Inboard engine boats do not naturally go in a straight line if the rudder is left un-attended. The reasons can be many, ranging from slight mis-alignments of the keel and/or driveshaft to the mysterious science of rotating propellers.
While fishing on a rocky shoal where no other boat could go, one dippy owner managed to fall out of his boat. Standing in calf-deep water, he watched his faithful Dippy putt away at a speed just slightly faster than a person can run in calf-deep water. In no immediate danger, but stranded, he pondered his rescue. Only a rowboat, canoe, or Dippy could get to him.
After some twenty minutes, help appeared. It was his own Dippy, returning from a grand circle. As it passed within a few feet of where he was standing, he hopped aboard and returned home.
...As told by John Thomson