Page 604 - Bob Holtzman "The Field Guide to Knots How to Identify, Tie, and Untie Over 80 Essential Knots for Outdoor Pursuits"

a minimum of three tie-downs: one across the boat at each end of the roof
rack, and a third from the bow of the boat to the front bumper, a tie-off point

under the hood, or a towing loop beneath the front of the vehicle. A fourth
line, from the boat’s stern to a suitable point on the back of the vehicle, is

also advisable. All should be high-quality rope that holds knots securely (not

cheap clothesline or hollow-braid polypropylene). Shoving against a
properly secured boat should rock the vehicle on its suspension without

shifting the boat on the rack. Beware, however, of pulling the lines so tight
as to damage the boat or the vehicle. Protect the car’s finish by padding bow

and stern lines with a rag or a piece of foam pipe insulation where they press

against the bodywork.

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