Seaworthiness

From SOUTH AFRICAN SAILING's Special Regulations and Safety Check List 2006
(Comprehensive and most interesting check list on recreational boats seaworthiness and preparedness)

http://www.rcyc.co.za/docs/sas-regs-check.pdf



BOAT STABILITY
5.1 It is recommended that all Monohull boats have a capsize screening value (CSV) of less than 2.0 as calculated as follows –
5.2 CSV = beam in metres divided by the cube root of (displacement divided by 1025)
A CSV of less than 2 will equate to positive stability of approximately 120º and less than 1.4 will equate to about 140º.
5.3 For racing only
The criteria for Racing is different and whereas the RORC have published a set of figures, experience has shown that these exclude certain vessels that have demonstrated their suitability in races such as the Sydney - Hobart. The IMS rulings permit entry of a wider range of boats falling outside the RORC guidelines and hence SAS have published a range of criteria in Appendix I. Race committees should decide which of these to use for their offshore races and publish their requirements in the NOR.”


I recommend a reading of the most interesting Annex I.

Cheers.

 

this CSV is penalizing light displacement ships....
i have now no clue whatsoever how this commitee got to their numbers an calculation - probably one here is able to explain it to me

just one example, that there might be something wrong:
the alubat cigale 14
http://www.alubat.com/?lang=en&keyRubrique=cigale-14
has 7 tons displacement (very light imo) and 4,4 m beam which gives us a CSV of 2,32 and according to this paper a positiv stability far less than 120°.

when i look at the GZ curve now of this aforementioned ship:
http://www.seilsamvirket.net/library/PBO-0408-Cigale.pdf
it shows that the cigale 14 has positiv stability up to 122°.....

what is now correct? what figures should i trust?
and what the heck has displacement to do with positiv stability?
i am no designer, just a skipper....

 

Here some parameters for the Cigale 14 with a displacement of 7000 kg, which I asume is in lightship condition,

Ballast/Disp Ratio W/Disp = 0,47
Displacement/Length Ratio D/L = 79,36
Sail Area/Disp. Ratio SA/D = 25,56
Power/ Disp. Ratio HP/D = 3,26 HP/ton
Velocity Ratio VR = 1,25
Capsize Safety Factor CSF = 2,37
Motion Comfort Ratio MCR = 14,65
Roll Period T = 1,86 Sec
Roll Acceleration Acc = 0,36 G's
Stability Index SI = 0,41

And here with a full load of 2304 kg plus 300 kg water ballast:

Ballast/Disp Ratio W/Disp = 0,34
Displacement/Length Ratio D/L = 108,88
Sail Area/Disp. Ratio SA/D = 20,70
Power/ Disp. Ratio HP/D = 2,37 HP/ton
Velocity Ratio VR = 1,16
Capsize Safety Factor CSF = 2,14
Motion Comfort Ratio MCR = 20,10
Roll Period T = 2,45 Sec
Roll Acceleration Acc = 0,21 G's
Stability Index SI = 0,54

Too light and stiff, to my taste, for an all around globetrotter. But that's just my taste.
I also think it has a significant amount of area under the negative side of the GZ curve.

It would be interesting to know her STIX. Do you know it?

Cheers.

 

unfortunatly i do not have any information on her STIX.
so i tried to calculate it but there were of course a lot of guestimates to make.....

i came up with the following figures:
LBS = 13.667
FL = 1.044
FDL = 0.929 (with 9600 kg of displacement)
FB = 2.2
FBD = 0,997 (with Bwl estimated at 3.5 m which is low with Bh being 4.4 m)
FR = 4.718 (here i had the most problems: downfloading angle was estimated 100° and the area under the GZ-curve was roughly calculated to be 63.1 from the pdf i posted above. sailarea was taken with 130.8 (according the pdf) and hc was estimated at 7 m)
FKR = 1.292
FIR = 1.025
FDS = 1.067 (with an area under the GZ-curve of 63.1 up to downfloading angle of 100°)
FDF = 1.111 (again with a downfloading angle of 100°)
FWM = 1

all this will give us a STIX of 50,5
when STIX = (7+2.25*LBS)*(FDL*FBD*FKR*FIR*FDS*FWM*FDF)^0.5 + 5

please tell me if my estimations are out of range or just plainly wrong.

