Witches, Storms and Egg Shell Ships

Apropos of nothing...while making daughter's lunch today I was reminded of an odd superstition of my grandmother's, she used to tell us never to throw away egg shells without smashing them first. Otherwise witches would go to sea in them and stir up storms to sink the ships and drown the sailors. A rather odd story to tell kids over lunch, but there you go! Has anyone else come across this superstition or anything like it?