Fatality - Northland

[Priscilla II 414]

Excuse my ignorance but with ports/windows of that size does getting Cat 1 make a issue about having on board alternative sources of shuttering the openings.

[Jon 397]

13.11 (K) Storm coverings are required for all windows more than 1852cm2 (2sq ft) in area.

[John B 109]

Metvuw screen grab I sent to a mate on the 9th Oct. It was always going to be awful

That is fascinating, do you know if it was being reported elsewhere on the wed, Puff?

It's valuable because as we all know tight isobars like that are always worse than the wind barbs say. Monday at 1 roughly where Essence was..30 knots. Clearly the plus 50 % rule applied there.

 

What model does James McGregor use as his base? GFS or EC?

[Deep Purple 530]

I don’t know where he gets it from but he is mostly right a week out and was in this case.

[Freedom GBE 27]

We left Fiji on a fastish cruising cat a few days earlyer than Essence.

 

The forecast on Predict wind when we left Fiji was moderate westerlies for the last few days of our trip. The forecast changed about three days in to our trip, we got light easterlies all the way instead. We still had a dream ride down.

 

We could see this storm on the predict wind maps when were about half way, from memory it was following about 400 miles behind us. Essence had possibly left Fiji already when the forecast changed. Then hoping to go around the east of the storm. Waves could have slowed them down.

[Chewing Gum 17]

You would have to wonder if there was some sort of structural failure as well as the windows. Sounds like the boat sunk quite quickly, that would need a lot of water ingress.

[DrWatson 382]

You would have to wonder if there was some sort of structural failure as well as the windows. Sounds like the boat sunk quite quickly, that would need a lot of water ingress.

Yeah, but I think with those big windows missing, and breaking waves on deck, you'll be taking +100 litres with every wave... doesn't take much before the boat is 3T heavier, then 5, etc etc.

[Black Panther 1,716]

The one time i have been inverted i was down below and my ears popped

[Fogg 427]

That is fascinating, do you know if it was being reported elsewhere on the wed, Puff?

It's valuable because as we all know tight isobars like that are always worse than the wind barbs say. Monday at 1 roughly where Essence was..30 knots. Clearly the plus 50 % rule applied there.

 

What model does James McGregor use as his base? GFS or EC?

 

Data from US National Weather Service

[mcp 34]

The one time i have been inverted i was down below and my ears popped

 

Pressure increase, rather than decrease?

[Fish 0]

Pressure increase, rather than decrease?

I would imagine (not from experience) that it is a little bit like scuba diving. Even a short distance underwater increases air pressure. If there is water in the form of a wave on top of the boat (1 or 2 m of green water?), or if the cabin is where the keel normally is (i.e. 2 m under water), that would likely have the same effect on air pressure as being 2 m under water in calm, flat water. 2 m = 20% of an atmosphere = lots of air popping.

[Fogg 427]

The one time i have been inverted i was down below and my ears popped

I’m surprised that’s your abiding memory!

[Black Panther 1,716]

Correct, pressure increase due to the weight of water above.

I also remember the owner stole my favourite beanie.

Ok i also remember the wave causing a sunset at 10 am.

[Black Panther 1,716]

Last one sounds plausible.

[Fish 0]

I could be convinced of both.

For the air pressure one, very few boats are fully watertight, specifically around dorades and companionway hatches (as well as various nefarious leakage paths). If the boat is on its side or upside down, you will get water forcing its way in (say) dorades, but with no exit in the hull, so the air pressure will increase immediately. Note that for ears to pop, you only need changes in air pressure (both up and down), and not for the air pressure to go up and stay up.

 

And the hydraulic shock of a wave:

Merfyn Owen of Owen Clarke Yacht Design (apparently knows something about yacht design) did an article in Yachting Monthly about all the Golden Globe boats getting smeared and smashed up. His underlying philosophy and guide to all his design principles is that, in the context of heavy weather, big waves hitting boats is all about the consideration of transfer of energy.

 

In short, if a waves slams into the side of a boat, energy has to be transferred, either by rolling the boat (rotational energy), or flexing stuff etc, so yes, Laterals hydraulic shock is well founded in yacht design considerations.

[DrWatson 382]

If the boat is inverted and two metres under, the increased pressure from the air inside the cabin being compressed should = the increased pressure from the water column pushing back against the windows, no?

 

I'm putting my money on hydraulic shock, and windows held in by only a row of screws and a bead of sika to seal it as opposed to being fully bonded to the cabin structure. 

 

Properly bonded windows should be as strong as the deck w.r.t. load transfer etc.

[Black Panther 1,716]

Anyone else remember when windows had frames screwed to the coamings?

[Fish 0]

If the boat is inverted and two metres under, the increased pressure from the air inside the cabin being compressed should = the increased pressure from the water column pushing back against the windows, no?

 

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...

[Brien 22]

Its also possible water from two colliding waves fell onto the deck or cabin top. I have experienced this and it was a massive force. Modern rounded decks and cabin tops are quite flexible.

[Island Time 1,286]

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...