ABS Yacht Rules & Plywood Construction

Totally agree. For a plywood panel of, say, 20 mm thick, an increase of 3 mm represents a 15% increase in weight which, for any designer, and even more so for a builder, is outrageous. Only with this saving in the total weight we have already paid the costs of the conceptual project.

 

Ok, this is getting farcical.

1- A NA on designing a vessel would not work their maths, then order special plate from the mill at custom thicknesses for every role on the vessel. They'd go to the nearest thickness and if their experience and knowledge suggested it, they'd adjust the framing slightly to make it work.

2- Plywood the same. A NA designs with existing material. Where the calculations show the ideal plywood doesn't exist, they up or down to the closest, and adjust framing or an extra layer of glass or something if their skill suggests. So in a recent example from someone who should know better, actual plywood sheathing would be no more than 7.5% off ideal.

3- NA's don't arbitrarily add 3mm of plywood for no reason. See above. They also know that the hull skin is not 100% of the fully loaded displacement of the completed vessel.

4- Flying Moth builders design and build with existing material, not wonderflonium, nor unobtanium. If they select the wrong material or the wrong design, it says nothing about anything other than that they are incompetent.

 

Probably not as hard as if you tell them to design a Flying Moth to ABS standards.

 

From F83MGM's original post in December:

"TIA, looking forward to interacting further in this forum."

Looks like F8MGM tuned out shortly after starting this, their second thread...

Never mind, carry on.

 

The responses seemed to drift away from my original question...anywho, was able to get in touch w/ someone at ABS and confirmed I was using the correct formula for plywood structure.

Onwards!

 

Good. Out of interest, how does it compare to the guide given in Gerrs formula?

 

***UPDATE***

Made an error in my ISO calculation! I knew it didn't seem right when I 1st did it...my bad.

For a plywood BHD:
s = 1830mm;
l = 3000mm;
σ = 28MPa;
h = 1.830m.

ABS Yacht Rules calculates 25mm
12215 calculates 19mm

I'm not familiar with Gerrs so I'd throw that back to you to crunch the numbers.

 

Gerrs requires knowing the length, beam and depth of hull to the topside of the keel to covert that into a volume calculated number that he uses in all his technical requirement graphs. a 25mm bulkhead would correspond with a boat having an SN of 14.
SN= LOA (ft) x beam (ft) x depth of hull (ft) divided by 1,000.

 

I wonder if that's akin to the Cubic Number used in the Seafish construction standard. For the fleet in Atlantic Canada it doesn't work as the calculated cubic number for the typical hulls fall outside the range detailed in Seafish.

 

No idea to be honest. For the designs I play around with, the SN is often less than 1.
Coming up with a "one size fits all" scantlings will still possibly have something of a heavy and light duty side to things. Some have suggested that the Gerrs scantlings are a bit on the heavy side. I treat it as a guide. Cross referencing some old Atkin wood designs it seems to average out.

 

I cannot understand that two boats with the same SN and very different speeds can have the same bottom thickness. Nor do I understand that, in the same boat, the entire bottom, whether bow, center or stern, has the same thickness. I don't understand many other things either. Maybe I have forgotten to read a chapter.
I am obsessed with achieving the minimum weight structure. When it is so simplified, very high safety coefficients must be adopted.

 

There are specified percentages for increasing the given sizes for things like speeds over 25 knots or heavy duty use etc.

 

Indeed, I know the book.