Seaworthiness. Do the maths tell the true story?

[[178529]]

I always love to hear storm tactic recommendations from people who have never experienced such conditions at sea and whose profound insight is based on the successful viewing of a Youtube video.

Why the smarmy dig?

 

[TernVI]

For any boat such as a Pogo to perform as specced it needs to be and remain light. That is not a likely scenario in a cruising boat and the smaller the boat is, the more improbable.

Sea state significantly reduces stability as does speed in a displacement hull. It should be noted that all the common ratios including the AVS curves are based on hydrostatics which, as the the name says, are just that, static. Dynamic stability is quite a different matter and much more difficult to assess.

In an AVS curve the steepness of the transition at zero and the relative size of the negative stability area below, indicate the inverted stability. The greater this inverted stability is, the more likely the boat will remain upside down and the longer it will take for a wave of sufficient energy to come along to right it. Statistically this is usually expressed in minutes; I guess it depends how long you can hold your breath in freezing cold water. A boat with an AVS of 150 degr. or more is considered to be pretty much immune to remaining upside down.
Various organizations that have concerned themselves with the safety of yachts, racing and otherwise have found that the minimum AVS for boats under 10m should be 130 degr.

I always love to hear storm tactic recommendations from people who have never experienced such conditions at sea and whose profound insight is based on the successful viewing of a Youtube video.

AVS alone doesn't tell you much, a log with a nail in it has an AVS of about 179 degrees....

 

[Laminar Flow]

Why the smarmy dig?

Because most of these discussions are about as reality based for the common sailor as pondering the merits of various types of martian rovers for your next family holiday.
While I have experienced such conditions several times, I would generally refrain from giving any recommendations other than stating what did and didn't work for a very particular boat in a very particular situation.

AVS alone doesn't tell you much, a log with a nail in it has an AVS of about 179 degrees....

Interesting. How many logs have you sailed on?

 

[savageseadog]

In the past a lot of yacht design was empirically based, experience and a degree of conservatism were the norm. I think a lot of caution needs to apply with regard to grounding where loads are hard to predict and of course shock loadings from slamming and breaking waves, this applies to rig hull and keel attachment. I don't think there are any easy formulae, it's more a case of a trade off between safety and speed.

 

[[178529]]

Because most of these discussions are about as reality based for the common sailor as pondering the merits of various types of martian rovers for your next family holiday.
While I have experienced such conditions several times, I would generally refrain from giving any recommendations other than stating what did and didn't work for a very particular boat in a very particular situation.

Interesting. How many logs have you sailed on?

Just oozing condescension

 

[Laminar Flow]

Just oozing condescension

Perhaps. And perhaps I've also grown a little thin-skinned about folk expounding second and third-hand opinions as mantra and thereby potentially endangering others by encouraging them to try out some of the rather more dubious procedures proposed.

As that may be, I would never suggest something I do not have personal experience of or have tried out for myself.

Re the log: a silly comment in what is essentially a technical discourse deserves an equally silly answer.

 

[jwilson]

Interesting that a UFO34 gets "red-lighted" on the capsize screening number. I can assure you that a UFO 34 self-rights fast both from totally inverted (once) and keel-well-in-the-air (three times). It was a litle breezy and the sea a bit lumpy at the time. If in teh same conditions again (which i have now no intention of being) I'd prefer a Nich 35, Rival 34 or Barbican 35, not for capsize resistance, but for having less "racy" a hull form, the fine bow of the UFO being IMHO much of the cause of the problems occurring when running with bare pole.

 

[geem]

Interesting that a UFO34 gets "red-lighted" on the capsize screening number. I can assure you that a UFO 34 self-rights fast both from totally inverted (once) and keel-well-in-the-air (three times). It was a litle breezy and the sea a bit lumpy at the time. If in teh same conditions again (which i have now no intention of being) I'd prefer a Nich 35, Rival 34 or Barbican 35, not for capsize resistance, but for having less "racy" a hull form, the fine bow of the UFO being IMHO much of the cause of the problems occurring when running with bare pole.

Maybe a change of technique might be better than changing the boat. Deploy a JSD?

 

[Birdseye]

"Do the maths tell the true story?" is the headline question but which maths is the OP referring to?

Its not maths but simply physics that tells you what is the AVS of a particular boat, what are the angles at which is it stable when inverted, and how much energy is required to invert the boat. Usually this is calculated by design software and it takes into account aspects like deck saloons which make the inverted stability for the OPs boat much greater than its non deck saloon variant. Inevitably, this wont / cant take into account the clutter on deck or even if the still hoisted sails are acting as roll dampers preventing roll back to mast uppermost. But above all, it cant and doesnt take into account damage like stoved in windows or even open hatches. So it cannot be any indication of your chances of sinking if thats what you mean by seaworthyness.

