Jump to content

Fatality - Northland


Recommended Posts

While that is technically correct fish, nothing BUT a submarine is that watertight. An inverted yacht in a stable condition will leak water into any opening/crack/vent until the internal air pressure is equal to the water pressure outside. 

In this case, the shock loading from the wave impact only has to flex the structure sufficiently to distort the windows  enough so the fixing systems fail. To pop windows out with hydraulic ram effect would require water inside the vessel already...

Link to post
Share on other sites

I would say no, because air is compressible and water is not. If pressure were equal then water would not pour in via vents and hatches etc. BUT, changes in air pressure is generally measured in milli-Pascals, i.e. tiny amounts.

 

milli-Pascals are potentially enough to make airs pop, but in the context of this discussion (forces to cause windows to suck in or pop out) I do agree that hydraulic shock is by far the more powerful force. In simple physical terms, with air, you have a weak easily compressible gas with which to transfer energy, with hydraulic shock, you have a solid, non-compressible water with substantial kinetic energy.

 

One proviso to add though, if you imagined a yacht being completely air and watertight, and being 2 m underwater, there is no way for the air pressure to equalise with the water pressure (assuming no way in for the water). In this scenario you are effectively turning the yacht into a submarine, and in physical terms in can be assumed to be a pressure vessel, with greater pressure on the outside. The structure of the yacht (windows) needs to be able to withstand that pressure differential. Its kind of getting into semantics now, as either way the windows need to be strong enough to not break when very big waves hit the boat...

My scenario was more the inverted boat being like a, let’s see, ... a teapot upside down. There are a number of holes that let water in from (now) below, until the air pressure inside = water pressure outside.

Link to post
Share on other sites

In this case, the shock loading from the wave impact only has to flex the structure sufficiently to distort the windows enough so the fixing systems fail.

This. So ideally you’ll want the windows and deck structure to flex at the same rate, and that any difference in flex be able to be taken up by the window fixings without failing. ie, decent polyurethane glue with a bit of flex.

Link to post
Share on other sites

Yes, I agree that hydraulic shock from waves crashing is the primary force that would pop the windows. I guess I'm still a few posts back as well and explaining what would lead to BP's ear's popping.

 

I agree the teapot analogue is the most accurate for an inverted boat DrW.

Link to post
Share on other sites

They reported 5+ meter waves. If it crashes through the hatch, I can imagine the force of the water and air inside the boat will be twice the height of the wave.

 

Like the kinetic forces in the blow hole at Muriwai, it spews out water twice the height of the wave. That will pop any window.

 

On some stormwater outfalls on the beach we construct air releases to stop our heavy cast iron covers  from popping.

 

Might be worth having hinge type hatches closed on a mechanical fuse and a spring to let air out but not let water in.

Link to post
Share on other sites

So would a compression of air inside, followed by rapid release blow out the windows ie Hydraulic action. Not sure of the physics of why decompression is so explosive but know it is an important process in shaping coastal geomorphology

 

"Hydraulic action refers to the action of water, generally from powerful waves, rushing into cracks in the rockface. This process traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rockface and widens the crack, so that more air is trapped on the next wave. This progressive system of positive feedback can damage cliffs and cause rapid weathering."

Link to post
Share on other sites

No mention of this boat rolling over in the account published. Neither is there mention of dismasting.

In the GGR several boats rolled and lost their rigs. None of them that I can recall lost windows or sunk.

I still think structural failure is a possibility.

Link to post
Share on other sites

No mention of this boat rolling over in the account published. Neither is there mention of dismasting.

In the GGR several boats rolled and lost their rigs. None of them that I can recall lost windows or sunk.

I still think structural failure is a possibility.

When you say structural failure I assume you include deck and cabin structure in that? Afaik there’s no mention of compromised hull integrity in the accounts so far given.

Link to post
Share on other sites

Only supposition of course but if crashing off a wave caused a crack in the hull to my mind it would more likely account for the rapid filling and failure of the pumps to cope.

