If you have one roller, then the strap goes Under. If you have two rollers, then the strap goes between the two.
The top roller, combined with the strap, should always help prevent the bow from lifting or shifting forward when hitting bumps in the road.
A second chain (or strap) should also go down to the trailer tongue as extra safety to prevent the boat from being able to move forward.
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings