Seaworthiness

Hi Mike,

Could you please expand on a couple of points you have made?

1. You have said that Metacentric height (GM) is "a significant design factor in any vessel". Can you detail how you use the GM in the design process of a cruising sailing boat ? What values would you be looking for, in say, a 40fter? How do you differentiate between the component of any GM that comes from the height of the CoG and that parts that comes from the Transverse Moment of Inertia (or beam in crude terms)?

2. You also say they do not endorse the use of STIX in Australia since "it's wave height of 7m for offshore is considered too low". Can you detail how wave height is used in the calculation of STIX ? What alternative do they use in deciding ISAF OSR Categories?

 

Probably many people here feels pretty much the same. A pity. May we all of us smoke the "pipe of peace" and debate whatever aspects without ofending each other? I offer that on my side, as I'm also tired of this kind of uncreative struggleness. So, I want to be the first one to do it:

Paulo, may I apologize to you? You have brought many interesting comentaries and posts to this thread and I tank you for that. I admire your capacity of gathering information about boats. I have been quite stupid being rude to you. Here my hand....

And this is extensive to Randy and whoever other who may have felt offended.

Cheers.

 

Guillermo, you are really incredible (and I am excluding negative connotations).
I am sorry if I have replied too you in a "stronger manner".

Frankly, I was going to do the same as Randy and I was leaving from this thread. But it looks that there are not many in this forum that likes to discuss the same sort of things...So I will give it a try and I will try to be...polite...providing you don’t call me ignorant or mentally confused or whatever .

PI, Many years ago, in the 70’s, ocean cruising boats didn’t also look like a Valiant.

The Valiant was then considered as a high performance cruising yacht. Conservators felt that the boat, with its racing hull and lightweight was not appropriated for cruising. They felt that it was a racing boat disguised as a cruiser. Fact is that the boat was also used as an ocean racer with success.

Sailors are a conservative lot and there is a reason for that. They don’t like to game with the sea, trying new concepts that can prove deadly. That’s why racing sailboats were always the development factor in cruising sailboats. Racing sailors are the ones that are more willing to try new concepts.

Boats like the Pogo don’t appear from nowhere, they are the result of 20 years of experiments with this kind of hulls, in the Minis and in the Open 60, experiments regarding speed but also seaworthiness.

Take a look at what they have said about the Valiant, today a conservative ocean going boat but once considered a disguised racer:

“The Valiant 40 has been described as a high-performance cruising yacht which, at least in the early 1970s, was clearly a contradiction in terms.

Valiant 40 hauled out would be enough to show you that Perry had cleverly managed to graft those traditional topsides onto a thoroughly modern underbody.

Using a similar hull section to the offshore racers he was creating in the Carter office....

One consolation for the owners of racer-cruisers who watched as the Valiant out-pointed and out-footed them was that, in spite of its appearance, the Valiant had SA/D and D/L ratios very close to the famed Cal-40. So it was no surprise that Valiants not only set records for cruising, but for racing as well. They competed with honors in several OSTAR and BOC Around-the-World races, and won events such as the Marblehead to Halifax, Miami to Montego Bay, Bermuda One and One/Two, and even the Swiftsure”.

http://www.boats.com/content/default_detail.jsp?contentid=10111


“Before the early 1970s, loads of teak, varnish, leaky skylights, long keels, big rudders and enormous undersides with enough wetted surface to require three gallons of antifouling defined the offshore passage-making cruiser.

Enter upstarts Nathan Rothman and Bob Perry, and a different concept in offshore ... Two Valiant 40s then entered a difficult singlehanded transAtlantic race. In the 1976 OSTAR, Francis Stokes aboard Mooneshine, hull number 122, became the first American in a monohull to finish.

Performance was terrific in comparison to more traditional designs, ... despite the contention of contemporary skeptics that you needed a full keel to tame any sailboat upwind in a seaway”...

http://old.cruisingworld.com/ssbk/valian40.htm

 

Paulo, there are two different aspects here. When the mass for maximum load condition is bigger than 1.15 times the one for the minimum operating condition, the ISO 12217 requires that the STIX at the maximum load condition shall be calculated too, and is required that both STIX satisfy the requisites for the intended category.
But the STIX to be asigned to the boat is the lower of those two values. The official STIX number is though only one of them (the lower).