The other branch of maths - statistics - gives you a better indication of seaworthyness though a statistician would shudder at the quality of the data involved. In an event like the Fastnet disaster, there wasnt enough data available to produce a mathematically sound analysis of seaworthyness but there was sufficient when added to human judgement and post event anayliysis to get a good idea of what characteristics were of benefit and what werent. Hence STIX. Perhaps the key statistical finding was that the seaworthyness of all the boats was better than that of the crews who on occasions gave up long before their boat did.

 

[TernVI]

Interesting that a UFO34 gets "red-lighted" on the capsize screening number. I can assure you that a UFO 34 self-rights fast both from totally inverted (once) and keel-well-in-the-air (three times). It was a litle breezy and the sea a bit lumpy at the time. If in teh same conditions again (which i have now no intention of being) I'd prefer a Nich 35, Rival 34 or Barbican 35, not for capsize resistance, but for having less "racy" a hull form, the fine bow of the UFO being IMHO much of the cause of the problems occurring when running with bare pole.

I don't think describing the UFO as 'racy' tells the story. It's a shape which you wouldn't design for a racer today. It's a rating rule shape. I suspect the shape of the back end is as much to blame as the front for instability when running?

 

[flaming]

Time and time again after big storms hit fleets of boats we have seen that when you are talking about small boats there probably isn't a magic design, or a magic tactic that guarantees the safety of the boat. And that not being in the very worst of the weather is by far and away the best survival "technique". In both the 79 Fastnet and the 98 Hobart, if you look at the casualties the main thing they have in common is the location, suggesting that before the days of weather routing being freely available and used widely in Ocean racing these boats were mostly simply unlucky with where they were. The same applies to the Golden Globe fleet who had no weather routing. Even the most "seaworthy" 35 footers using "proven" storm techniques were no match for the worst that the Southern Ocean could throw at them. And realistically even if they had been using the best routing available, they didn't have the speed to get out of the way of the worst of the weather. The 2017 Ostar is another example. The fastest boats were able to route around the worst of the weather, whilst the slower, more traditional, boats got hammered.

I'll maintain that the best storm survival technique in the modern era increasingly seems to be good weather routing and the speed to get the hell out of the way of incoming weather. Relying on the design of your boat, or your storm techniques, hasn't had the best record over recent times.

 

[doug748]

Interesting that a UFO34 gets "red-lighted" on the capsize screening number. ....

Yes, that has got to be an anomaly, note the OP says:

"The colour coding is based somewhat on the advice provided by Charles Doane in his book "The Modern Cruising Sailboat".

The UFO figures look as traditionally respectable as most. I don't know Mr Doane's book maybe he has other reasons or just a bee in his bonnet.

.

 

[TLouth7]

If a Pogo or similar turned turtle would it right it self? I wouldn't think so........

I couldn't find a curve for the Pogo 30, here is the curve for the Pogo 12.5, keel up and down.
By eye I would say that is comparable to other yachts, including many that are considered perfectly seaworthy. A Fairey Atalanta is fully self righting with her centreboards raised, but I know which boat I would rather face rough weather in.

 

[Chae_73]

Yes, that has got to be an anomaly, note the OP says:

"The colour coding is based somewhat on the advice provided by Charles Doane in his book "The Modern Cruising Sailboat".

The UFO figures look as traditionally respectable as most. I don't know Mr Doane's book maybe he has other reasons or just a bee in his bonnet.

It's just a function of beam and displacement. 2 is an arbitrary figure which came out of the '79 Fastnet. From Wikipedia:

The capsize screening formula (CSF) is a somewhat controversial figure. It is defined for sailboats as:
CSF = Beam / ((Displacement/64.2)1/3)
with displacement measured in pounds.
It came into being after the 1979 Fastnet race in England where a storm shredded the race fleet. The Cruising Club of America (CCA) put together a technical committee that analyzed race boat data. They came up with this formula to compare boats based on readily available data. The CCA characterizes the formula as "rough".
A lower value is supposed to indicate a sailboat is less likely to capsize. A value of 2 is taken as a cutoff for acceptable to certain race committees. However this is an arbitrary cutoff based on the performance of boats in the 1979 Fastnet race. The CSF does not consider the hull shape or ballast location.

 

[dunedin]

People try to read way too much into ratios and magic numbers.

