Scantling Rules

[Mat-C]

I've read through a number of the threads regarding scantling rules and the various bodies that have produced them... ISO, Lloyds, ABS, DNV... even Gerr.
I'd like some opinions as to which are the 'best' - by that I mean which would generally give the most sensible results?
It's a very wide subject, I know, so for the sake of (a little) simplicity let's consider pleasure power boats in the 30 - 50ft range... FRP, Composite, Alloy.
Still a wide net I know....

[Mat-C]

I should probably qualify that with another question...
Would it be 'normal' for a designer to use one of these rules, or would the calcs be done using 1st principles?

[Paul Kotzebue]

Originally Posted by Mat-C

I should probably qualify that with another question...
Would it be 'normal' for a designer to use one of these rules, or would the calcs be done using 1st principles?

It is normal for a designer to use one of the scantling rules. However, a knowledge of 1st principles is helpful in understanding the intent of the scantling rule.

[Mat-C]

Thanks Paul - so would this also be the case for a larger manufacturer - say the likes of Bertram, or SeaRay in the US, or Fairline, Azimut etc in Europe?

[gonzo]

They have to comply with scantling rules so it is not an option.

[Paul Kotzebue]

Originally Posted by Mat-C

Thanks Paul - so would this also be the case for a larger manufacturer - say the likes of Bertram, or SeaRay in the US, or Fairline, Azimut etc in Europe?

Recreational vessels sold in the U.S. under 300 gross tons (which is huge) are not required to comply with a scantling rule, however the use of Lloyd's Register Special Service Craft or ABS rules for motoryachts is typical for large custom motor yachts built in the U.S.

Boats under 24 meters long sold in Europe must be CE certified and therefore comply with the ISO scantling rule.

[Mat-C]

That would suggest then that there would come a point where engineering an ever lighter structure (but still strong enough to comply with the appropriate rule) would become impossible with current materials...?
And if it is legislative requirement to comply with ISO in the EU, what happens in the case of racing yachts?

[Paul Kotzebue]

Originally Posted by Mat-C

That would suggest then that there would come a point where engineering an ever lighter structure (but still strong enough to comply with the appropriate rule) would become impossible with current materials...?
And if it is legislative requirement to comply with ISO in the EU, what happens in the case of racing yachts?

I'm sorry I don't understand your first question, however the sacntling rules do consider the strength of the material used, so a carbon fiber skined cored hull will be much lighter than a single skin E-glass hull.

My understanding is racing yachts are exempt from CE certification ... but the ISAF (2008) requires compliance with ABS Offshore Racing Yachts or ISO 12215 Category A. So compliance with ISO or ABS is a de facto requirement for (sail) racing yachts.

[Mat-C]

Yes - sorry - that wasn't very clear was it....
Having to comply with the minimum requirements of a scantling rule would, I assume, mean that the structure is likely to be overbuilt to some extent... organisations are unlikey to expose themselves to liability by specifying absolute minimum structures. So, regardless of the material being used, the boat is likely to be more heavily built than necessary. I understand that there are safety factors involved of course... but if designers were free to engineer the structures themselves, then surely they could be lighter than if they complied with the various rules...?

[Paul Kotzebue]

Originally Posted by Mat-C

Yes - sorry - that wasn't very clear was it....
Having to comply with the minimum requirements of a scantling rule would, I assume, mean that the structure is likely to be overbuilt to some extent... organisations are unlikey to expose themselves to liability by specifying absolute minimum structures. So, regardless of the material being used, the boat is likely to be more heavily built than necessary. I understand that there are safety factors involved of course... but if designers were free to engineer the structures themselves, then surely they could be lighter than if they complied with the various rules...?

No I don't think so ... the ISO and classification society scantling rules are based on sound engineering practice. The safety factors are reasonable and not overly conservative. Designer's are free to engineer the structure themselves for the most part. The scantling rules are used primarily to establish loads and to define acceptable allowable stresses of the materials being used. The classification societies are not liable for the use of their scantling rules. Liability rests with the designer. I believe ABS and Lloyd's will accept an independent engineering analysis (such as FEM) for classed vessels in lieu of using the scantling rule. But doing that is a big engineering task that is beyond the scope of most small craft designers.

[Mat-C]

I started a thread a while back regarding what I call egg-crate stringers and frames - where these structures are more like 'floors': thin but tall, forming an egg-crate like structure that spans the bottom and the sole above it.
Egg-Crate Stringers and Frames

How does this fit in with these scantling rules? I don't really have a good grasp of any of them, but they seem geared to the more traditional approach incorporating top-hat type sections for stringers & ring frames..?

[Paul Kotzebue]

Originally Posted by Mat-C

I started a thread a while back regarding what I call egg-crate stringers and frames - where these structures are more like 'floors': thin but tall, forming an egg-crate like structure that spans the bottom and the sole above it.
Egg-Crate Stringers and Frames

How does this fit in with these scantling rules? I don't really have a good grasp of any of them, but they seem geared to the more traditional approach incorporating top-hat type sections for stringers & ring frames..?

ABS and Lloyd's have classed many commercial vessels and large yachts with similar (known as "double bottom") structure, usually steel or aluminum.

[masrapido]

Mat-C, while Paul is right about the scantlings, the question you probably want to answer yourself first is why are you after the scantlings? Are you going to build and sell boats, or is it just for your own boat?

If you are only building it for yourself, you do not need to follow the rules.

However, following the scantling rules ensures adequate safety at sea, and that is the most important issue to consider. The rules are a guide for a builder to ensure the boat will be safe for sailing. Either way, many different rules exist and it may be the best to leave it to a professional to determine the exact dimensions for your project.

You can buy the rules from ISO or Norske Veritas, for example, but you still need to convert the formulas into the numbers and that is quite a job.

[TeddyDiver]

Originally Posted by masrapido

you still need to convert the formulas into the numbers and that is quite a job.

In that sense I find D Gerr's scantlings one of the best for "amateur" one off projects.. Not too much complications in number crushing and not much to go wrong either..
For commercial boatbuilders in the target range "pleasure power boats in the 30 - 50ft" there's ISO rules whats most of the world to follows and ABS for the rest..

[MikeJohns]

For a small vessel Its a good idea (if you can) to calcutate from first principles using panel sizes pressure head and beam theory. Then run the figures through the scantling rule and compare.

You should get your scantlings checked either by the class society itself or by an experienced and knowledgable desiger. It's very easy to interpret a rule incorrectly or slip up on something. People tend to forget that class scantling rules only apply if the design is approved and they do this for a small fee, in effect consider it an unbiased peer review. Many design errors are due to incorrect interpretation and a beleif that the rule was followed correctly and this has resulted in death.

If you use Dave Gerr's guide then use it instead of the first principles approach but then run your figures through a class rule and compare. I've seen some very poor design by neophytes trying to apply Gerrs rules too. Just becasue they appear simple doesn't gaurantee a compliant vessel.

Germanischer Lloyd (GL) rules are very good and a lot of designers are adopting them around the world now that ABSORY has ceased to be valid.
see Germanischer Lloyd (GL) rules free

Cheers