- Andrew,

Everybody boat builder should have a copy of Elements of Boat Strength. It has a wealth of information. I think his scantling rules for wood are based on carvel construction but he says to use them for plywood too. As you've discovered, lapstrake is stronger than carvel because of the overlap (stiffness correlates with the square of the depth) and because it is skin stressed and forms a monocoque structure, whereas the structural strength of carvel is contained primarily in the framing. Carvel planking is there to keep the water out and provide impact resistance. Impact resistance is also a consideration for planking thickness. It can be improved by putting fiberglass on the outside (compression side) and kevlar on the inside (tension side) and just treating the wood core as a web. Then the wood could be of low density since it is on the neutral axis (as long as it has enough strength in shear).

fred s.On Wed, Mar 3, 2010 at 6:05 AM, Andrew <a.c.l.yen@...> wrote:Thanks Fred,

I've ordered a copy of Gerr's Elements of Boat Strength. I'll be interested to work this through. I had a look at the first chapter on Google Books and from that worked out that the Chebacco 25 would have a Sn of 25 x 4 x 8 / 1000 = 0.8 Google books didn't have the chapter on scantling rules for plank thickness so I will have to wait till the book arrives.

Out of interest I did some calculations/approximations of relative plank stiffness from an engineering formula I learnt in University and haven't used in 25 years since. I = b(d^3)/12 where I is the section modulus which is directly related to stiffness, b is the width of the plan and d is the thickness.

Assuming a 120mm (4 3/4")wide plank with a 20mm (3/4")overlap:

12mm (1/2") lapstrake is 73% stiffer than 10mm (3/8") lapstrake

15mm (5/8") lapstrake is 240% stiffer than 10mm lapstrake

But interestingly:

15mm carvel (i.e. a non-overlapping plank system) is only 56% stiffer than 10mm lapstrake, less stiff than 12mm lapstrake yet with 7% more wood. Lapstrake construction really does stiffen up a hull.

I started marking out construction moulds for Chebacco 25 today assuming 1/2" planking. (I haven't fully committed to building it yet but I will at least build the moulds and building frame and then decide if I still like the design and still think that I can finish it before I am too old to sail it ;-).)

Regards,

Andrew

--- In bolger@yahoogroups.com, Fred Schumacher <fredschum@...> wrote:

>

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

> 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.

> That would put the 25 foot Chebacco at a Sn of 0.33 and a planking thickness

> of 0.66". Even taking into account the flare of the hull, it still puts

> planking at 5/8." Lapstrake adds strength because of the overlapping

> planks, but I would think 3/8" is too thin for the Chebacco 25.

>

> fred s.

>

- 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.