Sailing boats' Stability, STIX and Old Ratios

[Vega]

It always amuses me that boat designers have this fascination with these complex formulae and their inherent trade offs.

For all this information to be accessible to the buying public, it should be presented to them in answer to all the natural questions they have when looking for boat. The figures are available anyway, so wouldn't require more disclosure by the manufactures, but at the moment the relevancy of this information is lost when stirred together into the 'pot purri' of STIX or Categorisation.

It's also claimed to have began to help the consumer. So let's build on this second point by developing an information system that is accessible, meaningful, workable and allows the consumer to really make sound choices in the boats they want to buy. The consumer's needs should be the driving force for this process and not just to allow another oppurtunity for boat designers to fiddle with formulae.

Yes I agree with most of what you say. But far meaningless than STIX (regarding stability assessment) were what Guillermo calls “Old Ratios”. Those were very inaccurate tools that are only applicable (and very roughly) to a certain type of boat. They are meaningless if applied to modern bulbed sailboats.

The information that you say is available, is not. The sellers will not have a clue about what you are talking about and the boat manufacturers don’t disclose that information to the public. You have to ask for it, and sometimes they give it to you, others they don’t.

I believe you are overestimating the average knowledge of the typical boat buyer. Fact is that even professional specialized press, completely disregard the information about safety stability in the boats they are testing for their magazines. Even the British press that usually publish the GZ or RM curves of the boats they test, don’t make any comments about them, neither when they are very bad, nor extremely good.

If these guys don’t really know how to interpret those data, how do you expect the average buyer to do it?

STIX has the advantage of simplicity in its interpretation, the bigger the better, and even if far from perfection, it can give you an approximate idea of the boat’s stability. Better than nothing, I would say.

 

[Vega]

.I am convinced of it because I sail 28 years ago, and the last 8 sailing alone.
I enterely agree with you except in the STIX number and weight. For me would be 50 the right number for maximum security and 8.5 T at least.
.

Thanks António,
Regarding STIX and Class A bots, I was not talking about Maximum Security, but about Minimum requirements for a Class A boat.

About the Oceanis 473, your intention is sailing it solo?

Cheers

 

[Guillermo]

But far meaningless than STIX (regarding stability assessment) were what Guillermo calls “Old Ratios”. Those were very inaccurate tools that are only applicable (and very roughly) to a certain type of boat. They are meaningless if applied to modern bulbed sailboats.

I'm afraid I do not agree. 'Old ratios' were and still are a good tool to judge how a boat will behave at sea for certain aspects. And they also apply to modern bulbed sailboats, of course. Read again post 151 about Antonio's boat numbers. Oceanis 473 is a bulbeb keeler and her 'Old ratios' give a pretty good idea about the boat. Even the estimated GM and the GZs at 10º, 20º and 30º (based in simple estimatives) coincide quite well with the real curve's values.
Only AVS has to be taken with care because it usually gives values under the real thing, as it doesn't take into account volumes over deck. Even though it's a nice tool to judge a boat against other.
'Old ratios' are incomplete from the point of view of ultimate stability (they are more oriented towards initial stability) except for the CSF. And even this one has proven to be quite a good guide. I've studied both the Final Report of the Directors and the Inquiry about the '79 Fastnet Race and I find the approach to the CSF formula at the report (named 'capsize screen value' there) to be very valid.
I find STIX a much more misleading info than 'Old' ratios, as we have seen thoroughly through this thread.
If you have examples to support your statement with numbers, I'd appreciate your posting here.
Cheers

 

[fcfc]

Fcfc, I was talking about force 10 wind and 7m waves. These conditions have nothing to do with the ones with force 8 and 4/5m waves.

With force 8 and 4m waves the Dehler would not have any problem.

I agree. I was simply pointing that the perception of bad/good weather could be changed by other characteristics than the boat, wind force and waves heigh.