The intention here is to check if when the load masses are added to the boat, the overall seaworthiness of the boat is not impaired. The masses to be added over the MOC (Minimum operating condition) are: all stores, consumables and other weights at their intended design positions (so tanks, lockers, etc) and the extra crew mass over the one mandatory for the MOC, to be considered at the mid length of the boat at rail height. This may, or may not, rise the center of gravity, and what the norm intends is that effect to be studied and so found if the "loaded STIX" number is not lower than the MOC's one (Which usually is not, because the increase in displacement influences more the STIX to rise its value, than the crew weight at rail to lower it, grossly speaking)

For light boats MTL (Maximum total load) can be a significative amount of MOC (in many cases bigger than 15%), so that's why for many modern light cruisers the STIX for the two conditions uses to be calculated. For heavy cruisers this is more rare, so only the STIX for MOC is usually calculated (Although the STIX for the loaded condition may be also calculated and displayed if desired)

I hope to have been clear enough. If not, please tell me and I'll try again.

Cheers.

 

Mike, I would certainly be interested in any studies that you might have time to dig up.

PI, Your opinion from a theoretical point of view would be appreciated.

 

Randy

I’m sorry I never meant to offend you in this way. It was not intentional. I do not think you a fool, your posts show that you are intelligent. I would have been more careful not to offend you but from your own style I am surprised you are so fragile.

We are apparently going about this discussion the wrong way and I am not making myself clear enough.

Hidden in some fairly long posts is the crux of the issue that I was debating; that of heavier rigs adding stability through increased roll inertia but we quickly get off the subject as I want to illustrate:

[Randy]Heavy rigs also reduce static stability

[] Heavy rigs reduce stability, from what condition? ………..Do you understand design metacentric height and all that entails wrt motion and stability?

[Randy] I think that a higher centre of gravity is less stable than a low centre of gravity.

[] You are confusing matacentric height and COG. We all know about COG, this wasn’t what I asked you.

[Randy] Metacentric height is not relevant when discussing angles greater than about 10 degrees.

Then you get stuck on GM. This is not what I wanted to discuss with you but simply the fact that a heavier rig is one way of adding roll inertia and consequently reducing the chance of knockdown. In fact the roll inertia is significantly more effective at resisting violent knockdown than the position of the COG. We’ve ended up with you on a limb way off the original subject.

Then PI joins the fray (and now Crag) jumping on GM.

[PI] GM is only an indication of initial (small angle) stability.

Now PI is an Engineer too and wanted to explain that I knew I was popularizing a complex system. I still think that considering what happens to the GM of a vessel if you changed from a heavier to a lighter rig to be a good illustration.

[me to PI] I wanted to know if he [Randy] really understands what happens if you remove a heavy mast and replace it with a light mast on the basis that it will improve stability. The easiest way of illustrating this is to consider what happens to GM. Anyone aware of the issues is immediately aware of the ramifications. ……. I shudder to think…..

Injury and death is what I am “shuddering” about, and I should have explained that in my head I saw a carbon-fiber pole being installed in a more extreme heavy displacement boat and I perhaps I should have illustrated the example, but I was addressing PI and this issue is taught to NA’s.
The resulting (horrible to contemplate) is people literally flung about due to a combination of lower gyradius and a lowered COG.

There is a classic in the shipping industry usually taught to all the budding NA’s, the Ore Carriers where the designers did not consider what would happen to the boat motion when in the loaded condition, the COG was fantastic when loaded but GM went through the roof (literally) many injuries resulted.

Frankly as I have said before I don’t want to get drawn into comparing existing boats because the comparisons are so subjective even the published data can be amiss unless it is a properly vetted scientific study. We have just seen Vega and Guillermo trying to do this.

I hope you have a hangover (from the drunken Rigger). Please come back, it shouldn’t end like this. Perhaps if I promise to capitalize Rigger. Mea Culpa.

 

Hey folks. No apologies needed.

I think that it is a measure of the posters in this thread that they were offered.

