The short answer

Bow and stern lines stabilize a long kayak against pitch and sway; they are not a substitute for wrapping the kayak to the crossbars. Route each line from an approved attachment point, keep the angle reasonable, protect the vehicle finish, and remove slack without over-tensioning the hull.

Inspect the route

Area Check Failure signal
Kayak attachment Handle, eye, or approved loop Line pulls on a weak deck fitting.
Vehicle anchor Tow point, hood loop, or approved point Line uses a plastic trim part.
Angle Clear and reasonably aligned Line twists the kayak or rack.
Protection Soft loop or edge guard where needed Paint or line rubs on a sharp edge.
Crossbar straps Separate, flat, and snug Bow line is used as the main tie-down.

Check straps after a short stop, keep loose ends secured, and avoid routing a line across a windshield view or into a moving wheel. If the kayak contacts the roof, spoiler, or hatch, stop and change the setup rather than tightening harder.

Limits

This guide does not rate a carrier or approve an anchor. Follow the kayak, rack, and vehicle manuals and local transport rules.