Was painful to watch. See around 2:15. Mast is almost jelly, hardly any rig tension. What forces are at play?

Just compression shortening the length of the spar in real terms.

Otherwise looks like they did not check the latest MSQ Notice to Mariners about the entrance.
Less than 1.0m lat

Looks to me like the backstays broke. Masts don't flex like that falling off waves when racing. I think I can see some stuff hanging from the rig. I would think the mast needs a really good check by a rigger as it could have been stressed close to its elastic limit.

That was really dumb, thankfully it was not me this time.

Flex seems to be mainly in one direction, some damaged/broken standing rigging?
PL

Quite likely that mast is damaged.
I can't see mast compression in those conditions causing that degree of flex. Looks to me like the backstay or a shroud or two have given up.

Select to expand quote

lydia said..

Otherwise looks like they did not check the latest MSQ Notice to Mariners about the entrance.
Less than 1.0m lat

When I went North last year about this time, they were dredging the main channel. The deep spot was where he went aground, just to the west of the channel. When I came back in September, the main channel was back to 2.5 LAT depth and the western area was very shallow. I assume that is where they dumped the spoils.
Pretty clear from the video where the shallow spot is, but perhaps he had been there for a while while the tide went out.

Happened on the 5th September 2017, so before warnings?

www.sunshinecoastdaily.com.au/news/major-warning-issued-over-coast-river-mouth/3180741/

channel is now good sailed a 37 footer out of there 2 months ago with no motor .Plenty of water in a straight line down the old leads it had just been dredged

I don't think anything broke. just looks like inertia causing compression as Lydia stated

I could not see anything broken. Might have to look again. Could not comprehend the amount of flex and movement under those conditions.

The flex is due to the boat hitting the sand while going down and sideways. The mast has alot of momentum causing the mast head to continue on its path slightly longer than when the boat stops.
When a mast bows like this all the pin to pin lengths on stays get shorter allowing the bend.
The hits on the bottom weren't strong enough to break the mast but it does happen in bigger waves.
If it was a frac rig the top probably would of snapped, not due to the initial bend but the snap back.
As soon as i got that boat to a safe spot is be getting up the rig for a look as no doubt there would be some stretched, elongated or torn stuff