A question of philosophy

"If it was only FEA anyone who can run the software could design light structure. "
I did not say experience is not necessary. It is in detail. With rulebook engineering one simply leave that much extra material here and there, just to be sure.

"I have worked with leading boat structural design companies; many of them still use rulebook, not FEA."
Do they design TP52?

 

Not sure about TP52 in particular, but they do design some very competitive classes.

 

Beware that technology is changing very fast.

Only 4 years ago , in 2007 :

No annex H in iso 12215-5:2002. No laminate analysis, no CLT software allowed. Only from 2008 version of the rule.

No ISAF requirement for ISO 12215 checked by notified bodies. It is a 2010 requirement. Before, ABS also was allowed, with compliance only written by designer and builder. Not externally checked.

No general CAD package with composite analysis builtin, like solidworks. First versions with composite builtin only appeared in 2009.

 

Actually ISO12215-5 is in force since 2008. So those were preliminary versions.

Yes, this is last what remember; did not touch racing sailboat designs for last 2 years.

 

I would not say preliminary versions, but previous versions.

Currently you are using ISO 12217-2:2002 for stability. It is currently under rewriting, and you will probably have within next year ISO 12217-2:2012 as a published rule, not under development : http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=55958

Rules continuously evolve.

 

Versions of ISO12215-5 before 2008 edition were draft standards.

Yes I know, even participate in development of some of them

 

You are right. I mistook a (old) national transcription for a international published norm.

 

Hello,

Thank you again for yours answers.

In fact, in my initial question, I thought to a "amateur" NA without big computer; i.e. without using FEA.

Can I do two observations ?

It seems that FCFC is agree with me : the standard ISO allow scantling more lighter than ABS ... but probably by the fact that ISO is more recent ... or ABS too conservative ?

In his answer, Paul Kotzebue said of the stiffness requirements in the ISO rules : when I see ISO 12215 part 5, it seem concern only the stiffeners in GRP. Is it true ? (I dont have the part 6 of the ISO 12215, but maybe there are others rules about the stiffness of stiffeners ?).

Best regards


Jacques

 

There are a couple of important issues with ABS.

The 1986 ABS ORY had inadequate hull shell thickness to prevent shear failure at the keel attachment backing plates. This was corrected in the 1994 version.

Consequently all composite sailboats built to pre 1994 ABS ORY should be have the laminate schedule checked and where appropriate upgrading to the 1994 requirement.

But the 1994 ABS ORY was deficient as well. After further structural failures of vessels built to ABS 1994 ORY analysis showed that ABS underestimated the possible deck loads and that decks built to that standard are actually inadequate for expected offshore deck loads in heavy weather.

So ABS simply dropped the standard and direct people to use ISO. Many designers have not received that message and it's an important one. If you design to ABS then you are directed to ISO to replace the old Offshore Racing Yachts.

That's why ISO has more robust decks than ORY. The minimum is adequate in ISO.

 

Thank you for yours observations : I am aware of the problem of lack of bottom thickness in way of the ballast, but not of the problem concerning deck scantling.

However, it seems to me that a problem exist with ISO about the floors in way of the ballast : in case of grounding, the load is nearly the half of the load calculated by ABS ...

 

Looking for the forces is half the part of the problem. The other half is what is the allowed stress ?

The third half is how do you compute/check the generated stress given the force. ABS is very very very short on this, like ISO keel without annexes. I do not undersand how you can compute the stress in a composite floor grid bottom system without either (heavy) FEA or experiments : building it and try to break (ie what is done for aircraft wings).

 

This is a very good point. Compliance with ABS does require a certain amount of engineering not explicitly covered in the scantling rule. Without FEA or experimental results with good data, the designer must make conservative assumptions regarding distribution of stresses and follow good engineering practice. The approach for a bolted keel attachment may vary depending on the structural arrangement and materials used.

 

Hello,

If I understand correctly, It is impossible by simple method (without FEA) to
Know the real stresses in case of grounding ; yes, I agree with you, ABS is not very explicit about the calculation of stress.

Do you think that the method of calculation of ISO is correct for resisting to a grounding at a speed of 5 or 6 Knt ? If yes, all is OK ...

Best Regards


Jacques

 

ISO neither, unless you switch to the basic cas described in annexes.

Dont know, but just do simple Newton physics.

F = m * gamma.

so for ISO (0.8) gamma = 7.8m/s² .

Theboat is 6 kts = 3m/s.


The boat is stopped by the force from 3m/s.

distance V² = 2 * gamma * x.

So x = 0.57m = 57cm.

The idealized force taken by ISO is a force that if constantly applied at the center of gravity, would stop a 10m lwl boat going at 6 kts on a length of 57 cm. I guess nobody is no longer standing up on the boat ...

Now, the real dynamic problem is another issue. The boat will rotate, heel, skid on the ground . There will be friction and deformations etc etc etc ....

 

Thank you for the physic lesson.

Can I said, in conclusion, that there are many incertitudes in case of grounding, and that the method proposed in annexe by ISO (with gamma = 0.8 * 9.81) is acceptable as statistical point of view for a cruising or racing boat ?

Good evening


Jacques