 

[Antonio Alcalá]

To Vega

Yes,Vega, I sail on 473 since 2005 March.Most of the times alone. I´ve been in a gale with 52 kts and 4 meter waves close to Isla de Alboran. I had no problems with the boat, very strong, very safe and even more very fast.I did escape of a low pressure system at 11.5 knots only with main sail 2 reefs and storm sail. All were perfect while steering at 120-130 º of wind. First example where I could prove to myself, the safety of speed for escaping a gale. Do you think that it had the same with a heavy weight boat?

Best winds

 

[rayk]

The answer to the most important question in the universe is 42. Simple enough?

overestimating the average knowledge of the typical boat buyer. Fact is that even professional specialized press, completely disregard the information about safety stability in the boats they are testing for their magazines. Even the British press that usually publish the GZ or RM curves of the boats they test, don’t make any comments about them, neither when they are very bad, nor extremely good.

If these guys don’t really know how to interpret those data, how do you expect the average buyer to do it?

Its a graph dude. The data (numbers) has been interpereted into a graph. If the buyer is reading new boat reviews, they have money, and reading graphs goes with the territory.

STIX has the advantage of simplicity in its interpretation, the bigger the better, and even if far from perfection, it can give you an approximate idea of the boat’s stability. Better than nothing, I would say.

A single number doesnt tell me more than a RM graph.


Average boat buyer is learning all the time and devotes a lot of time to studying. Boat owners understand more than you give them credit for. Buyers are more likely to understand a boats character and make a reasonable judgement, without this new layer of calculations.
STIX is a sum of so many variables it is meaningless. Two different boats with the same number,and one of them is going to look obviously more suitable for the conditions in mind to the prospective customer.
If all the input data for STIX was available to prospective buyers, they could make informed choices trading off good and bad points between boats. These basic numbers arent always available and old ratios are therefore difficult to compute when consumers really want to compare boats. If it is so difficult to obtain basic data for a boat in the market now, a new emperical number defining stability is not needed.

A broker selling foolproof boats with safety numbers/factors/ratios is the surest way of leading lambs to the slaughter.

By the way whats the STIX for Slocums Spray, it must be crap. Or Wanderer III how does that rate?

 

[Guillermo]

A single number doesnt tell me more than a RM graph.

Knowing the GZ curve, displacement and downflooding angle, one can get a lot of precise info on the stability based aspects of a boat's seaworthiness. I dare to say that more useful info than just knowing the STIX, if one knows what to look at.
Cheers

 

[Guillermo]

The Efes 56, designed by Dijkstra, is the type of cruising sailboat I would love to own (http://www.efesyat.com.tr/project.html)
(probably not water ballasted, but maybe lifting-keeled as precursor Bestevaer 56 is)

Here some estimated numbers for her:
(main characteristics as per YW magazine, not the designer's nor the builder's sites. All three differ from each other )

Length/Beam Ratio L/B = 3,44
Ballast/Disp Ratio W/Disp = 0,3 (with water ballast. without it is 0,25)
Displacement/Length Ratio D/L = 177,51
Sail Area/Disp. Ratio SA/D = 18,73
Sail Area/Wetted surface SA/WS = 2,31
SA (metric)/ Power (Imp.) SA/HP = 1,96
Power/ Disp. Ratio HP/D = 1,52 HP/ton (auxiliary engine)
Hull speed HSPD = 9,46 Kn
Potential Maximum Speed PMS = 10,34 Kn
Velocity Ratio VR = 1,09
Capsize Safety Factor CSF = 1,65
Motion Comfort Ratio MCR = 39,17
Heft Ratio HF = 1,19

Angle of Vanishing Stability AVS = 117 º
Roll Period T = 4,06 Sec
Roll Acceleration Acc = 0,08 G's
Stability Index SI = 0,88
Dellenbaugh Angle DA = 20,36

