ABS Yacht Rules & Plywood Construction

Looking to the brain trust for some clarification....

I see a lot of plywood construction in the industry I'm working in and I'm looking for confirmation (other otherwise) that I'm using the correct formula for plywood structure in the ABS Yacht Rules.

See: New Fluid Topics Tenant https://pub-rm20.apps.eagle.org/r/15/2024-01-01/Yacht-Rules

Reference Section:
3-2-3/7 Wood, Cold-Molded Laminate and Carvel

Thickness calculations for plywood structure, does that fall under 7.3.1 Cold Molded Laminate Thickness? Is plywood considered cold-molded laminate?

My (very limited) understanding is that 7.3.2 Single-Skin Carvel Construction refers to what I would call 'planked construction'?

We're moving away from the 1978 Glass rules (sad face...) and the Yacht Rules, under ABS, are the closest reasonable facsimile for the fleet in question.

Humbly submitted!

 

I have been using Gerrs scantlings as a guide, so can only offer his interpretations of a boat with the same SN for plank on frame.
Traditional plank 16mm
Cold moulded 12mm
Plywood 14mm (90% of trad plank thickness on topsides but equal to plank thickness on bottom)

Frame spacings can either reduce or increase skin thickness.

 

I'm sure that's what Gerr says but, honestly, I can't find any technical explanation that justifies these "equivalences."

 

Lots of people think plywood should be thicker than plank, as it only has half the strength in one direction, certainly multi laminated cold moulded can have better strength in more directions than plywood, so can be thinner for the equal strength and thinner than plank.
I heard Gerr used a combination of recommended scantling authorities, and averaged the findings. I dont know if that is accurate, but it would seem a logical thing if you can further break it down to a displacement figure.
Certainly many plywood boats on the 60s where built with much thinner material than "plank", but they often did have a more extensive framework.
Im sure there is no end of test videos on youtube. The thing with plywood is the gap between thicknesses, some people size up with a weight penalty, and some will size under. With plywood you buy a product, with planking you can mill to specific size if a scantling authority demands it to meet some class classification. Slapping fibreglass on plywood to make it stronger, usually ends up with a heavier skin than if one used the larger plywood to start with.

 

If you made an effort, you would find those explanations, so it's disingenuous to say you can't.

 

I am not going to say that you are not right, on the contrary, I am sure that you have your reason to assert this. It all depends on what each one understands as a "technical" explanation.
Cheers.

 

Again, since you haven't read his long detailed description of exactly how he did it, you can hardly suggest he hasn't provided a "technical" explanation. You just don't know.

 

Perfect, @DogCavalry . I agree with everything, are you happy?

Edited : Ok, I was wrong and I will try to prevent such a mistake from happening again.

 

From Gerr's description of the origin of his scantling system: https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH/BoatStrengthIBEX.pdf

How the Sn Scantling Rule Was Created

Where does one start to create a scantling rule? With real boats. We’ve already seen how difficult it is to know what the real loads are on a boat. What we can know with good certainty is the specifications of boats that have been built and have given good service over many years— dimensions, displacement, speed, type of service, material, and scantlings actually built to. These data can be analyzed (regressed) to obtain the desired rules. The data and the results are constantly checked and rechecked against other real-world boats, against basic class rule results and against first principles.

With any such process, the results are not going to and should not match any other specific rules’, or methods’ results, but the scantlings from the resulting new rule should produce consistently reasonable structures, that meet reliable criteria for strength. The real boat data available to work with came from:

- My own designs actually built and in service (and from the MacLear & Harris office and from Cape Dory Yachts)
- Boats from other designers for which I had data
- Boat data from the three-volume set of Fishing Boats of the World
- Additional wooden boat data from USCG NVIC 16-60, Scantlings for Wooden Passenger Vessels ​

This totaled hundreds of different vessels as a reference database.

Results were checked against:

- Engineering by first principles
- ABS
- Lloyds

Again the goal was not specifically to meet any of the class rules, particularly given the unknowns in loads, but to be close on the majority of the structure.

One of the interesting findings was how strong many real-world boat structures actually are, even without meeting the requirements of class rules. Stiffness and strength in real boats, often are considerably greater than such rules might indicate.
​

Previous thread about Gerr's scantling system: Dave Gerr Boat Strength Scantling https://www.boatdesign.net/threads/dave-gerr-boat-strength-scantling.60880/

 

Plywood thicknesses vary by 1/8" (~3mm) so the weight penalty is minimal.

 

Design to an specificity greater than 3mm would be an exercise in pendantic self deception anyway. Multiplying an approximation by an estimate, and dividing by an average and insisting on taking the answer to three significant digits because you'd done the math correctly.

 

@DCockey, I greatly appreciate your effort copying paragraphs from Dave Gerr's book but it is not necessary, at least on my part, for you to work so hard. It is enough for you to tell me the page you want me to read and I will read it. That said, I reaffirm in the comments I made in my previous post, not a single technical justification for the SN (I understand, but you can understand something else, that an accumulation of data from many ships is not technical reasoning).
I recommend that you read the note at the bottom right of page "ii", the one before the "CONTENTS" chapter. As I greatly respect all the members of this forum, I don't want to bore them by copying that NOTE here.

 

My post above was intended for anyone interested in the origins of Gerr's scantling. The portion I copied was from a paper Gerr originally presented at IBEX in 2010, not his book.

I am not making any statement about the applicability or validity of Gerr's book.

 

I do.
Read, please, the Note I mentioned in my post.
Finished, for my part, this conversation.
Cheers.

 

Tell that to a Flying Moth builder, and they will laugh at you.