Sailing boats' Stability, STIX and Old Ratios

[Guillermo]

Very interesting analysis on canting keels and stability (With their impact on STIX):
http://www.yachting.org.au/site/yac...the submission/Appendix 6 Reichel Pu~0008.pdf

Interesting to note how the STIX diminishes when canting the keel either the side, in spite of some increases in overall stability, as it is not conceived for this kind of keels.

A nice example of how we neither shouldn't do simplistic interpretations of the STIX number, nor of GZ curves:

From there:
"The negative values from the GZ curve are only an indication of the direction of rotation and not of inverted stability. The fact that, in a yacht with the keel on centerline position, the range of inverted stability coincides with the negative GZ values has led to the misconception that it is true for all GZ curves."

I also attach the document, just in case the address has any problems.

Cheers

 

[Guillermo]

Another interesting sailboat under 30' with STIX over 32, having a totally positive GZ curve:
The Scanyachts 290, derived from the LM27
http://www.scanyachts.co.uk/290b.htm

Cheers

 

[Crag Cay]

Hi Guillermo - I guess that's with the pilothouse door shut and all the windows remaining intact? Do you think both these factors are a certainty if its rough enough to roll 360?

With regard to the canting keel article, the 'villains of the piece' again seem to be excessive beam and inadequate engineering of the keel. Do you know of any canting keel boats that have remained inverted with their keels fully intact and the control systems still operating?

 

[Antonio Alcalá]

Excellent documents Guillermo, both are really interesting. Everyday I learn something in this forum.

West winds ( if your are on parallel 40º)

 

[Antonio Alcalá]

Could we imagine an ARC to Europe with any designs of Beneteau in bad weather?

Do we do the right without claiming more security to the ARC Head Office ?

Do we need a "little Sidney-Hobart" during the ARC for having clear ideas about what is right and what no?

ISO STIX 12217

Beneteau 57 53
Beneteau 50 41
Beneteau 473 51
Beneteau 461 37
Beneteau 44CC 34
Beneteau 423 38
Beneteau 411 37
Beneteau 393 43
Beneteau 381 38
Beneteau 373 36
Beneteau 36CC 35
Beneteau 361 32
Beneteau 351 35
Beneteau 343 34
Beneteau 331 28
331 New version 32
Beneteau 323 26
Beneteau 321 33
Beneteau 311 27
Cyclades 39 38
Cyclades 43 36

Figaro 2 33

Beneteau First

Beneteau First 25,7 22
Beneteau First 26 22
Beneteau First 27,7 28
Beneteau First 31,7 30
Beneteau First 33,7 32
Beneteau First 36,7 34
Beneteau First 40,7 37
Beneteau First 42,7 35
Beneteau First 44,7 41
Beneteau First 47,7 46


Any comments guys?

Antonio

 

[Guillermo]

Hi Guillermo - I guess that's with the pilothouse door shut and all the windows remaining intact? Do you think both these factors are a certainty if its rough enough to roll 360?

I guess that too. I'm not sure about the certainty, but we have to consider that to give a CE STIX number under the ISO 12217, boats must previously comply with watertightness as per ISO 12216. Relating the pilothouse door, this last only asks for it to comply with locking and securing exigencies, but to keep it shut when needed is a crew's decission. As a first approach, I have to believe in the honesty of the designer, the builder and the Notified Body involved in the boat's categorization. (Well, all of us, users, have to, haven't we? )

With regard to the canting keel article, the 'villains of the piece' again seem to be excessive beam and inadequate engineering of the keel. Do you know of any canting keel boats that have remained inverted with their keels fully intact and the control systems still operating?

Good question. The answer is no, or at least I don't remember at this moment, but I'll search for that. On your side...do you? Also other people's sharing their knowledge on this would be nice.

Cheers.

 

[Guillermo]

...Do we do the right without claiming more security to the ARC Head Office ?

Antonio, do they ask for a minimum STIX, Capsize Ratio, RORC's SSSN, or whatever?

Cheers.

 

[Guillermo]

I am wondering if we could use the 'Capsize Length' as defined by Karl L. Kirkman in the paper 'On the effect of size - as related to capsize resistance' included as Appendix A at the 'Final Report of the Directors' on the Fastnet Race '79, to give us a clue to seaworthiness to be used in parallel to the STIX.

