Angle of vanishing stability

[Alan1]

Hi
Does anyone know if a list exists of the AVS of different boats. I am looking to compare a Southerly 110 (avs 160) with other class A boats.

Thank you for any help.

Regards

 

[jwilson]

Hi
Does anyone know if a list exists of the AVS of different boats. I am looking to compare a Southerly 110 (avs 160) with other class A boats.

Thank you for any help.

Regards

I don't know of any list, but bear in mind that the Southerlies (just like RNLI lifeboats) achieve very high AVS due to the buoyancy of a substantial deckhouse. I do not think the Southerlies deckhouses/windows are nearly as structurally strong as lifeboats.

That high AVS is all lost of course if a window smashes (and I have seen one big deckhouse window on a near new yacht smashed by water). Without an intact and closed-up deckhouse the Southerly AVS would be much lower.

Of course a massive ingress of water creates a lot of free-surface water below, thus making an inverted boat less stable upside down and thus more likely to right even with a low AVS. The snag is a lot of water to shift (forget electric bilge pumps - they won't move it fast even if the electrics still work).

 

[Tranona]

There is a list available from the RYA. However it is is incomplete.

The 110 has probably the highest AVS of any boat of its size, partly because of the deck saloon providing buoyancy and partly the high ballast ratio. Think you will find little difference between the keel up and keel down AVS which suggests that the superstructure makes a big contribution.

You will find most Cat A boats of that size have AVS in the range of 120-140. My new Bavaria 33 is 126 and is typical of its type. 120 is normally the minimum, but some boats such as Ovnis are lower but still get into A. Boats higher than 140 usually have either wheelhouses or deck saloons.

 

[cmedsailor]

http://www.tomdove.com/sailcalc/sailcalc.html

It does not include AVS but probably it's useful in order to compare various boats.

 

[dunedin]

As Wilson says, a huge caveat is needed over the influence of a high coach roof on the Southerly figures.
These assume that all the coach roof openings / hatches are sealed, and that they remain that way in the crisis event.

Perhaps a reasonable assumption in an RNLI lifeboat with hugely robust locking doors and windows. Perhaps less realistic in the average sailing yacht at sea.

Not a reason to avoid a Southerly, which seem to be well proven boats. But equally not a reason to buy one over other boats. Incidentally, if a Southerly did ever hit 180, what would stop the keel crashing back down (and perhaps through the case)?

 

[awol]

Boats higher than 140 usually have either wheelhouses or deck saloons.

...... or have a decent ballast/displacement ratio and don't rely heavily (or should that be "lightly" ) on form stability.

 

[jwilson]

I agree that it's not a reason to avoid a Southerly, which are excellent boats for their purpose, which I don't think the designers ever expected to be heavy-weather ocean passages.

An unsecured lift keel would worry me too in conditions potentially likely to cause an inversion (and I have been inverted in a yacht and know just how violent an event it can be). I think in those circumstances I'd have a Southerly keel raised so it had nowhere to move, which would mean either lying ahull (mercy of the gods, though certainly better than lying ahull with a fixed keel) or running downwind.

 

[Seajet]

Re lift keels retracting if the boat is inverted by a big wave, this thought occurred to me with my Anderson 22; if she was rolled the keel could indeed slam back up, which could only add to one's considerable troubles.

On the A22 a simple thick walled stainless tube or rod 29" long can be fitted between the top of the exposed keel nib and the steel deckhead plate of the lifting assembly.

I also have a 20mm steel pin to secure the 950lb keel up on the mooring and take the strain off the lifting gear; on a lift keeler where I couldn't fit a down-lock rod, I'd definitely want a pin like this to keep the keel extended.

This also applies to hydraulically lifting keels, I wouldn't want a loss of pressure to leave the keel free to go up & down...

 

[Birdseye]

I have a copy of the original RYA list but its incomplete, in part because manufacturers like Benny wont reveal the numbers. Its easy to imagine why.

AVS is a long way from the whole story - if it were then big ships with AVS in the 30/40 degree region would be capsizing all the time and they dont. The reason is simple - size matters. Its rare apparently for a boat to capsize under the influence of sail force alone. What is also required is energy transfer from waves and this in turn is a function of factors like freeboard and vessel weight. Then there is the issue of grip on the water - the Southerly disparaged above has a big advantage over most fin keelers. It can raise its keel which in turn allows it to slide sideways rather than trip over the keel. Thats one reason why cruising cats of a decent size rarely capsize despite AVS in the 45 deg region.

So rather than use a simple avs as a guide you are better using a number like the STIX which also takes into account things like bridgedeck height. Category A requires a minimum of 32. A sun Od 52 creeps in at 39. Interestingly a Starlight 35 is 46.

 

[AHoy2]

I can't locate the original article at the moment but found these graphs for Cat A boats when researching the AVS of the Scanyacht/LM27 as it has no AVS!





Edit: found the article here:

http://www.troldand.dk/en/?The_Yacht:Stability

 

[JimBrown]

. . . researching the AVS of the Scanyacht/LM27 as it has no AVS! . . .

A perfect example of where the wheelhouse provides a significant contribution. You just need to ask how much faith you have in a pair of wooden patio doors to remain watertight when inverted.

If you think they would, then why does the RNLI, RN and all commercial ships used massive metal structures and interlinked dog latches on their waterproof doors if a pair of light wooden panels, a couple of piano hinges and a £5 lock together works as well to keep the sea out when inverted?

 

[Seajet]

This was all covered in the Fastnet 1979 Inquiry, one of the main conclusions of which being ' have strong washboards and a lock operable from inside & out '.

This must also prevent the washboards dropping out it the boat is inverted...

