The front & rear propellers are soft, and easily jam and stop spinning.  The trick appears to be to twist or untwist their eye-screws to get the right tension on them.  I don't like that--they should be firm, and should work perfectly out of the box without issue, as long as they're not jammed with weeds.  Haven't caught anything on this yet, perhaps because I use it less because of the propellers not both spinning properly all the time.  I'm disappointed.