@PiperPilotPaul A crash (uncontrolled/accidental) gybe, which is when the wind gets on the wrong side of the main and the boom comes flying across the cockpit at high speed. Bad for the rig and often deadly for anyone caught in the way.
On a 40 footer it will easily cave in a skull and then throw the corpse overboard. So best to avoid.
If sailing on a dinghy you just get a big swollen bump on your head and remember to keep it down next time 😃
Agree that an uncontrolled gybe is extremely bad for one's health.
How does a preventer help?
@PiperPilotPaul It's attached to the boom end pulling it forward. Opposite the main sheet. So it prevents the boom from moving. Only the sail backwinds.
You can then either correct the course (most crash gybes are caused by steering mistakes) or ease out the preventer and perform a controlled gybe.
OK - got it. It helped to see the preventer lines running to the bow in your picture.
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