I think Force 4 now sell such locks, and I use a sliding bolt to prevent the washboards moving vertically.

http://www.force4.co.uk/seasure-washboard-hatch-lock.html?sqr=hatch lock&#.VnAwSl7NSSo

 

[AHoy2]

A perfect example of where the wheelhouse provides a significant contribution. You just need to ask how much faith you have in a pair of wooden patio doors to remain watertight when inverted.

If you think they would, then why does the RNLI, RN and all commercial ships used massive metal structures and interlinked dog latches on their waterproof doors if a pair of light wooden panels, a couple of piano hinges and a £5 lock together works as well to keep the sea out when inverted?

I did say I was researching the data, not putting implicit faith in something!
Mind you, I dare say the piano hinges and cheapo lock are just as robust as the average companionway arrangement. As a matter of interest the doors are rebated into frames and do not rely on the fittings for closure under pressure, still not an imitation lifeboat I grant you but what Cat A boat is?

 

[Barbican]

I agree that it's not a reason to avoid a Southerly, which are excellent boats for their purpose, which I don't think the designers ever expected to be heavy-weather ocean passages.

An unsecured lift keel would worry me too in conditions potentially likely to cause an inversion (and I have been inverted in a yacht and know just how violent an event it can be). I think in those circumstances I'd have a Southerly keel raised so it had nowhere to move, which would mean either lying ahull (mercy of the gods, though certainly better than lying ahull with a fixed keel) or running downwind.

Agree with that - I definately plan to pin the keel up (4695lbs of it would definitely cause some damage in a free-fall in either direction) when there is a risk of inversion. There is no provision to lock it down, but I have heard that some folk have put the lock pin in place and raised the keel up to the pin. The boat will sail happily keel up, but makes a bit more leeway which in normal seas might be 10 degrees or so but I have no idea what it would be in the conditions that might cause inversion. I have no plans to test the theories........ As for the windows I was told a story that the leeward windows of a CO32 popped in during an inversion, so I guess if you want bullet proof stuff for such conditions, storm boards are worth considering.

 

[fisherman]

A minor point: a transatlantic rowing boat, with buoyancy pods fore and aft which contained the accommodation stayed inverted after a capsize because the men and equipment inside all fell onto the roof.

 

[lw395]

It's just one number, don't take it too seriously.
A log with a nail in it can have 179.99degrees AVS, it doesn't make it any use.

Also data for off-the-peg designs often does not include half the stuff boats tend to have, like radar and roller genoas.
A high AVS is not a bad thing, but a big 'eavy keel and a small companionway might be better in serious seas.

 

[Tranona]

...... or have a decent ballast/displacement ratio and don't rely heavily (or should that be "lightly" ) on form stability.

Unfortunately ballast ratio on its own does not necessarily lead to a higher AVS as many of the IOR influenced boats show. Remember that the IOR penalised stability, hence the wide beams and need to have crew on the rail to enable them to carry enough sail.

It is what you do with the ballast that matters and 1980's keels that are wide at the top, narrow at the bottom and made of iron are poor compared with today's keels which invariably have large bulbs at the bottom to get the ballast low, and often greater draft than similar size boats from the past.

As others have noted just one number does not necessarily tell you much and you can get the same AVS with a wide range of ballast ratios as the subject of this thread illustrates.

 

[doug748]

Hi
Does anyone know if a list exists of the AVS of different boats. I am looking to compare a Southerly 110 (avs 160) with other class A boats.

Thank you for any help.

Regards

Last time I looked for the RYA list I was unable to locate it. I have no idea why this interesting artefact has gone to ground except, as already observed, some makers have found it tricky to supply the necessary information. This is what the RYA think:

"To this end, by the time you read this, the RYA will have circulated a copy of this article to builders together with a request for stability data on their models. Where this has been received it well have been published on the Technical part of the RYA web site. If the data is not there then ask for it and also suggest it should be sent to the RYA. If it’s still not made available then smell a rat."

In a recent thread it was suggested (by a contributor close to the font) that Yachting Monthly no longer publish Stix numbers because of "lack of space" on their pages. Aye Aye.

 

[Tranona]

In a recent thread it was suggested (by a contributor close to the font) that Yachting Monthly no longer publish Stix numbers because of "lack of space" on their pages. Aye Aye.

It is a requirement of the RCD to include the AVS and STIX in the owners handbook that comes with the boat. Page 57 in my handbook.

No doubt if somebody had the energy it would not be difficult to collate this information for production boats. However suspect there really is very little interest in these figures amongst the general boating population and the keen ones are probably more interested in rating for racing. For cruisers ultimate stability is of little interest as few ever go anywhere near that limit and there has not been any significant number of cases of boats capsizing to raise concerns.

As has been noted on many occasions one figure on its own tells you very little and even the composite measures are compromised by the figures chosen and the weightings given. Much of the attraction of a sailing boat is derived from subjective feelings that take little account of technical analysis.

 

[grumpy_o_g]

I agree that it's not a reason to avoid a Southerly, which are excellent boats for their purpose, which I don't think the designers ever expected to be heavy-weather ocean passages.

An unsecured lift keel would worry me too in conditions potentially likely to cause an inversion (and I have been inverted in a yacht and know just how violent an event it can be). I think in those circumstances I'd have a Southerly keel raised so it had nowhere to move, which would mean either lying ahull (mercy of the gods, though certainly better than lying ahull with a fixed keel) or running downwind.

I thought that's exactly what the later Southerly's were designed for? They've got a large chunk of iron in the bottom the hull and the keel is operated by a hydraulic ram so won't drop in for quite a long time...