Not as excuse (since I sometimes winch at the tone of my own posts), but as explanation; my job requires that I bite my tongue when a customer wants to do something clearly idiotic to their boat. I'm afraid I allow my stress level from work to colour my posts here. Mike has been my target too often.

It is hard to make sure we are using terms in the same way. And for some of the posters English is not their primary language, thus adding to the difficulty.

In all honesty, I should try not to spend any more time on this thread for the next two weeks or so. I need to concentrate on a boat show. I cannot allow my frustrations with this thread effect my work, and I should not allow frustrations at work to effect my thinking and posts here.

One thing is abundantly clear to me. Sailing craft are beautifully simple and very complex at the same time. It seems the more I learn, the less I know. My ego is large enough to think that most (if not all) people that try to understand sailboats from a scientific standpoint must share that view to some extent. A better understanding of "why things work that way" has been my motivation for every pastime I've enjoyed over the years. I have taken breaks from sailing, but always return. Sailboats are an endless puzzle to me. Trying to reach a level of understanding that I am comfortable with continues to elude me. Racing motorcycles and cars, building successful racing engines, learning to build, fly and design successful R/C sailplanes all seem like trivial pursuits compared to the complexity of sailboats.

Not on topic in this thread, but one of the puzzles of sailboats is trying to evaluate overlapping sail area.

Another is the relationship between lateral hull area and sail plan and how that relationship effects helm balance.

Stability and motion could provide another decade of interesting research.

I'm running out of decades and it bothers me. I apologize for taking my frustrations out in this thread.

Walt Kelly's Pogo:

 

I would like now to do some analyzing for the Valiant. The presented numbers for its STIX are only a gross estimative, the same way the MOC's STIX for the Pogo a few posts ago were. Maybe both are wrong, but as I have worked them down both the same way, I expect them to be comparable. Let's see:
(Figures in brackets are from the Pogo 40, to compare)

VALIANT 40

Lh = 12,16 m
Lwl = 10,36 m
Bmax = 3,76 m
Bwl = 3,38 m
Draught T = 1,83 m
Body draught Tc = 1,17 m
Disp = 10206 kg
Ballast, fix = 3500 kg
Sail area = 71,7 m2
Heeling Arm = 7,50 m
Angle of vanishing stability = 125 deg
Downflooding angle 110 = deg
GZ at downflooding angle = 0,21 m
GZ at 90 degrees = 0,53 m
Area to flooding (Agz) = 42 m.deg
Area to AVS = 47 m.deg

From those I get:

Length/Beam Ratio L/B = 2,9 (2,74)
Ballast/Disp Ratio W/Disp = 0,34 (0,34)
Displacement/Length Ratio D/L = 256,01 (84,91)
Sail Area/Disp. Ratio SA/D = 15,49 (30,09)
Velocity Ratio VR = 1,05 (1,33)
Capsize Safety Factor CSF = 1,75 (2,55)
Motion Comfort Ratio MCR = 33,99 (12,92)
Roll Period T = 3,36 Sec (1,66)
Roll Acceleration Acc = 0,09 G's (0,44)
Stability Index SI = 0,89 (0,38)

STIX:

Base Length Factor (LBS) = 10,960 (12,080)
Displacement Length Factor (FDL) = 1,036 (0,891)
Beam Displacement Factor (FBD) = 1,061 (0,744)
Knockdown Recovery Factor (FKR) = 1,367 (1,075)
Inversion Recovery Factor (FIR) = 1,054 (1,044)
Dynamic Stability Factor (FDS) = 0,762 (1,500)
Wind Moment Factor (FWM) = 1,000 (1,000)
Downflooding Factor (FDF) = 1,222 (1,222)
Delta 0

STIX = 38,440

So, although being the 'Old ratios' much more appealing (to me) from a cruising point of view, Valiant 40's STIX (Something like 38) is lower than POGO's one (Something like 40) in spite Valiant's factors being better than Pogo's except for two: The Dynamic Stability Factor (FDS) and the Base Length Factor (LBS).