This 'Capsize Length' (Imperial units) is defined as follows:
Capsize length L' = L*((base B/B)^2 + (base C/C)^2)^.5 * I/base I

Where:
L = measured length (MHS)
Base B = L/4 + 2
B = measured beam
Base C = 2 ft (center of pressure above VCG)
C = 2 – CGTOT
CGTOT = center of gravity referred to DWL
I = moment of inertia
base I = 0.135*L^4.5

If we estimate moment of inertia I as Disp^1.744/35.5 (SNAME/USYRU Safety from Capsize Committee), using most usually available data, we may quickly get a (not MHS but STIX like data based) Capsize Length for any boat and then compare it with the STIX.

As STIX is somewhat also a corrected length, we may compare 'length versus length' and so have a comparative about how the two criteria categorize a particular boat. Could it be somekind of a 'correcting factor' to STIX?

Applying this to the 'Zara of Arran' classic boat I posted before (Post # 126), using L as 0.7*Lwl+0.3*Lh and based on the several estimatives for her, I find that the Capsize Length for this boat is 36.31 ft. It's STIX is barely 29.

The Final Report of the Directors upwards mentioned, proposed that the minimum AVS for a boat should be: AVS (min) = 160 - L' . In the case of 'Zara of Arran', this calls for an AVS (min) of 123.7 which in this boat's case is much lower than the real value, 152.

May we say, then, that 'Zara of Arran' in spite of having an STIX of 29 is an ocean going boat? Well, it fulfills all the requirements of the ORC regulations derived from the Fastnet Race reports: It has a Capsize Safety Factor (CSF) of 1.51 (well under 2), a Capsize Length over 30 and her AVS is much greater than the minimum required, so.....

I'll try to do this comparative for several boats and post here what I find. Perhaps somebody has already gone into this comparative and if so we all may save time. Anybody?

 

[Guillermo]

Some estimated data for the A35 type of boat:
(Basic data taken from: http://www.mussetplaisance.com/Pdf/A35.pdf . I'm not sure about her displacement condition)

Length/Beam Ratio L/B = 2,78
Ballast/Disp Ratio W/Disp = 0,4
Displacement/Length Ratio D/L = 133,66
Sail Area/Disp. Ratio SA/D = 24,01
Velocity Ratio VR = 1,24
Capsize Safety Factor CSF = 2,22
Motion Comfort Ratio MCR = 16,67
Heft Ratio HF = 0,61
Roll Period T = 1,91 Sec
Roll Acceleration Acc = 0,25 G's
Stability Index SI = 0,54
Angle of Vanishing Stability AVS = 120 º
Dellenbaugh Angle DA = 22,42 º
Initial Metacentric height GMo = 1,74 M

Nice boat for coastal racing courses, but saying this is a bluewater family cruising boat would be quite enthousiastic in spite of her CE A categorization. And being a single ruddered boat with that wide beam, things will become interesting when strongly heeled or under chute in strong conditions.

Just to state a not mine opinion (I do not subscribe it as I do not have enough info, but tend to coincide):
"L'A-35 est un très bon bateau IRC, son palmares en 2006 est impressionant. C'est probablement le bateau parfait pour le Trophée Atlantique. Inversement, ce n'est peut être pas le candidat idéal pour l'Offshore. Sa forme de coque nécessitant une bonne partie de l'équipage à l'arrière dans la brise n'est pas vraiment commode pour des courses longues. Il n'est probablement pas stable au portant dans la brise."

 

[Guillermo]

A quick look on two very close models from X-Yachts: The racer X-41 and the cruiser X-40

(Approximate numbers)

X-41 (Racing)
http://www.x-yachts.com/seeems/17834.asp

Beam/Length Ratio B/L = 3,07
Ballast/Disp Ratio W/Disp = 0,38
Displacement/Length Ratio D/L = 163,25
Sail Area/Disp. Ratio SA/D = 27,3
Velocity Ratio VR = 1,28
Capsize Safety Factor CSF = 1,91
Motion Comfort Ratio MCR = 24,22
Downflooding angle: Fd = 105 º
Angle of Vanishing Stability AVS = 121 º
Roll Period T = 2,55 Sec
Roll Acceleration Acc = 0,15 G's
Stability Index SI = 0,7
Heft Ratio HF = 0,85
Dellenbaugh Angle DA = 32,82
Initial Metacentric height GMo = 1,16 M
Righting Arm 10º RA10 = 0,2 M
Righting Arm 20º RA20 = 0,37 M
Righting Arm 30º RA30 = 0,49 M


