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Fatality - Northland


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Stuff story updated with video footage from the Rescue Helicopter.

Gives some perspective of the sea state (although cameras never do it justice).

The raft didn't look at all secure with a breaking wave going over it...

 

This must be particularly traumatising for those involved, the families, and also numerous boats currently in the Islands planning departure and weather windows back to NZ.

 

https://www.stuff.co.nz/national/116582701/bay-of-islands-yacht-rescue-reports-vessel-was-returning-from-fiji-investigation-launched

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Terribly sad.

 

Sometimes you see footage of rough weather and it doesn't seem that bad due to angle/camera etc. But this time, the footage from the chopper looks like the conditions were really really nasty!

 

Fingers crossed the remaining person in hospital recovers too.

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20 nm NW of Cape Brett is the Cavilli's, but 20 nm due north is the 500 m contour. I wouldn't be surprised if the sea state around there 'coming onto soundings' was nasty with this easterly and deep low pushing in.

 

Sounds like a terrible ordeal.

Dont know much about this sort of detail but are you suggesting at 500 metres deep the bottom is still effecting the sea state?  If that is the case at what depth does it have no effect?  or I guess that depends on a whole heap of factors. 

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Talking to a well known sailing ex submariner the other day...he recounted a story of a RN submarine rolling violently 400ft submerged in the Bay of Biscay during a storm..so yes I think it could affect things if the water shallows

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Dont know much about this sort of detail but are you suggesting at 500 metres deep the bottom is still effecting the sea state? If that is the case at what depth does it have no effect? or I guess that depends on a whole heap of factors.

 

Absolutely. It's the same reason you stay away from the Kermadecs outbound if it's rough. The Colville Ridge 70 miles out from or the Kermadecs themselves are ridge/seamounts that emerge from miles deep. Same with the washing machine/ compression zone south of Fiji. Add wind and a few thousand miles of sea piling up in a thousand metres and its nasty.

We were a bit beaten up leaving Fiji Thursday ,10 days or so ago. Like Motuihe channel wind against tide for 36 hours.

That rescue helicopter footage, man, the guys who did that are heroes plain and simple.

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Dont know much about this sort of detail but are you suggesting at 500 metres deep the bottom is still effecting the sea state?  If that is the case at what depth does it have no effect?  or I guess that depends on a whole heap of factors. 

There is no science to it (that I'm aware of) and almost the opposite, oceanographers have tried to study it and can't determine the key factors. BUT, its known to happen, fairly much as John B says.

The best known example is the Bay of Biscay, being notorious for bad sea state as the big Atlantic swells roll in and steepen up as they approach the continental shelf during classic Atlantic storms. The other moderately well documented spot is the Labadi Bank (spelling) in the Celtic Sea, from the 79 Fastnet race. Its a spot on the rhumb line from Fastnet rock to the Isles of Scilly, where it shallows up a little bit. Known as being an area with sharper / steeper waves, but physicists etc cant determine how the bottom has an effect there. (refer the book Fastnet Force 10, for details on that, I've got a copy but can't be bothered finding the page reference, give me a shout if you are interested, its got a good discussion of bottom depth on wave shape and how it impacted certain boats in the 79 Fastnet).

 

From my basic understanding of physics, if you have a deep low such as was this storm, pushing a lot of water infront of it, and a big swell set up over 1,000's of nm, and then it comes up hard against a coast line (and shelving bottom), the energy / waves don't have anywhere to go, and steepen up. My assumption is, if the storm is big enough / the fetch long enough, you will get an effect much further out than is initially logical.

 

Or to put another way, old mariners are always wary when they 'come into soundings' or when the bottom shallows up from open ocean.

 

Either way, the swell in the rescue video looked frightful, and I can only imagine how traumatised the survivors are, both from having their boat sink, and for ending up in that water.

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There's plenty of science on shoaling effects and sea states. And no I'm not going to outline it. Start with wikipedia.

Well yes, if you must. But the phenomena isn't easily predictable, in that you can say a certain depth will or wont have an effect. Or which other factors are the most dominant factors, such as wind speed, fetch, rate of change of bottom depth.

 

Rossd specifically asked if the bottom would still have an effect at 500m. Probably not on a calm day, or in a Sou wester...

 

Of interest, it is always the shape or steepness of waves, but  this isn't reported, measured or forecast. Swell height is, and so is wave length. From those two you can deduce wave steepness.

But then accuracy of the forecasting gets so marginal that it is effectively better to use rules of thumb, or old mariners sayings...

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A guy I once knew who was on a US Nuclear Sub said that in a severe storm above, they could still significantly feel the wave effects at 600ft down.

BP, maybe you are correct as far as a wave energy is concerned. We all know water cannot be compressed. But there must be some other physical factor involved.

Here's an additional thought to that. The Sea State can be vastly increased in magnitude over a Shoal Bank that may still be considered deep water. I am thinking of the Banks along the East Coast off Gisborn to East Cape region. They are still deep water, but no where near the depth around them. The Sea State over them in a blow can be horrendous. There could perhaps be a tidal flow over them maybe that is not out in the deeper water maybe. I don't know, just a thought

 

 

Then 1200ft between crests.

There is no compression only rotation. A particle of water describes a circle as a wave passes. Those circles get smaller with depth. Motion effectively disappears when depth is half wavelength.

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The BOP Regional Council have a wave bouy out off the coast. It records a lot of variables, one is the period. Check it out as the significant period has ranged 3-11sec over the last month, mean period 3-8 sec and height around 3m and a max of 8m etc.

 

http://monitoring.boprc.govt.nz/MonitoredSites/cgi-bin/hydwebserver.cgi/points/details?point=822

 

Considering its fixed these are large variations in water behaviour ....... 

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Hi Bp Great clip. Pretty sure though as as waves start to "feel bottom" the orbit slows an they become eliptical and slow down. This changes the wave length,waves closer together and the waves eventually become translatory and break. These eleptical orbits can be felt snorkelling in a swell. you are moved backwards and forwards rather than in a circular orbit. When diving a more an circular orbit.

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