Mark-1
Well-Known Member
They won't have mounted one sideways to measure downdrafts.
Bayesian Interim Report
- Thread starter DreadShips
- Start date
They won't have mounted one sideways to measure downdrafts.
RunAgroundHard
Well-Known Member
That is just a ridiculous statement and implication.
Mark-1
Well-Known Member
I said they are unlikely to have a way to accurately determine if they experienced a downdraft.
Ridiculous or not, I'm standing by it.
Fr J Hackett
Well-Known Member
Whatever people may or may not understand about a"downdraft" whatever laid Bayesian over must have had a horizontal component which as the MAIB report indicates must have exceeded 64kts. that will have been measured, whether it was recorded and is now recoverable is another matter.
RunAgroundHard
Well-Known Member
The vessel may have been fitted with ultrasonic wind instrument that has no moving party's and detects wind speed via sound transit time changes due to wind velocity between probes. In my opinion, very likely for the type of systems that this vessel would have been fitted with. As the vessel healed, the instrument would have become more accurate (they are calibrated for certain degrees of heal), thus measuring the downdraft that was causing the heal. Even if not a traditional wind instrument would have started to become more accurate as the vessel heals.
Mark-1
Well-Known Member
I can imagine that they could correlate wind speed and direction to heel angle, but obviously a downdraft can't go direct into the sea so it's going to have a massive horizontal component as it gets anywhere near the water. At best, it's just going to be a mess open to any interpretation.
Plus the MAIB make no mention of any recorded data being available or useful to them despite wind speed and direction being critical here. They do go into massive detail about the potential wind it might have experienced. Hard to see why they'd investigate in detail the wind it might have seen if accurate data was available from the ships.
Regarding the other vessel, do we even know exactly what hit them? How would they know? They experience of it would just be "loads of wind". They aren't to known that 30 metres up it's going straight down.
I think this is one of those things where, if they don't know the nature, direction and speed of wind that hit each boat right now, we will never know. The other vessel has no need to keep their experience of it secret.
Either way It not ridiculous to assume two random pleasure vessels in tbe med didn't have dedicated kit to detect and measure downdrafts. I have no idea how meteorologists measure it.
doug748
Well-Known Member
It would be interesting to know that information but just as easy to speculate that the Sir Baden Powell would be the more seaworthy boat. I suspect it was.
This is an interesting read:
https://irp.cdn-website.com/5ae73933/files/uploaded/TS+Stability+pamphlet-UDJ-v10a.pdf
Essentially a practical guide for masters of tall ships, it tends to use two broad exemplars:
Ship A - with a righting arm that runs out about 90deg and:
Ship B - where the righting arm runs out about 70 deg
"No specific recommendations or limiting values have yet been defined for ships like “B”. The easy way out(for the regulator) is to refuse or reduce the right to sail for “B” (as is the case in the UK, for example).The fact that a number of sisters of “B” have been plying the seas for decades without mishap, may provenothing other than that they have been lucky so far.....
.......As a consequence, reaching the angle of deck edge immersion (as a mean heeling angle) is a wakeup call for most experienced Tall Ship sailors. Going beyond that may spell a lot of trouble, staying below that should keep even ship B reasonably safe. This rule-of-thumb is applicable in any situation, day or night.
The critical angle(s) revisited Experts tend to agree today that the positive range of stablity should be in the region of 90° or more rather than 80° or less, with good reason. In fact, the latter would no longer be allowed in a number of countries."
.
billskip
Well-Known Member
Can you please show your calculation
Mark-1
Well-Known Member
The velocity of the water through the gap 0.5 m under water:
v = sqrt{2gh}
v = sqrt{2 * 9.81 * 0.5}
v = 3.13
Flow rate through a 1m opening:
Q = Av
Q = 1 metre * 3.13 = 3.13
So it's about 3.13 cubic meters per second.
BUT... that assume it flows through cleanly with no friction so I reduced it by a fiddle factor I made up 50%, I think.
So that's ~1.5 tons a second so 90 tons a minute. The vessel weighs about 600 tons. So 15pc.
Happy to be corrected.
bedouin
Well-Known Member
I believe those were recovered shortly after the accident.
franksingleton
Well-Known Member
My impression is that a surprising number of posters seem not to understand some rather basic limitations to weather prediction. In part, I put this down to unrealistic claims made by some 3rd party providers.
In a nutshell, small Weather features have short lifetimes. A Weather model cannot predict that a thunderstorm will occur over a speific location a few hours ahead. A model will do better if it knows that a storm is forming. A model can predict groups of storms pretty well. That is clear when you are using the Met Office rain radar app.
