AVS is a Moveable Feast

[zoidberg]

Downflooding and AVS is again in the news, with publication of the MAIB's Interim Report into the fatal 'Bayesian' sinking.

The good souls at the MAIB want us to 'learn lessons' and apply them to our circumstances. Experience tells me that's akin to 'herding cats'.
Here's what I posted on an Owners' Group, relevant perhaps to some very much smaller boats...

I'll 'raise a hare' which many will not want to be reminded of.

News emerging from an interim MAIB report into the fatal downflooding/sinking of the 'Bayesian' draws attention to the oft-ignored Angle of Vanishing Stability. Neither the owner nor the crew were aware that her AVS was as low as 70 degrees.

Our Cutlass 27, in the 'light ship' configuration as designed, is excellent in that respect - due, for the most part, to the relatively large ballast:displacement ratio. That's a reassuring safety factor - but many do not realise that is NOT a fixed value....

The C27, due to its modest size, is particularly sensitive to changes having significant effects on AVS, and therefore ability to recover swiftly from a knockdown before downflooding sinks the boat. Roller-furled headsails, mast-mounted radar, gantries with PV panels, 6-man liferafts mounted on cabin top.... all add weight and 'lever arms' above the Centre of Buoyancy where least appropriate.

That's why my 6mm Grade 80 anchor chain is lighter than most others' and is stowed in the deep bilge, and my liferaft stowage is also below the waterline - not perched up high.

This MAIB report is a 'wakeup call' for those who will hear it and pay heed.

Well?

 

[ylop]

This was a point that was very clearly made on my Day Skipper course. Its a point that does come up from time-to-time in other MAIB reports as fisherfolk often make modifications affecting stability.

 

[boomerangben]

The question is how much is stability affected by various bits and pieces. Without a naval architect and or surveyor, assessments are purely conjecture. I agree that we know a radar is detrimental and stowing chain down low is good but does one counteract the other? What effect do crew have? Empty vs full tanks? When the chain is over the side, what then? Do our modifications make a significant difference to our stability? And what is a significant difference?

I can’t see anything to learn from the interim report for those of us who measure our boats in 10s of ft rather than 10s of metres, except perhaps beware the cruel sea and her twin sister weather.

 

[billskip]

Downflooding and AVS

Ok so if one doesn't know what downflooding is, and learns about it, what more can a person do than ensure everything is closed and watertight, what happens next of course is guess work...

AVS well once again one learns about it,
What can you do to prevent going beyond it if the conditions create the situation?

Sailing is a fairly dangerous pass time, and all the knowledge in the book will not prevent the human factors, panic,stress,tiredness, exhaustion, disorientation, injury, confusion, inexperience of conditions at a time when things unexpectedly go tits up.

All I can say is if I do the best I can and enjoy my pass time and best ensure others on board safety.. what else would you expect...
As I said in another post, it makes me shudder when I read threads advising people to downsize ground tackle.

I really don't think many of us go sailing thinking there is going to be a knock down

 

[Stemar]

Sailing is a fairly dangerous pass time, and all the knowledge in the book will not prevent the human factors, panic,stress,tiredness, exhaustion, disorientation, injury, confusion, inexperience of conditions at a time when things unexpectedly go tits up.

Sailing involves an element of risk, yes, but I still reckon that, when I lived in London, the most dangerous part of my sailing weekend was negotiating the M25 and the A3

 

[SaltyC]

Ok so if one doesn't know what downflooding is, and learns about it, what more can a person do than ensure everything is closed and watertight, what happens next of course is guess work...

AVS well once again one learns about it,
What can you do to prevent going beyond it if the conditions create the situation?

Sailing is a fairly dangerous pass time, and all the knowledge in the book will not prevent the human factors, panic,stress,tiredness, exhaustion, disorientation, injury, confusion, inexperience of conditions at a time when things unexpectedly go tits up.

All I can say is if I do the best I can and enjoy my pass time and best ensure others on board safety.. what else would you expect...
As I said in another post, it makes me shudder when I read threads advising people to downsize ground tackle.

I really don't think many of us go sailing thinking there is going to be a knock down

I disagree that sailing is 'dangerous' yes there is a risk - as with crossing a road, cycling and l other daily activities. Knowledge and understanding can mitigate the risks education and understanding has tought you.
It will never be 10p% safe, nature is unpredictable, hopefully your understanding of the risks can mitigate its effect.

 

[billskip]

I disagree that sailing is 'dangerous

Which you are fully entitled....they say that flying is the safest form of transport.. well I suppose it is if the plane doesn't crash.

 

[ylop]

I agree that we know a radar is detrimental and stowing chain down low is good but does one counteract the other? What effect do crew have? Empty vs full tanks? When the chain is over the side, what then? Do our modifications make a significant difference to our stability? And what is a significant difference?

A detailed analysis might need some robust calculations but you can get a rough idea of the magnitude yourself. A radar (or other object) 10m above the centre of rotation needs 10x the weight 1m below the centre of rotation. Now how much impact the difference has on the boat depend on the boat - both its shape and it displacement, but it’s not particularly hard maths if you want to make rough approximations. Understanding impact on windage is probably harder.

