Angle of vanishing stability

[jwilson]

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...

I really don't think the designers ever intended any Southerly - even the later big ones - as a high-latitudes cruiser. They did advertise them as "blue water cruisers" but I doubt they meant "go anywhere". And remember that size matters: in really heavy weather I'd rather be in a production GRP 50 footer than a built-to-be-bulletproof steel 30-footer.

 

[KellysEye]

I was talking to a surveyor about AVS and what he said the French manufacturers Benetteau and Jenneau, then the biggest selling brands, hijacked it so their boats and all AWBs got a Cat A Ocean. I had no reason to disbelieve him.

 

[JimBrown]

I was talking to a surveyor about AVS and what he said the French manufacturers Benetteau and Jenneau, then the biggest selling brands, hijacked it so their boats and all AWBs got a Cat A Ocean. I had no reason to disbelieve him.

Utter rubbish. The STIX requirement was a highly British endeavour, building on and continuing the work of the Wolfson Unit and the RORC work on stability.

Having lauded the Contessa 32 as being the epitome of 'seaworthiness', this was then deemed to be the benchmark for the bottom end of the range. It's STIX was 32 so 32 became the minimum for CAT A.

If Beneteau had had any any undue influence, they would have wanted it at 30 so as to include the Figaro 1 as an ocean going boat. As it was it only got CAT B, so requiring Beneteau to completely redesign it and launch the Figaro 11 in order to be eligible for ocean racing.

 

[Tranona]

I was talking to a surveyor about AVS and what he said the French manufacturers Benetteau and Jenneau, then the biggest selling brands, hijacked it so their boats and all AWBs got a Cat A Ocean. I had no reason to disbelieve him.

Absolute rubbish - as usual. Why do you always believe just one person just because he says it is true. If you were at all interested in the subject you could at least look at the origins of the RCD and then you will find that the prime movers in establishing the standards were in fact British - and not directly connected with any manufacturers.

 

[jwilson]

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.

Agree with all you say except about AVS and STIX being in handbooks. My 2004/5 Jeanneau SO35 handbook definitely does not give either, just quotes Cat A for six etc. Maybe newer boat handbooks do.

I strongly suspect the AVS is quite low, the fact that the design is heavily reliant on form stability is evidenced by the fact that a near-identical except for LOA Jeanneau SO 37 lost it's keel and was sailed quite a way further before it was discovered. I'm not worried: the boat does what it says on the tin very nicely: a good fair weather coastal cruiser that will also get from A to B extremely slammily if needed in a "yachtsman's gale".

 

[dom]

Tremendous strides have been taken in stability and yacht design over the past twenty years or so and it is largely predicated on hard science - it is also perfectly plain to see.

For example last Saturday a friend and I decided to go for a sail from Portsmouth to the Nab then back to Cowes, really just to see how two very different yachts performed, side-by-side, both with an AVS of around 130 degrees and both around 45'. One was a 1980s Swedish cruiser racer built by a near legendary boat-builder and the other a modern French equivalent. Wind was westerly about 35-45kts.

On the run down to the Nab the Swedish boat became hard to manage as the waves built - its stern would be picked up by the waves and unable to gain speed the force was quickly turned into a yaw and a strange nose-down attitude. Nothing in the least dangerous here, just uncomfortable, slow and a bit weird really. The French boat just flew away, happily surfing under complete control.

We turned around as the tide began to set to the west with the usual East Solent wind over tide bump. To both our surprises the French boat effortlessly walked away from its Swedish elder, 2-3kts faster, higher and under much better control. Test was unfair as French boat sported a laminate blade #4 jib set off an inner say whereas the Swedish boat a heavy duty Dacron affair. When the French boat's blade was replaced with a small heavily reefed roller genny, both boats behaved, well like pigs really, the French one was still faster due to better foils, laminate main, slightly longer LWL, who knows?

Now in 35-45kts out near the Nab, what was the honest chance of a wave/wind inversion? The answer has to have been nada, zip, zero. The truth is boats get knocked down in truly violent conditions with big breaking seas. A 120 deg+ AVS and a small section underneath the inverted AVS curve will ensure a swift recovery. The only serious danger is a rogue wave in the aftermath of a storm which is much bigger than its neighbours, which may not be big enough to right the boat. If this happens one either either needs the RNLI approach (not really possible on a sleekish yacht which ever hopes to make to windward in a blow), or that ultra-modern invention - a canting keel!

 

[doug748]

I was talking to a surveyor about AVS and what he said the French manufacturers Benetteau and Jenneau, then the biggest selling brands, hijacked it so their boats and all AWBs got a Cat A Ocean. I had no reason to disbelieve him.

There may well be a grain of truth in this. It was certainly a process that involved a lot of "negotiation" always a fertile area for conspiracy and indeed conspiracy theory.

Rule formulas and minimum standards often produce anomalous design. In this case, club footed keels and/or very deep draughted family family cruisers which are being pinched up[/I ]into the bottom of Cat A.

More worryingly, I feel, some 34ft+ boats with very poor stability which are drawn up with little regard to seaworthiness and, in effect, are working down to the rule.

 

[Tranona]

Agree with all you say except about AVS and STIX being in handbooks. My 2004/5 Jeanneau SO35 handbook definitely does not give either, just quotes Cat A for six etc. Maybe newer boat handbooks do.

Mine is a 2015 boat. There is a section on stability in general with a table showing the AVS and STIX for the two versions. Interestingly the shallow draft version (which I have) is virtually identical to the deep draft, perhaps because it has an extra 200kgs ballast mostly in the bulb.

