The tie downs are for the abnormal situation. Yes, gravity is going to keep
your boat on the trailer. Until you have to make a sharp turn to avoid an
accident or get in an accident.

I saw my buddy's 23' Rabalo slide about 300' down US 41 using that logic.
Fortunately it tilted left a little and came to a stop on the grass before it
came off "plane". Virtually all of the road rash was along the keel.
We all got a lesson on cheap tie downs and tongue weight that day.
Two quick trailer whips was all ot took to launch that boat onto the pavement,
straightening out a 3/8" hook and shearing off the winch arm.

Kudos again to the Lee Couny Sheriff's department who helped us get it back on
the trailer (with 2 body builder type deputies) and sent us off with a verbal
warning.