I don't know the model wiper motor you have but in the past I have had wiper motors that to adjust the range , a cover was removed and there was a cam shaped plate with a series of holes along the curved side and the drive arm was pinned to the appropriate hole for the desired range. That was many years ago. In your case I wonder if the blade is hitting the frame at one end of its stroke and has extra room on the other end of its' stroke then the motor may have just been mounted in the wrong orientation. Adjust the unit as necessary. It could be that someone has installed a blade that is too long for the windshield. That you can figure out. The problem may be solved by carefully bending the existing arm to conform to the needs of the mounting position. Hopefully someone with the same model boat will chime in with their experience on this. Good luck.