 

[Tranona]

There may well be a grain of truth in this. It was certainly a process that involved a lot of "negotiation" always a fertile area for conspiracy and indeed conspiracy theory.

Rule formulas and minimum standards often produce anomalous design. In this case, club footed keels and/or very deep draughted family family cruisers which are being pinched up[/I ]into the bottom of Cat A.

More worryingly, I feel, some 34ft+ boats with very poor stability which are drawn up with little regard to seaworthiness and, in effect, are working down to the rule.

See posts #23&24. The standards were established independent of the manufacturers.

Of course they will then try to design and build boats that get into the category that fits the market they are aiming at.

The reality is that it is very marginal to get boats under 10m into Cat A, primarily because STIX has a heavy weighting for LOA. As noted earlier the STIX value of 32 as a minimum was set with reference to the CO 32 which is unlike most modern boats and probably the only sub 35' boat of its era that would get into Cat A, despite their high ballast ratios.

 

[Just_sayin']

I was impressed by this:

https://www.youtube.com/watch?v=b0TxpxCyMno

 

[Deleted member 36384]

STIX Numbers on the IRC website for some yachts that I assume have been rated under IRC; not as comprehensive as the RYA list (which I had a copy of but cant find now): -

http://www.ircrating.org/technical-a-certification/sssn-a-stix

 

[Tranona]

I was impressed by this:

https://www.youtube.com/watch?v=b0TxpxCyMno

That boat of course is one of the type with "poor stability" that some on here worry about!

The clip illustrates that all the talk about high latitudes and ocean cruising etc is misleading. The danger points for capsize are situations like this one - restricted entrances to lee shore hambours with long fetches to windward, areas where there are sudden changes in depths with strong winds and currents, often close to shore. So Needles Channel, western approaches, Biscay ports, Bass Straits, Anghulas current etc. All scenes of documented founderings due to sea conditions.

 

[Robin]

I was talking to a surveyor about AVS and what he said the French manufacturers Benetteau and Jenneau, then the biggest selling brands, hijacked it so their boats and all AWBs got a Cat A Ocean. I had no reason to disbelieve him.

And the STIX and AVS figures for your heavy steel boat are what exactly? my 1998 Benny 36CC is 35 STIX and 126AVS with it's shoal draught wing keel, not that I give a damn, Neptune doesn't do spreadsheets, saltwater got into his lappie years back!

 

[olly_love]

So, this was a question that was asked alot about the keel slamming through the boat.
we had a model in the sales office to show what would happen.
no boat even goes from upright to 180 in 0 time it takes a while(normally after a knockdown) the keel would gradually go into the hull.

also we calcs to prove that the keel couldnt break through the keel box,

it is about 30mm thick in that area some times more as there alot of overlaps which all tie down through the hull.

also when we designed the later boats (57-47-535-49) all were designed as go anywhere boats. with heavy laid grp structures with single skin below the waterline ( about 20mm thk)
quite close frame spacing etc,

 

[dom]

So, this was a question that was asked alot about the keel slamming through the boat.
we had a model in the sales office to show what would happen.
no boat even goes from upright to 180 in 0 time it takes a while(normally after a knockdown) the keel would gradually go into the hull.

also we calcs to prove that the keel couldnt break through the keel box,

It would be interesting to see those calcs or at least their underlying assumptions. I agree that rollovers aren't that quick, but I'm not sure that necessarily implies a damped retraction of an unrestrained swing keel.

 

[macd]

The danger points for capsize are situations...Needles Channel, western approaches, Biscay ports, Bass Straits, Anghulas current etc. All scenes of documented founderings due to sea conditions.

Please don't forget Portugal. Its harbours regularly claim yachting victims. The toll is perhaps partly a result of the sense of relief, and easing of vigilance, at having left Biscay behind. It can be a nasty place even without wind if the swell is high.

 

[30boat]

Please don't forget Portugal. Its harbours regularly claim yachting victims. The toll is perhaps partly a result of the sense of relief, and easing of vigilance, at having left Biscay behind. It can be a nasty place even without wind if the swell is high.

+1

 

[doug748]

STIX Numbers on the IRC website for some yachts that I assume have been rated under IRC; not as comprehensive as the RYA list (which I had a copy of but cant find now): -

http://www.ircrating.org/technical-a-certification/sssn-a-stix

Thanks for that an excellent resource.

Tho the original poster seems to have lost interest.

 

[jwilson]

. snipped .....
no boat even goes from upright to 180 in 0 time it takes a while(normally after a knockdown) the keel would gradually go into the hull.

Having been inverted I can assure you it CAN be very fast.

First time knocked down I was at the chart table. First a couple of seconds of sound of express train approaching, then the train hit. Went from sitting at chart table to landing on underside (now inverted) inside of coachroof in maybe at most two seconds later. and standing up. The roll back upright was slower, perhaps 5-10 seconds.

I'm assuming Olly is referring to later bigger Southerlies about which I am not very familiar: AFAIK the earlier Southerlies had the keel held down only by weight, the hydraulic ram pulled kevlar lines to lift. In a rapid inversion the force exerted could be catastrophic. If the later boats had directly attached rams there could be good damping.

 

[GrahamM376]

I really don't think the designers ever intended any Southerly - even the later big ones - as a high-latitudes cruiser. They did advertise them as "blue water cruisers" but I doubt they meant "go anywhere". And remember that size matters: in really heavy weather I'd rather be in a production GRP 50 footer than a built-to-be-bulletproof steel 30-footer.

When we visited Northshore to look at Sountherlies some years ago, we were asked if any long passages such as Biscay were planned. When we replied in the affirmative, the advice was that they were not suitable and we would be better looking at Vancouvers. Whether they made more profit on those I don't know.