I have been asked to comment on this thread and will insert responses where appropriate. As was pointed out in the post below the Sheathed Strip Rule only goes down to 1,728 lbs. displacement. Thus arbitrarily following the formulas lower will at some point get you to the point where concentrated loadings for a human being will require modifying the design. The author of this post that I'm responding to was confused that the Fiberglass Scantlings Rule goes lower but requires heavier skins in small sizes. He is correct that no explanation is offered for this. The reason is two fold. At the time of doing the engineering the most common glass cloths sold were 10 ounce and as we went down into "dinghy" sizes I was more and more concerned with abrasion. Therefore I set a lower limit on glass weight for this rule. So you see this was less for engineering reasons than practical ones. In the wooden boat world lighter glass cloths are more common. Although they are often intermittently available for some reason.

Tom MacNaughton http://www.macnaughtongroup.com

> I have received my copies of MacNaughton's scantlings for both
> wood-strip/glass and wood-core/glass. As I said in my initial posts - read
> the footnotes and apply the rules in the correct domain.

> I have several comments

> Footnotes::

> Under the heading "General Intent" - "Dimensions should be
> increased where special considerations of design or service make it
> advisable". I believe that the possibly of surf landings and more
> than occasional rock impacts are special considerations of service.

> Domains of the Rules:

> Nick was correct in his computations from MacNaughton's FORMULA - the
> wood-strip/glass scantlings indicate that for a 300 pound displacement a
> layup of 6.1oz/.251"/6.1oz is appropriate. MacNaughton's TABLES of
> displacement only go down to 1728 pounds and that displacement indicates
> 11oz/.450"/11oz. It is possible that the formula was not intended to
> be applied to such small boats or boats configured like kayaks.

> From MacNaughton's FORMULA - the wood-core/glass scantlings indicate that
> for a 300 pound displacement a layup of 15.1oz/.251"/15.1oz is
> appropriate. MacNaughton's TABLES of displacement only go down to 373
> pounds and that displacement indicates 20oz/.270"/20oz. The 20oz per
> side is the minimum for this type of construction - appearing at all
> displacements below 4096 pounds. We can probably safely apply the formula
> down to 300 pounds.

> The descriptions of the two construction methods include epoxy as
> "glue", epoxy not polyester resins, and core material
> specifications. The core materials for wood-strip/glass boats are listed
> as cedar, pine, fir, and Honduras Mahogany. The core materials for boats
> are listed yellow pine, spruce (Sitka and white), Douglas fir, and
> Honduras Mahogany. I see no significant difference in the core materials.

> Cloth specifications are somewhat different. Wood-strip/glass allows does
> not specify a type of cloth. Wood-core/glass specifies layers of 10oz
> cloth for 300 pound displacements . I expect that their is no significant
> difference caused by the type of cloth.

> The two rules were designed to produce boats "equivalent in strength
> and fatigue resistance".

> Since both rules are designed to produce boats equivalent in strength and
> fatigue resistance but the "correct" amount of glass is either
> 6oz, 15oz, or 20oz, someone went astray.

> --------------------------------

> It is really hard to find where someone goes astray.