The predictability of any weather feature from gusts to major systems depends on the forcing features, a sea breeze should be predictable days in advance, for sea breeze effect we see on the French coast north of Ile de Re for example. However, there is far sea breeze is ndependent on their not being enough cloud to prevent the earth getting warm enough to create a sea breeze. Systems such as as tornadoes and waterspouts oprobably depend on just the right amount of latent heat release. Microbursts and other small scale effects are no doubt similar with vast amounts of latent heat release, possibly enhanced by local geographical effects.
We are a long way from being able to predict such small events and may well never succeed whatever forecast system is used.
When it comes to larger weather systems, our lows and highs there is still a fair amount of uncertainty although often less so than when I was in the hot seat. It will be interesting to see whether AI makes a significant change. I would not put much money on it.
In a nutshell, small Weather features have short lifetimes. A Weather model cannot predict that a thunderstorm will occur over a speific location a few hours ahead. A model will do better if it knows that a storm is forming. A model can predict groups of storms pretty well. That is clear when you are using the Met Office rain radar app.
The predictability of any weather feature from gusts to major systems depends on the forcing features, a sea breeze should be predictable days in advance, for sea breeze effect we see on the French coast north of Ile de Re for example. However, there is far sea breeze is ndependent on their not being enough cloud to prevent the earth getting warm enough to create a sea breeze. Systems such as as tornadoes and waterspouts oprobably depend on just the right amount of latent heat release. Microbursts and other small scale effects are no doubt similar with vast amounts of latent heat release, possibly enhanced by local geographical effects.
We are a long way from being able to predict such small events and may well never succeed whatever forecast system is used.
When it comes to larger weather systems, our lows and highs there is still a fair amount of uncertainty although often less so than when I was in the hot seat. It will be interesting to see whether AI makes a significant change. I would not put much money on it.
S04
Well-Known Member
I'm not sure of the applicability of the formula v=√(2gh) in this case. What you've calculated is the terminal velocity of an object with initial velocity of 0 m/s after falling under gravity from a height o 0.5 m.
If I drop a pork pie (for example) from a height of 0.5m it will be travelling at 3.13 m/s when it hits the floor. I can't see the correspondence with water entering an orifice so would welcome enlightenment.
Mark-1
Well-Known Member
I wondered that but according to google it's already built into Torricelli's Law. Water Pressure and Acceleration due to gravity are related and when you cancel it all out you get that 2gh.
But I make no claim to understand all this. No doubt a physicist will be along to offer the correct calculation if my attempt is wrong. (Having said that 90 tons a minute seems *very* plausible to me - having seen the result of a log being taken out.)
S04
Well-Known Member
That's surprising isn't it, thank you. We live and learn.
ylop
Well-Known Member
But forecasts do seem to quite accurately predict a risk of thunder/lightning in a general area and time period. I don’t think anyone expect to be able to say there will definitely be extreme weather at this exact spot at 0406. But is it reasonable to predict, within say a 5km radius that there is a significant* risk of extreme weather?
*significant is obviously open for debate - it needs a balance between false positive and false negatives which aren’t always equal => if you have too many false alarms nobody bothers, if you have a small number of missed opportunities and they result in deaths everyone gets hysterical.
franksingleton
Well-Known Member
From posts above, it seemed to me that some were thinking that it should be possible to give a high risk for a specific area at a useful time scale ahead.
Total lifetime of a single thunderstorm is about 6 hours. Allowing for the time taken for satellite and radar data to be received, it is difficult to see how warnings of a specific storm could be disseminated usefully in time for action to be taken.
Fine scale models use grid lengths of around 1.3 km . Given smoothing on models, the effective resolution must be nearer 10 km. However, looking at output from models, I cannot detect detail any way near 10 km. If an ensemble of fine scale models was run, it might be possible to quantify severe storm risk on a scale of several tens of km. Maybe an AI scheme could be developed to generate probabilities on a useful space time frame but we are not there yet.
WindyWindyWindy
Well-Known Member
I think people are overstating their certainty of the weather. Where are you getting this "direct hit from a tornado" from?
franksingleton
Well-Known Member
Not me.
Graham376
Well-Known Member
Bayesian Raised -
Cerebus
N/A
It may be just me but I found that YouTube clip distasteful (people perished).
Nothing to see really, just someone talking over a bit of film taken from a distance.
Official reports may be one thing (informative perhaps).
This clip was not to my taste; that could just be me of course; we are all different.