So we can realize here the huge effect of the bulbed keel (Influencing FDS) and the waterline length (Influencing LBS) of the Pogo 40, allowing her to reach STIX values higher than the Valiant 40 ones, being the Valiant a proved oceangoing boat with probably hundreds of thousands of successful miles under her keel. This is one of the reasons I do not like how STIX works.

 

That does sound horrific. As carbon replacement masts are not uncommon, you must have links to accounts where this has happened. And what about where a manufacturer offers a carbon mast option - surely those choosing the more expensive 'upgrade' must be beating a path to the law courts if it results in 'death and injury'? Do you have any case studies?

We're all aware of the research done after the Fasnet on rig inertia preventing rolling, and rig loss' contribution to being rolled repeatedly, but do you have any more contemporary studies on these problems with light weight rigs? What values of GM do you use when designing a 40ft sail cruising boat? What trade offs do you make to achieve this? Do you prioritise achieving your ideal GM over, say, righting moment or AVS?

If you have the time, I would also appreciate more information about the limitations Australians have about STIX and "it's wave height of 7m for offshore being considered too low". Any feedback is useful.

 

Crag Cay
Here is a link to a document titled

'Submission to Modify the Australian Yachting Federation Special Regulations Part 1, Appendix D Resistance to Capsize’


Prepared by Richard Slater
PO Box 800, Mona Vale NSW 1660
Phone: +61 2 9973 4111
Mobile: +61 (0) 414 752 837

http://www.yachting.org.au/site/yachting/ayf/downloads/Technical/Agendas/Stability%20Submission%20Paper.pdf

I don’t know if this document will help, but it references STIX as it is viewed in Australian racing circles. The document was prepared in 2003; it is primarily about the effects of the AYF 10 degree rule regarding movable ballast, and its insufficiency on determining and /or increasing capsize resistance.

An excerpt:

The section that deals with STIX reads as follows: (2003):

You may be able to contact Mr. Slater and get more info.

 

Thanks for that, however I was hoping someone had some information that updated the situation since that was written.

I was wondering if Appendix K to the ISAF OSR 2006 had addressed any of their concerns with regards to screening vessels with movable ballast.

As to the general reservations expressed concerning STIX, I'm still unclear exactly where the problem lies. The STIX requirement for ISO Cat A is pretty much irrelevant to racing (and certainly beyond the control of the RORC), except that there is the option of allowing ISO (CE) Catorgarised vessels to use that as their proof of STIX. (Para 3.04.4) However this is only 'allowable' if the race organisers consent.

The RORC is emphatic that " ... Organising Authorities may incorporate a minimum qualifying STIX for entry to a race. Responsibility for selection of the minimum qualifying value(s), and for any AVS requirement(s), lies with the Organising Authority for a race. It is not possible for the RORC Rating Office, nor any other body remote from the organisation of a race, to lay down firm recommendations or guidance. Only the organisers of a race can be fully aware of the circumstances of a particular race. The RORC does not accept responsibility for the consequences of the mis-use of these systems ( SSSN, STIX or RORC STIX), which are a guide and not a guarantee."

Surely the Australians can set their minimum STIX requirement at whatever level they feel appropriate for racing in their area?

 

Crag
Sorry for the delay , I have some work to finish and what you are asking requires some effort.

The STIX consideration is knocking around along with a discussion document not for racing but for non-commercial craft. I will try to find it. As with the US we currently have no such labeling but so far there is no inclination to adopt the EU STIX we may be far better to make full stability info mandatory. My own concerns are similar to Guillermo's on this issue. In that it is a rule that can be manipulated.
(I have decades of papers poorly filed and I have a cleaner who punishes me when she tries to find the floor in my office by stacking documents anywhere ) or it may be a pdf I will look later.

As for metacentric height (both trans. and longnl.) we have used this a lot particulalrly redesigning vessels, in new designs it is always a considered although the general equations are reasonably accurate there is a more involved method, and I'm sure it is presented in one of the common yacht engineering books proably Marchaj's earo-hydro?.

We have some fairly sophisticated sreadsheets and though the numbers are used the underlying maths is sufficiently complex to require some explanation and some re-understanding on my part.

