Originally Posted by shrew
This is with all boats, but slightly more pronounced with I/O's and outboards. The bow will always attempt to wander. The degree to which it wanders is a product of oversteering.
If you adjust after the bow has wandered, you've reacted way too late. The second it starts to go to port, turn to stbd. The second the bow starts going back to center, center the wheel back again. If you wait to straighten the wheel once the bow is straight, you've again, reacted way too late. The other key is not turning the wheel too much. That is what causes the behavior you're experiencing.
as a test, in a fairly wide canal i set and held the wheel exactly in the middle for a couple of minutes, so without any correcting.
what happens is the boat goes in the same direction but the bow sways from left to right... like a perfect sinus wave... so i'm failry sure it has nothing to do with correcting the course.