- Hi Fred,

> My copy of Gerr's Elements of Boat Strength is at home, and I'm at my son's

...snip...

> house, but I would suspect that 3/8" is pushing it. As I remember it,

> planking thickness is the Scantling Number raised to the 0.37 power, and

> Scantling Number is length times beam times midship depth divided by 1,000.

I don't have Gerr's book (yet), so I have to ask if this formula is intended to apply only to certain hull types or uses like, say, ballasted keel boats intended for offshore voyaging. Certainly it seems that the displacement should play a factor. For example, I get values of approximately 0.54 for the Light Schooner and 0.61 for Birdwatcher, both of which have 1/4" side panels. I don't recall anyone complaining about the strength of the side panels on these boats, but of course neither of them are intended for offshore voyaging either. A more extreme example is a hypothetical 16' x 2.5' x 1' kayak. I get a value of 0.30 (7.62 mm), yet most are built with 4mm ply.

It seems that a second consideration must be planking species. If the formula is intended to apply to all species, then it must be biased toward lighter or weaker wood. Are two carvel planked boats the same "strength" if one is planked with red cedar and one with teak of the same thickness?

Nevertheless, I agree that 3/8" sounds light for a 25-footer of Chebacco's displacement. It also sounds like I need to get this book - if it's as thorough as "The Propeller Book" it should be quite useful.

Jon Kolb

www.kolbsadventures.com - On Sun, Mar 7, 2010 at 4:37 AM, Andrew <a.c.l.yen@...> wrote:In my calculations I assumed that the stiffness of the hull will be related to the stiffness of the planks in bending. As we know, the round boat shapes increase stiffness considerably over flat planks stiffness - but I still think that if you double the stiffness of the planks then you will - more or less - double the stiffness of the boat.

Stiffness is related to the square of the depth of a beam, so doubling planking thickness would quadruple stiffness. In calculating the scantling number, the product is directly related to the product of length, width and depth. In the case of the rowing gig in relation to the 25 foot Chebaco, the length is 7 feet longer, but width and depth are much less, so its scantling number is lower. It also doesn't have to carry the loads imposed by the mast and sail. Clinker built gigs are skin stressed, monocoque boats that don't need framing except to add some marginal stiffness and shape retention under stress. A carvel built gig would need substantial framing.

fred s.