Initial Metacentric height GMo = 0,73 m
Righting Arm 10º RA10 = 0,13 m
Righting Arm 20º RA20 = 0,23 m
Righting Arm 30º RA30 = 0,31 m

CG height, KG = 1,13 m
Position CG to WL, KG – D = 0,24 m (over floatation)

Heeling Moment at 20º HM20 = 36948,94 Ft*pound
Wind pressure coefficient WPC = 1,01


Does anybody have her stability curve, to try to calculate her STIX? (not available anywhere)

By the way, at YW magazine's last issue there is a nice comparative analysis of Dehler 44, Grand Soleil 43 and Maxi 1300, and STIX is not even mentioned for any of them. Interesting.

Interesting also what YW says about the target market for this new breed of light and fast cruisers: "...(their builders) identify their typical customers as sailors for whom the chunky go-anywhere yacht is overkill for an annual diet of weekend sailing and two week family holidays..."

Down here a picture from Efes 56. Beautiful!

 

[Antonio Alcalá]

Stix in the Vellux 5 Oceans

Could anybody bring us, the stability curves and STIX of the Vellux 5 Oceans 60 feet?. We could compare designs. Hugo Boss capsized!

Best winds

 

[Guillermo]

Rm 1200

Let's analize here one of those modern fast french cruisers, 'A' categorized, from a top designer:
The interesting tween-keeler RM 1200, designed by Marc Lombard (http://www.marclombard.com/)

Input data:

Overall Hull Length = 11,99 m
Length Waterline = 11,42 m
Flooded Buoyancy (Y/N) = N
Beam Waterline = 3,8 m (asumed)
Beam = 4,22 m
Displacement MOC = 6876 kg
Displacement Max = 7800 kg
Height of CE above DWL = 6,51 m
Height of CLR below DWL = 0,51 m
Angle of vanishing stability = 116 deg
Downflooding angle = 100 deg
GZ at downflooding angle = 0,21 m
GZ at 90 degrees = 0,33 m
Sail Area = 86,67 sq.m
Area to flooding (Agz) = 53,7 m.deg
Area to AVS = 55,71 m.deg


Here some estimated numbers and Ratios for her:

Length/Beam Ratio L/B = 2,75
Ballast/Disp Ratio W/Disp = 0,33
Displacement/Length Ratio D/L = 146,08
Sail Area/Disp. Ratio SA/D = 22,39
Hull speed HSPD = 8,20 Kn
Velocity Ratio VR = 1,19
Capsize Safety Factor CSF = 2,15
Motion Comfort Ratio MCR = 20,94
Heft Ratio HF = 0,71
Angle of Vanishing Stability AVS = 115 º
Roll Period T = 2,13 Sec
Roll Acceleration Acc = 0,26 G's
Stability Index SI = 0,5
Dellenbaugh Angle DA = 9,07

And now, asuming the downflooding angle is the minimum required for Category A, i.e. 100º (as this data is not available), we can estimate her STIX at this angle, what we may call the STIX(100)

Base Length Factor (LBS) = 11,610
Displacement Length Factor (FDL) = 0,937
Beam Displacement Factor (FBD) = 0,957
Knockdown Recovery Factor (FKR) = 1,053
Inversion Recovery Factor (FIR) = 0,961
Dynamic Stability Factor (FDS) = 0,981
Vaw = Not aplicable
Wind Moment Factor (FWM) = 1,000
Downflooding Factor (FDF) = 1,111
Delta = 0

STIX (100) = 32,920
Very low figure for the size. Realize how most of STIX's factors are under 1,000. Increasing Dfl to 116º (=AVS) only brings STIX up to 33,531

An interesting boat, no doubt, with some interesting features. But from my point of view, with that huge beam it will result cruel to her crews when at displacement speeds or at anchor, and not safe at all when at open ocean, with that high negative area at the GZ curve (32% of the positive!), maximum negative GZ slightly bigger than the 50% of her maximum positive GZ, maximum GZ at around 50º and low GZ at 90º.
I wouldn't like at all to experience a B2 knockdown with this boat!