If there was a crack in the hull below the cabin sole it may not be evident as to where the water ingress was coming from.

Having said that of course as everyone here has been commenting it could be just the windows that were the cause of the flooding.

I guess we will never really know for sure as the yacht is unlikely to be salvaged.

Link to post
Share on other sites

"Hydraulic action refers to the action of water, generally from powerful waves, rushing into cracks in the rockface. This process traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurized air cracks away fragments at the rockface and widens the crack, so that more air is trapped on the next wave. This progressive system of positive feedback can damage cliffs and cause rapid weathering."

Pipeline design needs to consider 'column seperation'. This is where you have a column of water moving (in a pipe) and it suddenly stops (i.e. with a power failure to the pump driving the pipeline). The kinetic energy means the column of water wants to keep moving forward, but the water closest to the pump suddenly stops. Where the column separates a vacuum is formed. The vacuum causes the two columns of water to crash back together. This then sends high energy shock waves crashing up and down the pipeline, and will break it somewhere.

 

The column separation is the exact same principle a mine uses to sink a warship. Its not the explosion that sinks the ship (although it can do). The explosion forces the water apart, when it crashes back together, the shock waves break the ships hull, and it sinks.

 

I think in your example the presence of air allows for rapid expansion and contraction (or compression and release of pressure) at the crack face, similar physical phenomena. At the end of the day its about wave energy being transferred to something solids, like a pipeline, warships hull, or a cruising yacht. I'm sure with a chaotic seastate there is sufficient energy around to cause issues.

Link to post
Share on other sites

I see that it is Stuart Pedersen's funeral today. My thoughts and condolences are with his family and friends .

 

And thankyou Matt, for keeping this thread open for what has been a thoughtfull and respectfull discussion.

Link to post
Share on other sites

The funeral was a great tribute to Stu, he was a person who liked to help the under-privileged and youth selflessly. I consider myself lucky and fortunate as well, to have sailed with him on Ballistic for a season and to meet socially while cruising at various times and at the TYPBC events.

It was also very sad, hearing the trauma of the event and feeling the sorrow for all involved. The rescue personnel did an amazing job in very difficult circumstances, huge waves and 60+ kn winds.

RIP Stuart 

  • Upvote 1
Link to post
Share on other sites

Happier times, Stu in his favorite green tee shirt enjoying a sundowner on Comfort Zone, Pam on the right

20190612_182832.jpg

 

RIP Stu, you were one of nature's gentlemen

                                         

  • Upvote 1
Link to post
Share on other sites

I just noticed an article in STUFF (17 minutes ago) about this possibly a report from Maritime New  Zealand released today?... Has the Coast Guard released it's final report? Anyone have a link? Thanks

Link to post
Share on other sites

So the cat 1 rules have changed, requiring boats to have storm shutters fitted to any window over 2 square feet (not just have them available).  Also a bit more emphasis on making sure life rafts are securely attached if they are externally mounted.

Link to post
Share on other sites

Essence yacht sinking: Storm covers could have prevented loss of life, Maritime NZ investigation finds

An investigation into the fatal sinking of a yacht off the Northland coast has revealed storm covers - fitted to protect cabin windows - could have prevented loss of life.

Safety regulations have now been updated to prevent similar incidents from occurring in the future, Maritime NZ said.

https://www.nzherald.co.nz/nz/essence-yacht-sinking-storm-covers-could-have-prevented-loss-of-life-maritime-nz-investigation-finds/OUJXB276XEYRRYX4ZJWIF746MY/

 

Link to post
Share on other sites

So the significant change is the covers,are now fitted rather than carried?

Then in the report a photo of how it should be done that looks to me to be utterly useless. Essense lost her windows from the inside out  the covers shown are ply over the windows , the ply against the coamings would stop them being stovevin, but a few little screws wouldn't stop them popping out.

  • Upvote 3
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...