Possible Clues:

• After struggling free, bird makes secure place in the sails
• Sounds as if the tide is not for Granny
• Not sound, Granny not
• One may easily be undone by hazard, sailing at speed
• Starts under the 17 across and gets tied
• Granny would never, by the sound of it make it
• Rocks this not, by the sound of it, not for Granny
• Not rocks, not Granny
• One's tied up securely, a mile or so beyond maritime hazard
• Not, by the sound of it, not granny