Bayesian Interim Report

[oldbloke]

I'm sure it has already been said, but not recently that I can see, but the thing about downdraft is that as the boat leans over the force on the mast, sails etc increases and is at it max when the boat is horizontal, unlike horizontal winds where as the boat leans its exposure to the wind decreases.

 

[DreadShips]

that's why I don't have such things on board ;-)

Apparently you can park next to a superyacht and wait for them to crash overboard in the first good blow. Best of luck catching them, mind.

 

[lustyd]

Whilst you guys bicker about stuff that really will make no difference to survivors/families, and likely do nothing to prevent future disaster the MAIB report was only an interim one, and I note the full investigation is considering aspects beyond those in this report including escape routes and emergency procedures. Better stability information MIGHT help avoid a repeat, but the conclusion seems to be that in freaky enough winds any boat may get knocked down and if those winds are unexpected might have big issues. The obvious questions to my mind are, should a dragging anchor have resulted in a general alarm, did the passengers know what to do once it was in trouble, and presumably the reason they went down with the boat was they were trapped in their cabins - from knockdown to sinking she was afloat on her side for at least 18 minutes on the timeline narrated by the MAIB.

I agree but it has to be said none of that is groundbreaking stuff. Escape routes, escape hatches etc. have been common design elements for decades. The question for me is why did that stuff fail or why wasn’t it there.

 

[ylop]

I agree but it has to be said none of that is groundbreaking stuff. Escape routes, escape hatches etc. have been common design elements for decades. The question for me is why did that stuff fail or why wasn’t it there.

Or was it there but nobody briefed the guests when/how to use it? I very much don't sail a superyacht but I do a safety briefing for everyone new who comes aboard (I've never actually explained the escape hatches). I get three typical responses (not always out loud sometimes just facial expressions):
- why is he telling us this, is this really dangerous, am I going to die
- oh jesus this guy is anal, I just want to have fun
- I don't know why you are telling me this, I have the attention span of a gnat and expect you to shout at me if required

I can imagine that typical superyacht guests really don't want to be imposed upon with safety briefings.

 

[Bouba]

Or was it there but nobody briefed the guests when/how to use it? I very much don't sail a superyacht but I do a safety briefing for everyone new who comes aboard (I've never actually explained the escape hatches). I get three typical responses (not always out loud sometimes just facial expressions):
- why is he telling us this, is this really dangerous, am I going to die
- oh jesus this guy is anal, I just want to have fun
- I don't know why you are telling me this, I have the attention span of a gnat and expect you to shout at me if required

I can imagine that typical superyacht guests really don't want to be imposed upon with safety briefings.

I always go through the life jacket routine....and finish with, ‘if you see the captain put on his life jacket....that’s a good sign to do the same’

 

[penberth3]

I can imagine that typical superyacht guests really don't want to be imposed upon with safety briefings.

I don't know, but there's a simple answer. If they're not willing to have a safety briefing they don't go aboard.

 

[Boathook]

I don't know, but there's a simple answer. If they're not willing to have a safety briefing they don't go aboard.

You can make them listen but will they hear.

When I did training we reckoned that the persons being trained only remembered 20% of what you told them.

 

[billskip]

I don't know, but there's a simple answer. If they're not willing to have a safety briefing they don't go aboard.

I don't think it's as simple as that...
It's a comercial business, it makes money,
It could suffer dramatically if people realise the possibility that a near death experience is possible. I fully agree with safety briefing, but when in practice it is required it's a bit late when panic sets in.

 

[KevinV]

You can make them listen but will they hear.

When I did training we reckoned that the persons being trained only remembered 20% of what you told them.

Not a very good trainer were you?,

 

[Poecheng]

To answer earlier questions (Fr Hackett #110), Southerly 38 owner here:
Keel up AVS c.125 degrees
Keel down AVS c. 135 degrees
Source: https://itboat.com/uploads/857e/1eaa85828280.pdf and it accords with my recollection but not, thank God, my experience.
It is a Stephen Jones design

Stability assessed using maximum total load (persons, equipment and liquids) of 2082kg Light craft condition (all standard equipment) is 9921kg. RCD Category A and designed for crew limit of 8 persons.
Source: Southerly 38 manual by Northshore.

There isn't a stability problem sailing them with the keel up, just a leeway problem. I have sailed a S115 and this S38 with the keel up going downwind.
Ordinarily, unless lifting it to avoid touching stuff or occasionally when going downwind, it remains down, including when motoring; it wouldn't occur to me to do otherwise tbh.