and i am with you in one regard:
i realy like this vessel very much, but i would not take her on a world tour and that for various reasons. ;-)

btw - i figured out that it is not the first time that this ship has been discussed in this particular thread.... ;-)
http://www.boatdesign.net/forums/stability/sailing-boats-stability-stix-old-ratios-13569-15.html

 

Sorry for the delay in answering, but I've been out sailing for some days.
Still having to refine my numbers, I get an STIX of around 51,1 which is close to your figure of 50,5. This is pretty good for a 46' Lh boat, in my opinion.

Nice boat and nice STIX, but still not my preferred choice for an all around globetrotter.

Cheers.

 

mine neither....

although i like the unconventional layout with the saloon in the aft, it bears some major disadvantages.
- less storage in the stern/cockpit
- no space for additional equipemet like watermacker, generator
- no generator -> no diesel-electric propulsion system
- low tankage (only 200 l diesel, 500 l water)
- saildrive (i am not happy with that at all)
- the fast acceleration would give a windvane-steering system major troubles -> electrical autopilot beeing the only option -> no generator -> low on diesel....
- no pilothouse (which is just one of my preferences)
and even if i would enjoy a fast and agil boat, my wife definitely wouldn't. her sailing experience is so far very limited....

it is a marvelous boat for costal cruising and the one or other race, but i would not make long passages with her - at least two handed - with an quite experienced crew... any time!

 

Csv

The CSV was a regression analysis using lots of production and one off boats. The USYRU (now USSailing) Safety at Sea Committe, led by Karl Kirkman and with input from many yacht designers including Olin Stephens, gathered data from many different designs from many eras (including up to the mid 80's when the study was done). They analyzed the data and found that boats with CSV above two generally had lower limits of positive stability, while those with values lower than two generally had higher LPS. Yes, it is no substitute for a good stability analysis. But if you are going to look at 20 boats to go offshore, you can use this as a quick guide to see if the LPS will be in range or not. Obviously there can be exceptions.

What does displacement have to do with it? They found that boats with lower beam/hull depth ratio had generally higher LPS. So if you have two boats with the same beam, lwl, and one had 50% more displacement, it usually means two things. First, the hull is deeper in the water. Second, the deeper hull has more stability, both dynamic and static.

I am skeptical of a lot of the stability data. You have no idea what is included in the calculations (and the regulations provide minimal guidance nor do they require you to report in that kind of detail). So for example, is the boat in lightship condition? Did it include radar halfway up the mast and double roller furling? Or did it just include one halyard and hank on sails? Life raft included or not? Where did they assume these things are placed? A manufacturer, if they desire, is allowed some flexibility in these assumptions and there is certainly some pressure to report a high number. Couple this with some other inconsistencies - for example IMS stability numbers assume no volume for cabinhouse or cockpit - just a flat deck. So those numbers amy be different from a manufacturers numbers. I am always skeptical of high sided designs with big cabinhouses - if you make the right assumptions they will always show as being ultimately very stable, but of course that assumes the cabin windows don't implode and the companionway door is robust enough to survive (not always true).

A real world example: One boat, with a very enviable performance record over the last 30+ years (and still in production) at one time reported these values for stability: For the standard model, LPS at 134 degrees. For the pilothouse model, LPS at 180 degrees. Then I saw an IMS certificate for a sistership using real inclining data - the LPS was 109 degrees (radar, roller furling, dinghy davits, liferaft on cabinhouse, etc.). So if you are truly interested in safety then a call to the designer might be in order (most of them are happy to talk about these sorts of things if you are considering buying the boat).