Cheers

 

[Guillermo]

And here some first numbers for POGO 40 Cruiser (I have no data to estimate her STIX, but I guess is probably as low as the RM's one):

Length/Beam Ratio L/B = 2,74
Ballast/Disp Ratio W/Disp = 0,34
Displacement/Length Ratio D/L = 84,91
Sail Area/Disp. Ratio SA/D = 30,09
Hull speed HSPD = 8,42 Kn
Velocity Ratio VR = 1,33
Capsize Safety Factor CSF = 2,55
Motion Comfort Ratio MCR = 12,92
Heft Ratio HF = 0,44
Angle of Vanishing Stability AVS = 115 º
Roll Period T = 1,66 Sec
Roll Acceleration Acc = 0,44 G's
Stability Index SI = 0,38
Dellenbaugh Angle DA = 16,57

Things are even worse. Really dangerous when not in planning conditions (*). I'm really curious to know the GZ curve of this boat.

Cheers

(*) Or even in planning if you lose control, whatever the reason. And when running fast and furious at 20 kn, I wouldn't like to hit a whale, container, log or whatever!)

 

[Guillermo]

Some statistics for the Capsize Safety Factor and the Motion Comfort Ratio:
(Taken from Johnsboatstuff. Analysis based on 777 boats)

Capsize risk, usually below two for cruising boats, is strongly related to LOA. The longer boats are heavier and have less beam, which greatly reduces capsize risk. Its fairly uncommon for a boat longer than 40 feet to have a capsize value greater than two. For MCR again longer boats clearly have an advantage.

See attached charts of CSF and MCR plotted vs LOA (ft)

 

[Mikey]

And here some first numbers for POGO 40 Cruiser (I have no data to estimate her STIX, but I guess is probably as low as the RM's one):

Length/Beam Ratio L/B = 2,74
Ballast/Disp Ratio W/Disp = 0,34
Displacement/Length Ratio D/L = 84,91
Sail Area/Disp. Ratio SA/D = 30,09
Hull speed HSPD = 8,42 Kn
Velocity Ratio VR = 1,33
Capsize Safety Factor CSF = 2,55
Motion Comfort Ratio MCR = 12,92
Heft Ratio HF = 0,44
Angle of Vanishing Stability AVS = 115 º
Roll Period T = 1,66 Sec
Roll Acceleration Acc = 0,44 G's
Stability Index SI = 0,38
Dellenbaugh Angle DA = 16,57

Wow! Seems like the Sea Worthiness thread is more needed than what most people think

Guillermo, Thanks for your effort in this area

Mikey

 

[Antonio Alcalá]

About Beneteaus

Here you have a complete list of Beneteau, except some models, with their STIX. As you can see, the best relation between price and security is the 473. Very impressive how some models do not surpass the right STIX in comparation with LOA. Very impressive too how much of them have Category CE A, and however score with a minimum limit of ISO STIX 12217.

Let´s cross oceans with them and let´s pray for us!

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?

Best winds

Antonio

 

[Guillermo]

Thanks for your kind words, Mikey

Antonio: Nice Job!
Only the 473 and the 393 have a reasonable STIX for their length. Beneteaus are rather conceived as light, comfortable (home-like) and relatively cheap coastal cruisers than all weather bluewater globetrotters. I'll try to join info from other manufactures going the other way round (as Pacific Seacraft, i.e) and post it here.

As finding Pacific Seacraft info for their STIX is not going to be easy (somebody may help?), in the mean time here some info for Island Packet Yachts models http://www.ipy.com/

Model ------ LOA (ft) --STIX
SP Cruiser --- 41' 1" ----- 39
IP 370 ------- 37' 10" --- 43
IP 440 ------- 45' 9" ---- 52
IP 445 ------- 45' 9" ---- 53
IP 485 ------- 51' 7" ---- 66

Cheers