ISO 12215-5 and Principles of Yacht Design

Hello all,

I'm currently working out the scantlings for a wood/epoxy 40 foot ketch. After quite a bit of head scratching and contemplating, I've finally come to understand most of the ISO standard and have made a nice huge spreadsheet to enter my data in.

There are a couple of questions I have so far, but the most important (and reason for this post) is the following:

In the second edition of Principles of Yacht Design (PYD) by Larsson & Eliasson, there is a chapter devoted to the scantlings determination of a boat. However, when the book was printed, the ISO standard was not complete yet.

The interesting thing is that they mention an area near the centerline called "K" (c.f. p 278) which is defined as 80*Bh on each side of the centerline (where Bh is the hull beam in meters). They say that this area needs to be reinforced to 1.5 times the thickness of the normal scantling determination for the panels.
They also mention on page 286 that Stiffener calculations have a similar exception called SMk, which is the Section Modulus at the keel.
And finally, they introduce a constant C in their calculation of the SM (on top of this SMk) that is 183 for floors at the centerline of sailing craft, and 83.3 otherwise.

Now, none of these factors are to be found anywhere in the 2008 version of the ISO 12215-5 document.

The magic number C is still in the SM determination btw, but it is simply 83.33. There is no mention of 183, nor of K, nor of SMk, nor of tk.

What gives? I know some of the formulas are different from what is in PYD, and I haven't gone through the differences thoroughly enough to say that the current ISO naturally deals with keel areas using the height reduction factor (kz).

Anyone know what's up with this? Can anyone who has the 3rd edition of PYD confirm that these keel constants are still there, or if they have changed it?

Alternatively, can anyone familiar with ISO 12215-5:2008 tell me if I'm completely missing something if the keel isn't being mentioned? Or if in fact, this is normal? (The only reference to keels I see is that section 11.9 says to refer to ISO 12215-9, which weirdly enough happens to have a status of "Deleted" on the ISO site)


The second question is this shorty: in calculating the SM of my stiffeners, would I be missing something important if I omitted the SM of the skin, and just calculated the girder SM to be greater than the expected design loads?

Thanks for your time.

Regards

 

Never heard about 2008 version of standad...

Advice: buy HullScan from Wolfson Unit and forget about those problems. This software uses ISO12215-5 for scantlings.

 

This won't specifically answer your question but will give you some insight. ISO standards are consensus standards. What the heck does that mean? It means that peple from about 70 different countries sit down to gether at a meeting usually in some place you and I can't afford to go, like Paris, Rome, Helsinki, and they all put in their best thoughts on what the standard should be. Some stuff gets included, some gets left out, but after four or five years of hash and rehash, they eventually come up with something they can all agree on. So, the stuff from PYD simple got left out.

This does not mean you can't use it. It just means the Recreational Boat Working Group couldn't agree that it was necessary.

 

Clarification...

Let me put it this way: I know more or less exactly what the hull is going to be. It's 3 layers of 1/16" veneer over 1/2" pine. The stiffener positions are pretty well determined.

This however doesn't mean that it can't be ISO compliant. In fact, if it is to be sold in Europe, it has to be ISO compliant.

So to put in context: I am stuck interpreting the ISO standard, I'm not stuck in building the boat.

Oh, regarding the PYD stuff: the book was simply quoting from the ISO standard of the time (which wasn't finalized). What I'm curious about is where the keel factors went to in the current incarnation of the same document. It would be surprising if the keel/centerline factors got altogether erased. I'm wondering if they've been integrated into other factors (like kz) or if they are still explicitly somewhere but buried in a foot note that I missed.

 

Are you sure about this? I can be wrong, but it's not mandatory to follow the ISO 12215-5 for the boat to be approved, it just needs to be approved from the notified body; here in italy (but I think in other countries also) the RINA has emitted its own rules for scantlings etc., and you can follow these rules and get your boat certified.

Moreover, you can just dimension your boat with your own rules, then you can present to a notified body a technical relation that demonstrates that your boat has all the needed requisites, and if you convince the notified body that you're right, your boat will be certified.

Obviously the latter is very complicated to achieve and usually is not worth, but it can be done. If you are already in contact with a CB you can ask them and see what they say.

