. This will leave a semi-circle that can be extruded along the forward profile curve to form the first rounded surface. Extrude the circle along the mast profile curve a distance greater than the finished surface will be. You will be trimming this surface later, so it is good to have excess. You can either extend the profile curve or extend the surface after extruding it.