Agreed. Many / most of the “magic formulas” are just the individual’s prejudice put into a very primitive set of ratios.

And many are ludicrously simplistic
- a large glass doghouse can artificially improve the AVS, but be useless in practice if doors/hatches open or windows smashed in (as quite probable with a southern ocean knockdown when AVS starts to be relevant).
- ballast ratios taking no account of centre of mass is also useless - less weight in a lead bulb much more effective than iron plate higher up.
Etc

Maths can and does work, but needs much more sophistication based upon wider models. This stuff is just prejudice in numbers.

 

[Quandary]

What the table seems to demonstrate very effectively is the direct relationship between high stability and slow performance, no surprise there of course.

 

[TernVI]

What the table seems to demonstrate very effectively is the direct relationship between high stability and slow performance, no surprise there of course.

Among a certain group of boats from a long time ago.....

 

[Robin]

I don't think describing the UFO as 'racy' tells the story. It's a shape which you wouldn't design for a racer today. It's a rating rule shape. I suspect the shape of the back end is as much to blame as the front for instability when running?

UFO 34 Black Arrow (RAF club boat) not only survived but finished and won her class in 1979 Fastnet I'd have one in a heartbeat.

 

[Laminar Flow]

Interesting that a UFO34 gets "red-lighted" on the capsize screening number. I can assure you that a UFO 34 self-rights fast both from totally inverted (once) and keel-well-in-the-air (three times). It was a litle breezy and the sea a bit lumpy at the time. If in teh same conditions again (which i have now no intention of being) I'd prefer a Nich 35, Rival 34 or Barbican 35, not for capsize resistance, but for having less "racy" a hull form, the fine bow of the UFO being IMHO much of the cause of the problems occurring when running with bare pole.

First of all I would like to thank jwilson for his contribution; For once his is based on personal experience and I appreciate his conclusions as well.

In regards to the math: all the "numbers" are based on hydrostatics which in turn are based on the assumption that the water is flat and the boat is not moving, hence static. As my wife will happily assure anyone, there is nothing static at sea.

Dynamic factors such as wave action and shape, as well as the boat's own wave system, as it develops with increasing speed, significantly reduces stability. When you ad other elements to this, such as wave or aerodynamically induced rolling, it gets even more complicated.

This does not render the hydrostatic numbers entirely invalid, but one should view them simply as benchmarks to establish minimal safety. They do not actually tell us much about how a particular boat behaves in rough water or even all that much, and in some certainty, about whether she might capsize or not. Much of that has to do with the design and general shape of the boat, her balance, buoyancy in her ends, displacement, propensity to roll and her ability to resist it.

I appreciate all the comments as to "active" storm management such as weather routing etc.
These all assume that one has the technology to do so and that it is all functioning as it should, which is not a given in a violent salt laden environment, that the crew is well and fit and not exhausted, seasick or injured or short-handed. Things simply do not always work out as we like or planned; especially not at sea.

The idea that we can always outrun trouble and hence, we can get away with a less seaworthy design, is akin to claiming that because your vehicle is more maneuverable and speedy, you do not need airbags, seat belts and a crunch zone.

I have always found the argument that the boat was much stronger than the man, when it is found weeks later afloat and abandoned, not very reassuring. The main purpose of a boat is to convey it's crew safely across a body of water. If, for reasons of too violent a motion, or for lacking a degree of psychological shelter and reassurance, it fails to do so, then it is conclusively as failed a design, as if it had sunk, IMHO.
"Man is the measure of all things. Who measures differently, measures falsely" Oskar Kokoska.

 

[Laminar Flow]

It's just a function of beam and displacement. 2 is an arbitrary figure which came out of the '79 Fastnet. From Wikipedia:

The capsize screening formula (CSF) is a somewhat controversial figure. It is defined for sailboats as:
CSF = Beam / ((Displacement/64.2)1/3)
with displacement measured in pounds.
It came into being after the 1979 Fastnet race in England where a storm shredded the race fleet. The Cruising Club of America (CCA) put together a technical committee that analyzed race boat data. They came up with this formula to compare boats based on readily available data. The CCA characterizes the formula as "rough".
A lower value is supposed to indicate a sailboat is less likely to capsize. A value of 2 is taken as a cutoff for acceptable to certain race committees. However this is an arbitrary cutoff based on the performance of boats in the 1979 Fastnet race. The CSF does not consider the hull shape or ballast location.

I agree that it is a very rough number. It is, however, based on the well established (mathematically and by extensive tank testing) fact that wide beam and light displacement are the two greatest factors in regards to capsize.