Sailing 101: Crash gybes

"Crash gybe" sounds like a violent thing, and it can be, but it isn't
necessarily. Ours wasn't, but it was still bad. So what is it?

Let's start with what a gybe, non-crash, is: a turn where the wind comes
across the stern. Picture yourself standing with the wind coming from over
your right shoulder. Then turn to the left. At one point, the wind is
directly behind you. Continue, and it's coming over your left shoulder. Do
that on a boat, it's a gybe. (The contrasting turn is a tack, where the
wind comes across the bow. It's also, of course, possible to turn so the
wind stays on the same side of the boat. This is much simpler and is called
"heading up" if you go more upwind or "bearing away" if you go more
downwind.)

So, if you're sailing downwind, your sails will be swung out wide, away
from the centerline of the boat. In particular, the mainsail and the boom
will be out, maybe even nearly perpendicular to the boat. When you gybe,
you first bring the boom back to the center, under control. Then you turn.
Then you let the boom out again. (You do other things with the headsails
to move them across, but set that aside for the moment.)

If you're sailing along and you accidentally gybe, the the boom can swing
right across the boat as the wind angle changes. Booms are very heavy. Best
case, you get a massive crash with huge strains on boom, mast, and rigging.
Worse case, people get hit. This is, quite simply, one of the most
dangerous things that can happen on a boat. (An accidental tack isn't
nearly so bad, since the sails will be very close to the centerline and so
don't move much. And accidental tack is just embarrassing for the helm.) We
sail with a line called a gybe preventer, or just "preventer", that keeps
the boom from swinging if you accidentally gybe, which helps with safety.
It's important enough that we actually rig two.

Two reasons our crash gybe wasn't so violent then. First, we had the
preventer, so the boom didn't swing. But also, this didn't happen as we
were sailng along. The wind died. We drifted around without realising. Then
the wind came back up, and it was on the wrong side of the boat. Still
called a crash gybe, but not the classic version.

That said, we now had the spinnaker on the wrong side of the boat, blowing
back into the rigging, where it go wrapped. Thus, it turned into a big
problem, even without the big crash.