X-40 (Cruising)
http://www.x-yachts.com/seeems/12235.asp

Beam/Length Ratio B/L = 2,94
Ballast/Disp Ratio W/Disp = 0,41
Displacement/Length Ratio D/L = 177,1
Sail Area/Disp. Ratio SA/D = 21,96
Velocity Ratio VR = 1,19
Capsize Safety Factor CSF = 1,93
Motion Comfort Ratio MCR = 25,03
Downflooding angle: Fd = 105 º
Angle of Vanishing Stability AVS = 121 º
Roll Period T = 2,57 Sec
Roll Acceleration Acc = 0,15 G's
Stability Index SI = 0,68
Heft Ratio HF = 0,86
Dellenbaugh Angle DA = 24,03
Initial Metacentric height GMo = 1,24 M
Righting Arm 10º RA10 = 0,21 M
Righting Arm 20º RA20 = 0,39 M
Righting Arm 30º RA30 = 0,53 M

Interesting to find out that the racing boat appears to be slightly slender (L/B) and less stiffier (GMo) than the cruising one (Although bigger mass influences T, making it slightly bigger for the cruising boat). Interesting also the diverging tendency showed by SI and Heft Ratio.


And now the new cruiser-racer J-122:
http://www.jboats.com/j122/j122dimensions.htm


Beam/Length Ratio B/L = 3,04
Ballast/Disp Ratio W/Disp = 0,36
Displacement/Length Ratio D/L = 168,89
Sail Area/Disp. Ratio SA/D = 23,09
Velocity Ratio VR = 1,21
Capsize Safety Factor CSF = 1,9
Motion Comfort Ratio MCR = 24,5
Downflooding angle: Fd = 105 º
Angle of Vanishing Stability AVS = 120 º
Roll Period T = 2,56 Sec
Roll Acceleration Acc = 0,15 G's
Stability Index SI = 0,71
Heft Ratio HF = 0,86
Dellenbaugh Angle DA = 28,29
Initial Metacentric height GMo = 1,14 M
Righting Arm 10º RA10 = 0,2 M
Righting Arm 20º RA20 = 0,36 M
Righting Arm 30º RA30 = 0,49 M


I like much more this tendency than the A-35 and the like one, from the cruising point of view. I would like to have a look to their real stability curves and know their STIXs. If someone has info on this, will be greatly appreciated.

Cheers.

 

[Guillermo]

Let's make another comparative on STIX vs Capsize Length, both expressed in feet, following the idea in post 188.

Taking the RM1200 I posted at post 175 we have for her:

Lh = 39.34 ft
Lwl = 37.47 ft
And from those we get:
Base Length Factor (in feet) = 38.09 ft

Now, asuming her downflooding angle is 100º, we have:

STIX (100) = 32.92 ft
Capsize Length L' = 22.57 ft

So we see that the Capsize Length is even smaller than her STIX in ft, which is already smaller than the Base Length.

This boat would have not been allowed for blue water racing under the ORC regulations, as her CSF is as high as 2.24, her Capsize Length is much lower than the minimum required of 30 ft and her 119º AVS is also lower than the 137.43º required by the ORC in this case.

This numbers need a more accurate study, as probably the real moment of inertia of a bulbed boat may significatively differ from the estimated by the formula I = Disp^1.744/35.5 used for the calculation of L'. I need to work on this.

Modern bulbed boats with their very low VCOG and high moment of inertia compared to their light weight, should probably qualify higher, both at ORC and STIX. I think we'd need to revise the way of estimating the moment of inertia for its use at the Capsize Length formula, and also a way of taking it into account for the STIX.

 

[Guillermo]

Something basic but interesting for further discussions

 

[Guillermo]

Open 60 stability curves

Someone (Antonio?) asked about Open 60's stability curves. The only clues I've found in internet about Open 60's stability showing some curves, are at Finot Group's pages:
http://www.finot.com/general/index_ang.htm

Interesting to compare it with the other I've previously posted:
http://www.yachting.org.au/site/yach... Pu~0008.pdf

Cheers.