Bye

Andrea

 

http://www.boatdesign.net/forums/showthread.php?p=194784&highlight=iso#post194784 look post #108 by Guillermo
Keels are in the part 9 (someday)

 

Good design practice (despite what the rules say) would indicate substituting a denser/tougher species along the centerline. At a minimum I would use Douglas Fir or Mahogany. Pine seems an odd choice unless there is a structural glass skin inside it. A 40' boat with only 3/4" total skin/core will require lots of framing and glass inside and out. The close-spaced framing causes trouble with the inside glass unless it's installed after the skin is off the mold. 1/2" pine will be very flexible and the molds must be close (18") together to get it fair.

I have a draft of 12215-9 and it details an increase of 180% in the plating design pressure around the ballast keel. I would guess the standard has been dropped for now because of the current fervor surrounding keel design. See the "Keels and Keels again" thread.

 

Thanks TeddyDiver, that thread contains pretty much exactly the info I was looking for.

 

On a more philosophical note, I'm surprised to hear so many answers disparaging the ISO standard. I find it especially odd to hear it being referred as a "rule" as if some sort of a law one must obey...

e.g.

or

A standard isn't meant to be a limiting factor. In fact, a standard is nothing more than a "scantling". A rule of thumb under a more democratic and modern structure.

Without getting too philosophical, the reality of the matter is that most boats will be by fact ISO standard compliant, since that standards compliance really only indicates that many many intelligent engineers have agreed that a boat with at least those specifications won't break in half. The odds are also quite high that these same boats will also be compliant with many if not all the other good standards.

My 2 cents,

And thanks again.

 

I wasn't disparaging the ISO rules, I was just saying that, as far as I know, you're wrong when you say that "In fact, if it is to be sold in Europe, it has to be ISO compliant."

I'm almost sure that Wally yachts does't respect the scantlings resulting from the ISO standard: they do their own scantlings ant then they submit a technical relation to the notified body, and the certified body accepts to certify the yacht if they are convinced that the boat is safe. And I dare to say that Wally yachts are very well done and safe.

Philosophically speaking, you're right, a standard is just a scantling; however like every engineer has its own sensibility and is more or less conservative, each standard is more or less conservative: a boat can be conform to a standard and not to another.

Regarding "good" standards, all are good by definition: if a standard wold permit the approval of a ship that is in some way unsafe, it wouldn't have become a standard in the first place IMHO.

Bye

Andrea

 

The "Rules" and/or "Standards" are most certainly meant (IMO) to be a lower limit, beyond which, except under special circumstances, your vessel will not be certified as compliant.

I did not intend disparagement of ISO 12215, I was only commenting on the fact that the standard does not cover every nuance of construction technique. The various rules and standards cover many points in general terms, methods change/develop faster than the standards.

Your definition of "Rule" is rather narrow. Merriam Webster defines rule as "1 : a guide or principal for governing action:" and "2: the usual way of doing something:" When I started designing small craft I had two scantling rules to work with, "Nevins Scantling Rules For Wooden Yachts" and "Herreshoff's Rules For Wooden Yachts". Later I would work with ABS "Rules For Building And Classing" and Lloyd's "Rules and Regulations for the Classification of Special Service Craft". Thus to my poor old mind, they're all rules and always will be.

 

Are not you sure 12215-5 deals with globals loads, and 12215-6 deals with local loads, such as keels and chine and so?

sailboat keel and bolts are in another specific 12215-7 -8 -9 I do not remember wich one. (keel, multihull and rudder, but I have lost the numbering).

 

ISO 12215 PARTS OF STANDARD
ISO 12215-1 Materials – Thermosetting resin, glass-fibre reinforcement, reference laminate
ISO 12215-2 Materials – Core materials for sandwich construction
ISO 12215-3 Materials – Steel, aluminium, wood and other materials
ISO 12215-4 Workshop and construction
ISO 12215-5 Design pressures for mono-hulls, design stresses, scantlings determination
ISO 12215-6 Structural arrangements and details
ISO 12215-7 Multi-hulls
ISO 12215-8 Rudders
ISO 12215-9 Sailing craft attachments

 

Where can I buy a bound paper copy of ISO12215? Or electronic if bound is not available.

 

www.evs.ee
Chears Teddy