 

[Mark L]

Well?

You make some excellent points, but wrt liferaft cannisters: they tend to have a buoyancy that is greater than their weight. So, whilst they would adversely affect stability (as generally understood), in most yachts, a liferaft on the cabin top would reduce the AVS and in almost any yacht would lower the (negative) area under the GZ curve.

 

[justanothersailboat]

Stability is definitely mentioned in Day Skipper. It's a small bit among a mass of other things but it's there.

Sailing seems to work out pretty safe overall, but that does rely on most sailors choosing what they let themselves in for, and being constantly vigilant. Some things are easier to keep an eye on than others. I can easily believe many popular boats fall in the range "heavy enough that stability changes are not immediately obvious in most conditions, but light enough that things you can lift yourself still meaningfully affect stability", thereby making stability hard to keep an eye on. I can't believe a liferaft on the coachroof has an effect on stability worth worrying about on any seaworthy boat - my standing up to go forward must have a worse effect and I don't cause capsizes - and I wish it wasn't popularly cited as an example.

 

[billskip]

Stability is definitely mentioned in Day Skipper. It's a small bit among a mass of other things but it's there.

Sailing seems to work out pretty safe overall, but that does rely on most sailors choosing what they let themselves in for, and being constantly vigilant. Some things are easier to keep an eye on than others. I can easily believe many popular boats fall in the range "heavy enough that stability changes are not immediately obvious in most conditions, but light enough that things you can lift yourself still meaningfully affect stability", thereby making stability hard to keep an eye on. I can't believe a liferaft on the coachroof has an effect on stability worth worrying about on any seaworthy boat - my standing up to go forward must have a worse effect and I don't cause capsizes - and I wish it wasn't popularly cited as an example.

Be careful going for a wee to leeward

 

[Marsali_1]

Be careful going for a wee to leeward

Do you not have to be more careful to windward?

 

[billskip]

Do you not have to be more careful to windward?

No..you can't fix stupid...

 

[zoidberg]

I s'pose my objective, in posting my contri above, is much the same as comments in an MAIB Digest - if it jolts/bounces/encourages a handful of owners to give the topic a bit of deep thought i.r.o. their own boat, then that's a success.

Whether guys immediately reach for the phone and book an urgent Inclining Test.... or yawn and turn the page.... is not my responsibility, but theirs.
Me, I try not to make assumptions which encourage me to 'swerve' doing something I know I oughta.

 

[Chiara’s slave]

I think I’ll skip the inclining test. We are acutely aware our boat has 2 stable positions, and how it goes from one to t’other.

 

[Mark-1]

I disagree that sailing is 'dangerous'

Agree. Sailing is about as safe as any sport or activity gets. The numbers bear that out. Part of the joy is that it can 'feel' dangerous but the actual objective risk is tiny.

1 in 200 of us die on the UK roads. The drive to the boat is near suicidal, the time on the boat is safe.

 

[Supertramp]

Which you are fully entitled....they say that flying is the safest form of transport.. well I suppose it is if the plane doesn't crash.

I think sailing is intrinsically safe. Boats don't generally sink, explode or crash. The hazards come from the decisions you make as skipper and the extent to which you take safety seriously. Downflooding precautions and monitoring forecasts are my takeaways.

Agree. Sailing is about as safe as any sport or activity gets. The numbers bear that out. Part of the joy is that it can 'feel' dangerous but the actual objective risk is tiny.

1 in 200 of us die on the UK roads. The drive to the boat is near suicidal, the time on the boat is safe.

I feel far more at risk cycling or driving - though 1 in 200 feels over pessimistic!

I am happier trusting my own precautions, maintenance, decisions and judgement despite the obvious hazards and challenges of life afloat.

Aren't more people injured putting socks on than anything else?!

 

[Mark-1]

I feel far more at risk cycling or driving - though 1 in 200 feels over pessimistic!

It's written on the back of my copy of the highway code, and it's a classic pub quiz question becaise it seems so implausible so people react exactly as you did - "Can't be true.". But no, road use really is that dangerous. (I *think* I first became aware of it from a post on here.)

What chance of being killed in a road accident? - Page 1 - Speed, Plod & the Law - PistonHeads UK

 

[oldbloke]

Everything has a Sanger, but doing nothing is more dangerous than most things. Relative risk is a well researched but often ignored subject. Look up "micromort"

 

[ylop]

It's written on the back of my copy of the highway code, and it's a classic pub quiz question becaise it seems so implausible so people react exactly as you did - "Can't be true.". But no, road use really is that dangerous. (I *think* I first became aware of it from a post on here.)

What chance of being killed in a road accident? - Page 1 - Speed, Plod & the Law - PistonHeads UK

It’s a little bit dated but orders of magnitude it’s right - roughly 500K people die in the U.K. each year (pre covid), roughly 2000 of them on the roads - so 1:250. Of course if you did the analysis by age group you’d get different ratios. As you would with genders.

Translating it to boating risks is harder - almost all the population frequently drive/cycle/cross the road etc so no need to factor that in. You’d need to consider the participation denominator. But then just as with the roads you’d need to consider different types of boating - eg transocean racing versus rowing on the local lake.