 

[CT 249]

Could we imagine an ARC to Europe with any designs of Beneteau in bad weather?

Do we do the right without claiming more security to the ARC Head Office ?

Do we need a "little Sidney-Hobart" during the ARC for having clear ideas about what is right and what no?


Any comments guys?

Antonio

Well, in the "big Sydney Hobart" we had (from memory) one death basically from wave impact; one death from drowning in an inversion (LPS about 116, I think); one death from head injury in a roll over; and the remaining 3 deaths from the sinking of an old heavy boat.

We also had severe damage from rolls to and S&S 34 and a Cole 43 (pre-IOR boats, long, slender and heavy);

a man overboard from a heavy 43 footer, saved only by the extremely lucky chance that a helicopter was only minutes away;

a steel cruiser hove down till it lost its liferaft (the cruising sailors on board had a raft equipped for 6 crew in a boat with a crew of 8, and the raft didn't fit the chocks);

a heavy canoe-stern cruiser sank;

a non IOR lightweight rolled with massive deck damage (later sank, I think)

two IMS racer/cruisers rolled with severe damage;

a 65 foot light/medium racer rolled with severe damage;

etc

Given the fact that the fleet was mainly composed of IOR boats or IMS racers or racer/cruisers, there seems to be little evidence that there were any "clear ideas about what is right". The heavyweights suffered by far the worst death toll and sinking rate, pro rata.

Three of the designs that rolled (the S&S 34, the Cole 43, and the Farr 40 IOR) have done round-the-world singlehanded passages around Cape Horn and the Cape of Good Hope. Similarly, another '98 competitor, the Brolga 33 Berrimella, was rolled and was dismasted with other damage on her return from the latest Hobart - after she had sailed around the world two-handed via the great Capes last year.

The fact is that designs that rolled with damage in Hobart '98 and while returning from Hobart 2006 have completed something like 7 solo or doublehanded circumnavigations via Cape Horn without significant damage. It therefore seems that saying that boats that failed in the '98 Hobart are unseaworthy is raising the bar ridiculously high. "Unseaworthy" designs rarely complete 3 or more singlehanded or doublehanded circumnavigations via the Southern Ocean without significant damage.

Similarly, it often seems to be ignored that we have had precious little experience about how "seaworthy" designs would have fared in such conditions, as they were not there in great numbers. Two of the few heavy cruisers that competed sank through construction problems and one was knocked down with minimal damage. A Swan 46 was rolled about 180.

Similarly, while hte '79 Fastnet report noted that smaller boats had many problems (although the smallest did better than the second-smallest) this has not been the case in any of the bad Hobarts, which surely may indicate the problem with making decisions based on such small statistical samples; the problem with Class IV in the '79 Fastnet may well have been that they were in the wrong place for the weather.

Of interest, I checked one submission to a conference held after the '98 Hobart. It had a chart showing the capsizes of boats int he NZ-Tonga event, the Hobart and the Fastnet. It said that heavy cruising boats were safer, without actually defining the type. The real eye-opener was that it said that only one heavy cruising boat had capsized in these races. It ignored the several rolls from pre-IOR boats of a type very similar to the renowned Contessa 32, used so often as an example of a superior boat.

This is clearly wrong; several traditional heavy cruisers rolled or otherwise got into trouble in the Tonga event (the only one in which they were not vastly outnumbered by lighter racing boats, which is one reason using the Hobart and Fastnet to attack lightweights seems flawed in some respects). There does seem to be a fair bit of poor research from the heavy brigade.

 

[Guillermo]

....may indicate the problem with making decisions based on such small statistical samples...

Well, I agree and I don't. I think there has been quite a lot of investigation on the matter, done by scientifics (and not only) all around the world, based not only in real events, but also on tank testing. As I told in some other posts, I'm missing more accurate and complete statistics and studies from real life (like the Masuyama, Nakamura, Tatano and Takagi's one) based not only in catastrophic knockdowns, which by a matter of probability should be only the 'top of the iceberg' (Gauss bell), but in all kind of losts of control, course keeping instabilities, etc. These aspects have been repeatedly studied at tank tests, with quite clear results, but it would be very important to collect accurate data from real life. There are factors not measurable in tank tests, one of which is crews factor and their decission-making interference with results. Most interesting matter.
Cheers.