Jump to content


Photo

Fatality - Northland


  • Please log in to reply
122 replies to this topic

#111 DrWatson

DrWatson

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 2,259 posts
  • LocationLand locked

Posted 22 October 2019 - 07:47 AM

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.
  • 0

"Si Dieu n'existait pas, il faudrait l'inventer"

 


#112 DrWatson

DrWatson

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 2,259 posts
  • LocationLand locked

Posted 22 October 2019 - 07:57 AM

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.
  • 0

"Si Dieu n'existait pas, il faudrait l'inventer"

 


#113 Fish

Fish

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 3,518 posts

Posted 22 October 2019 - 08:01 AM

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.


  • 0

#114 Freedom GBE

Freedom GBE

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 327 posts

Posted 22 October 2019 - 11:09 AM

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.


  • 0

#115 mcp

mcp

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 184 posts

Posted 22 October 2019 - 04:34 PM

I suspect a few tons of water propelled the liferaft into the windows which were probably glass.   


  • 0

#116 ScarecrowR31

ScarecrowR31

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 169 posts

Posted 22 October 2019 - 07:00 PM

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."
  • 0

#117 Chewing Gum

Chewing Gum

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 895 posts
  • LocationAuckland

Posted 22 October 2019 - 07:36 PM

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.


  • 0
PK

#118 DrWatson

DrWatson

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 2,259 posts
  • LocationLand locked

Posted 22 October 2019 - 07:54 PM

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.
  • 0

"Si Dieu n'existait pas, il faudrait l'inventer"

 


#119 Chewing Gum

Chewing Gum

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 895 posts
  • LocationAuckland

Posted 22 October 2019 - 08:04 PM

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.


  • 0
PK

#120 Fish

Fish

    Advanced Member

  • Advanced Members
  • PipPipPip
  • 3,518 posts

Posted 22 October 2019 - 08:05 PM

"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.


  • 0




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users