As others have said,
(a) Southerlies have a massive ballast plate in a recess in the bottom of the hull through which the swing keel swings - it is all massively engineered.
(b) those numbers, like all other boats, are before you load it up with radar reflector etc.

The Southerly manual states:
Users of this boat are advised that:
All crew should receive suitable training;
Any change in the disposition of masses aboard (for example the addition of a radar, in-mast
furling, etc) may significantly affect the stability, trim and performance of your craft;
The boat should not carry more than the manufacturers maximum recommended load;
Bilge water should be kept to a minimum;
Stability is reduced by any weight added high up;
In rough weather, hatches, lockers and doorways should be closed to minimise the risk of water
ingress;
Stability may be reduced when towing or lifting heavy weights using a davit or boom;
Breaking waves are a serious stability hazard.
Specific Information:
The stability of this vessel is not particularly dependant on the skill of the crew, and capsize is
considered to be unlikely provided that normal care is taken.
This vessel is unlikely to sink if the cockpit is swamped, but may do so if holed.

 

[Boathook]

Not a very good trainer were you?,

Work creation. They had to attend another session !

 

[doug748]

To answer earlier questions (Fr Hackett #110), Southerly 38 owner here:
Keel up AVS c.125 degrees
Keel down AVS c. 135 degrees
Source: https://itboat.com/uploads/857e/1eaa85828280.pdf and it accords with my recollection but not, thank God, my experience.
It is a Stephen Jones design

Stability assessed using maximum total load (persons, equipment and liquids) of 2082kg Light craft condition (all standard equipment) is 9921kg. RCD Category A and designed for crew limit of 8 persons.
Source: Southerly 38 manual by Northshore.

There isn't a stability problem sailing them with the keel up, just a leeway problem. I have sailed a S115 and this S38 with the keel up going downwind.
Ordinarily, unless lifting it to avoid touching stuff or occasionally when going downwind, it remains down, including when motoring; it wouldn't occur to me to do otherwise tbh.

As others have said,
(a) Southerlies have a massive ballast plate in a recess in the bottom of the hull through which the swing keel swings - it is all massively engineered.
(b) those numbers, like all other boats, are before you load it up with radar reflector etc.

The Southerly manual states:
Users of this boat are advised that:
All crew should receive suitable training;
Any change in the disposition of masses aboard (for example the addition of a radar, in-mast
furling, etc) may significantly affect the stability, trim and performance of your craft;
The boat should not carry more than the manufacturers maximum recommended load;
Bilge water should be kept to a minimum;
Stability is reduced by any weight added high up;
In rough weather, hatches, lockers and doorways should be closed to minimise the risk of water
ingress;
Stability may be reduced when towing or lifting heavy weights using a davit or boom;
Breaking waves are a serious stability hazard.
Specific Information:
The stability of this vessel is not particularly dependant on the skill of the crew, and capsize is
considered to be unlikely provided that normal care is taken.
This vessel is unlikely to sink if the cockpit is swamped, but may do so if holed.

Excellent information there. I have seen nothing to approach it from continental builders, where stability data seems to be treated as a state secret.


.

 

[Tranona]

Excellent information there. I have seen nothing to approach it from continental builders, where stability data seems to be treated as a state secret.


.

Southerly published that to help differentiate their approach from that of the French builders who mostly used unballasted or lightly ballasted boards. Ovnis for example just scrape into Cat A and some smaller 32/33' boats would make Cat A in fixed keel but B in lift keel.

Others have mentioned that the Ovni approach works in extreme conditions because the boat slides sideways rathe than tripping over its keel. This ides is not new as Maurice Griffiths discussed it at length nearly 100 years ago.

Suspect that an important factor in the Bayesian scenario is that the boat was not free to yield to the wind but was effectively tethered by the anchor at a dangerous angle to the wind. This is hinted at in the table of forces relative to angle to the wind in the Wolfson simulations. I expect there will be more on this when the full report is completed.

 

[DreadShips]

Suspect that an important factor in the Bayesian scenario is that the boat was not free to yield to the wind but was effectively tethered by the anchor at a dangerous angle to the wind. This is hinted at in the table of forces relative to angle to the wind in the Wolfson simulations. I expect there will be more on this when the full report is completed.

There was a suggestion above that the height of the mast itself might have prevented it rounding up, as once it was cocked over it'll act like a tail. That thought has been haunting me since.

The full report is also likely to go into areas that the interim was largely silent on due to the criminal investigation.