 

thank you very much for that explanation mgpederson.
it is more or less on the lines of what i thought....

as for displacement and stability... it is somewhat only logical, that light displacement hulls stability is more affected by the load of the boat especially if some of it is placed at considerable hight (ie radar half up the mast, windgenerators, solar-panles mounted on an additional structure (don't know the english term now...) in the aft).
but non of this is represented in any calculation at all, isn't it?
if we take the STIX formula for example - you calculate it with a loaded displacement but the GZ-curve derives most probably still from light/unloaded displacement with the center of gravity beeing considerable lower.... giving you an overoptimistic STIX in the end.
am i right?

another thing is that modern design is obviously also influenced by calculations and formulaes not only by fashion.
overhangs are penalized in some formulaes as far as i know, so all modern designs avoid overhangs wherever possible to accomplish good stability indeces - and larger Lwl for better speed....
but does this mean that a hull with overhangs is lesser stable considering that the underwater body and Lwl or Loa/Lwl, Loa/displacement ratio is somewhat combareable?

i appreciate your reply very much and will follow your advise and contact the designer once i made my choice.

 

That makes many of us, marketing hype seldom relates to the real world and the manufacturers are not required to demonstrate actual stability via inclining tests. I have not been able to get information in the past from the big production boat designers as to what was included in the derived COG and displacment fed into the stability module when the curves were produced, it's the intellectual property of the manufacturer not the designer and they won't tell you.

Reading that PDF http://www.seilsamvirket.net/library/PBO-0408-Cigale.pdf I don't think he is being objective.

Cruising boats accumulate enough equipment spares and provisions to seriously effect both stability and performance. ULDB's COG very quickly rises as load is added, its impossible to stow anything well down in a flat bottomed skimming dish.

It's an interesting claim that ULDB's can still be high performance boats in a heavily loaded condition. The big problem is that the SA/D ratio (and SA/WSA) are not the only defining factor; the power to carry sail has more to do with overall performance than the SAD. A heavily loaded boat with compromised stability will have to reef much earlier, it also quickly loses it's ability to 'semi-plane' once the D/L ratio rises and then it becomes a displacment hull with various coefficients not well suited to displacement performance or particulalry good handling and controll attributes.

This is the true advantage of well designed heavy displacment boats that can absorb a big load with little change in stability ratios and coefficients and offer comfort and seaworthiness in a loaded condition.

 

Just to clarify: STIX has to be taken as the lesser of two figures, one calculated for the Minimum Operating Condition (MOC) and the other for the Maximum Load Condition (MLC), which are quite precisely defined at the ISO 12217, if MLC > 1.15 * MOC

 

guillermo:
you want me to pay 150 CHF for a pdf file?

 

It's significant that Abby Sunderland is now awaiting rescue in the Southern ocean on a racing boat she had a lot of problems controlling. The boat of all things for a 16 year old girl is an Open 40.

It's interesting that her competition Jessica Watson in an S&S 36 had a much more suitable boat for a 16 year old.

Both girls apparently experienced similar bad weather and sea states. So considering what's written in much in this thread, particulalry the poorer attributes of seaworthiness of boats like Abby's open 40 is this a clear example of the boat letting the girl down?

They bought into an argument that said it's a class 0 ocean racer so it's built for the southern ocean. But the boat has all the compromises of a go-fast semi planing racer so vividly explained here.

She would have been much better off with a boat similar to Jessica's. Pity they didn't read this thread before sending a schoolgirl off for the most miserable time of her life .

Why on earth couldn't they have bought a comfortable bombrproof slow heavy boat that she could lie ahull and collect her wits and energy and do repairs? Racing boats under 60 feet have a poor record in the southern ocean.

 

Let us pray that this brave girl survives this adventure!

 

Usually from excessively wide sterns and lean bows , in other words, grossly unblanced hulls.

 

Bombproof, slow, heavy,

seakeeping? seaworthy?

yes, there are a lot of questions why did´nt they.

And one answer:

They did not really care and have no idea of the task.

Poor Girl, lets hope she survives.

Regards
richard