I'll try and write something for you or find a synopsis. Generally you want around 900 to 1000 (trans) for a sailboat. Before I go any farther do you specifically want this? There is quite a complexity in this relationship of motion, stability, mass distribution and hullform.




cheers

 

Crag
If you have "Design of sailing yachts" PGutelle 2nd ed revised
Have a look at page 199. This should give you a good primer.

Cheers

 

Hi Mike, Don't go digging out any references as I probably have most of them here. (Edit: I wrote this before seeing your reply above) However as you came across as a very passionate advocate of GM as a basic design tool that prevents death and serious injury, I was really after an insight into how you use GM in practice when designing a cruising sailboat (of say 40ft as that is the sort of size that has caused so much debate).

What priority do you place on achieving your ideal figure (900-1000mm) and what other parameters do you compromise to achieve this? Is it roll period and comfort levels you are trying to achieve? How do you differentiate between a boat like a Contessa 32 and Grimalkin which had identical GM's but very different desirable and undesirable characteristics?

Over the years I've noticed a shift in the emphasis that GM gets in text books. Gutelle had us constructing 'metacentric curves' although suggested that 'the real stability is best shown by a righting moment curve' and Marchaj made extensive reference to it, but more recently, there's mention of it in Larsson & Eliasson and even less in Claughton, Wellicome & Shenoi.

So I would appreciate your insight as an active practitioner of its use. I must confess that in all my designs I have not given it much consideration beyond using the steepness of the 0-10/20 degree portion of the RM curve to indicated initial stiffness. It kept me awake last night worrying that perhaps the reason I have not had any complaints from clients is perhaps they have all perished in those accidents you foresee! The only solace I could muster was to think that there no combination of monohull and lightweight rig that could achieve the GM of a Wharram cat and the last time I saw James and Hanneke, they seemed okay.

The only example of a sailing yacht design being driven by a target GM that I could think of, was the two editions of the Chay Blyth Challenge yachts. I know his design brief included a stiffness limitation so Lewmar 65's could be used as the primaries. This was his way of ensuring loads (and accelerations) did not get above a 'safe level' for his paying crew clients.

As to the other matter, I appreciate the RCD does not extend to Australia or the US. Using more detailed stability information may indeed serve the boat buying public better, but the quote was from someone concerned about the screening of race entries. In this context STIX should be seen as the current manifestation of the old SSSN system or IMS Stability Index, and has precious little to do with ISO, CE, Cat A, 7m waves or the like. As I have quoted earlier, local organisers are encouraged to set a level of STIX and AVS that they feel is commensurate with the conditions in their race area. If people (Guillermo ) feel that some parameters inputed into STIX are unduly weighted, then I'm sure these will be considered in either revising ISO STIX (unlikely) or its variant RORC STIX. A lot of work has been done since Richard Slater voiced his concerns in 2003 with both a revision of the ISAF OSR and IMOCA regulations. I'm not sure quoted him really moves the debate about seaworthiness forward in any meaningful way.

 

A boat that hits something and then sinks is not a seaworthy boat. IMO.

There is more to crossing oceans than stability, steerage, and speed. The ocean is getting more littered every day. People say the ocean doesn't change. Well, it does and in this case not for the better.

Speed and light weight don't work too well for impact resistance. Never has, and never will. Either gain weight and go slower, or ignore the risk. As I see it, those are the only two options.

As soon as you design for impact resistance, unless your client's wallet is like the department of defense, at LOA 40 feet you are going to end up with either a medium or heavy displacment sailboat. That displacement restricts a lot of design options. Cruising sailboat design takes on a center of gravity, so to speak.

IMO, you can deviate from this center of gravity only by ignoring impact resistance, which is exactly is what is done as best as I can see it. That's what liferafts, EBRIPs, and helicopters are all about.

Note: There are cruisers who sail the oceans without liferafts (Lynn and Larry Pardey come to mind) and to the best of my knowledge not one of them do it in a light go-fast boat.

Which brings to mind a line out of one of the HANK THE COWDOG series of books that my kids used to like so much: "Junior, how many times have I got to tell you, speed kills. And the speedier you go, the killier you get. If you're not careful, one of these days you're liable to wake up dead."

There, I've done it. Quoted Hank the Cow Dog in sailboat forum. I'm pretty sure that's a first.