 

[TiggerToo]

Do you think that's a helpful comment that adds value to a serious thread?

then start by stopping using misogynistic tropes in a very serious thread

 

[Mark-1]

There was a suggestion above that the height of the mast itself might have prevented it rounding up, as once it was cocked over it'll act like a tail. That thought has been haunting me since.

...as I'm sure you know, unless there's some boat speed it wouldn't naturally round up anyway, and *if* this was a downdraft then all bets are off.

 

[potentillaCO32]

Or was it there but nobody briefed the guests when/how to use it? I very much don't sail a superyacht but I do a safety briefing for everyone new who comes aboard (I've never actually explained the escape hatches). I get three typical responses (not always out loud sometimes just facial expressions):
- why is he telling us this, is this really dangerous, am I going to die
- oh jesus this guy is anal, I just want to have fun
- I don't know why you are telling me this, I have the attention span of a gnat and expect you to shout at me if required

I can imagine that typical superyacht guests really don't want to be imposed upon with safety briefings.

Also inthe MAIB report it alludes to the rate of downflooding being a torrent that a crew member and the owner only just managed to escape from the upper saloon. I don't think anyone would have been able to use an escape hatch with the boat instantly on its side and filled with water.

 

[Laser310]

Others have mentioned that the Ovni approach works in extreme conditions because the boat slides sideways rathe than tripping over its keel. This ides is not new as Maurice Griffiths discussed it at length nearly 100 years ago.

AVS and other common measures of stability are for the _static_case.

It's pretty unlikely for a boat just sailing along to be gradually heeled past it's AVS, and so it's not that clear that the AVS of a sailboat is really that great an indicator of safety.

The far more likely case is that a boat gets rolled by a breaking wave, and the AVS does not really give much of an indication of how likely a boat is to resist rolling, where dynamical effects come in to play.

even a high AVS boat will get rolled by a big breaking wave.

The Ovni sliding idea is more applicable as a way to handle the dynamic effects of a breaking wave, rather than as an aid to a boat with low static stability.

Given that these, and similar low static stability boats, successfully sail all around the world, it's worth asking whether high static stability is really an important safety factor.

There is the issue of a rolled boat staying upside down.., but I also think this issue is exaggerated.

Static stability clearly helps performance - stability is the most important determinant of sail carrying power, it reduces heel and thus reduces leeway, and has other benefits. A potential downside is that it increases rigging loads, and may make rig care and inspection more important.

 

[flaming]

AVS and other common measures of stability are for the _static_case.

It's pretty unlikely for a boat just sailing along to be gradually heeled past it's AVS, and so it's not that clear that the AVS of a sailboat is really that great an indicator of safety.

The far more likely case is that a boat gets rolled by a breaking wave, and the AVS does not really give much of an indication of how likely a boat is to resist rolling, where dynamical effects come in to play.

even a high AVS boat will get rolled by a big breaking wave.

The Ovni sliding idea is more applicable as a way to handle the dynamic effects of a breaking wave, rather than as an aid to a boat with low static stability.

Given that these, and similar low static stability boats, successfully sail all around the world, it's worth asking whether high static stability is really an important safety factor.

There is the issue of a rolled boat staying upside down.., but I also think this issue is exaggerated.

Static stability clearly helps performance - stability is the most important determinant of sail carrying power, it reduces heel and thus reduces leeway, and has other benefits. A potential downside is that it increases rigging loads, and may make rig care and inspection more important.

True - however boats with AVS less than 90 will not pop back up once the wave / downdraft has gone away.

 

[Laser310]

Or was it there but nobody briefed the guests when/how to use it? I very much don't sail a superyacht but I do a safety briefing for everyone new who comes aboard (I've never actually explained the escape hatches). I get three typical responses (not always out loud sometimes just facial expressions):
- why is he telling us this, is this really dangerous, am I going to die
- oh jesus this guy is anal, I just want to have fun
- I don't know why you are telling me this, I have the attention span of a gnat and expect you to shout at me if required

I can imagine that typical superyacht guests really don't want to be imposed upon with safety briefings.

as much as we all might want to hate superyacht guests.., and feel the need to disparage them.., there is nothing to suggest that these guests were stupid.

on the contrary, they were pretty successful people working in a range of highly competitive fields.., where they are probably all accustomed to delegating important work to others and then making good, and profitable, use of this work. There is no reason to believe they would not listen to advice or instructions from experienced seamen concerning their safety.

There has been a lot of online discussion about "escape hatches", but as far as I know, nobody has definitively shown there were any.

In any case, as the MAIB report makes clear, this event happened really quickly, it was dark, and many of the victims were likely sleeping when it began.

I think that, sadly, for most of the victims it was not survivable.