# McNaughtons Scantlings - fibreglass weight sheathed strip construction

Discussion in 'Boat Design' started by rwatson, Feb 21, 2009.

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### rwatsonSenior Member

Hi all

Just working my way through the Mc's scantling rules I bought recently (they are cheaper to buy as PDF instead of books - great idea !)

I would love to get verification that I inderstand the formulae.

Their scantlings for 'glass weight over strips planking are expressed like this

"If ordinary balanced weave glass cloth is used, the total glass cloth weight of each skin in ounces per square yard, equals 3.67 multiplied times the cube root."

So, if the boat is evaluated to a cube root of 3.8 (3500 lbs displacement),

or
3.67 x 3.8 = 13.9

or 13.9 ounces of glass per square yard on each side

This sounds reasonable, but have I worked it out correctly do you think ?

Any and all thoughts welcomed.

2. ### apex1Guest

Hello rwatson,
have no idea if that works well, but I fear it does calculate the glass only?
Real figures are provided here:

http://www.r-g.de/en/
change language, goto onlineshop, select your product and find laminate weight per m² incl. resin, for every single sort of fiber, weave etc.
a program to calculate laminates: (must register)
http://www.r-g.de/laminatberechnung_registrierung.html
and this is a REAL test:
unfortunately (for you) in German only.

I hope I could help a bit further!

Regards
Richard

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### AlikSenior Member

I wonder why use these amateur 'scantling rules' if official standards (ISO12215-5, ISO12215-5 simplified, GL, etc.) are available?

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### TeddyDiverGollywobbler

Maybe a fear of VOR's and Vendee boats having ISO scantlings
Reckon however they aren't

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### diwebbSenior Member

have a look at Dave Gerr's book The Elements of Boat Strength for builders, designers, and owners. This book synthesizes the formulas and scantling rules in to simple tables and graphs for those who do not wish to spend a long time and a lot of effort to get a viable construction layup for their boat. I find it very useful and relatively easy to understand. Also difficult for a non professional to make an eror in selecting a suitable weight and thickness for the hull and other parts of a boat.

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### AlikSenior Member

Provided that there is no error in the book
Actually, the method in that book does not work for planning boats.

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### PARYacht Designer/Builder

Dave Geer's "scantling rules" work rather well, including planning hulls. What they don't do is produce the lightest boat you can get away with, which is an intentional side effect of his rules. He clearly states the rules are designed for well founded hulls, not light weight machines. This covers the vast majority of desires when looking for this type of information. He also points out that the rules can be shaved to a degree without harm, but understandably rejects any ill will, that may come about by someone looking for this type of short cut. If you need this type of hull, then the reasonable designer works out the loads and scales everything accordingly.

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### AlikSenior Member

They don't really have planning hulls in Gerr's book as they do not define planning speed. Gerr uses increment of thickness for over 10kts(?), and this sounds rather strange because there is no dependence from Froude number. And actually there is no definition of design loads...

I mean Gerr's books are always great reading - I study his "Mechanical systems" right now, that is excellent. But for structural calculations there are better professional sources supported by software. I still believe for amateur designers simplified method presented in ISO12215-5 is the best.

This is my calculator for planning boats, based on ISO simplified method, works for craft under 12m.

#### Attached Files:

• ###### Easy_structural_calculator.xls
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9. Joined: Aug 2007
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### rwatsonSenior Member

So many answers .... but no solution to the question. I dont know why people assume I have no other source of calculations than McNaughtons.

Also, funny how close McNaughtons comes to the calcs on the 'easy ISO calculator' since they are 'amateurs' !!!

Still, if anyone wants to throw in a comparative figures, feel free

Length 8.5 metres

Beam 2.24 metres

Lightest displacement 1200 kilos

Heaviest Displacement - 1800 kilos

Draft around .3 metres

Freeboard - around 1 mtr

Power Source - 50 -up to 70 hp outboard

Top speed around 20 knots with lighter load

Building materials - Ply ,or epoxy over strip planking

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### ricardoribeiroJunior member

Você tem o easy structural calculator xls planilha para sailboats?

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