Best resource for learning to calculate scantlings

What books would you recommend for learning to calculate scantlings for a new design. More engineering and less this is what we've always done.
I'm interested in larger (30-60ft ish) skin on frame boats particularly sailing multihulls.
Trial and error is much better made paper and pencil on a desk than on wood and canvas on the open sea(at least if you are the test pilot)

Many people say it can't be done because of the high torsion loads, but we built skin on frame bombers in the second world war which were built to handle major torsion and dynamic loads so it would seem possible.

 

After wednesday rum ration , I have to ask why such a concept?

 

There are no books specifically on what you are asking. However, you can start with first principles of physics and engineering. To start with a 36 foot boat is a very different design than a 60 foot boat. Write down a Statement of Requirements (SOR) to get the basics of what the design will be. The first and most important constraint is what your budget is. That should include engineering, testing, and building.

 

30' easy folding, light, cheap materials available. S.O.F

 

So I understand you are talking literal "skin" on frame, not tortured ply. So like two umiaks side-by side. And really, pressure loading normal to the skin is a magnitude higher in a boat than even a Hawker Hurricane. So from a practical perspective, do your homework and learn how umiak hulls are constructed then marry it to the flexable aka connections used in the polynesian voyaging canoes (like Hokule'a). Realistically, it is two independent space frame hulls (vakas) that are connected by a flexible deck that floats on the two akas forming a wa'a wa'a. This will not be a fast vessel because of the flexiure between the hulls, and I think that trying to stiffen it up to modern multi-hull strength and stiffness will result in you chasing your own tail due to the torsion loads.

Edit: Sorry, didn't answer the first question...try this.
http://freeit.free.fr/Structure Engineering HandBook/13.pdf

 

Thanks for the replies.
I'll dive into the pdf this evening.

Re budget use SOR etc. it is not so much a specific design as trying to learn how to scale designs from a kayak to an umiak to a catamaran, but using commonly available materials, seal skin and coconut fiber cord are both really hard to get in the pnw. Obviously what works on a coracle would be stove in just at anchor on a 60ft version.
I think (leaving the skin part out for now) learning to calculate the strength of various truss structures is probably the way forward.


Or, just doing stitch and glue, which I already have the books for

 

Actually, thinking about it, there is probably a practical limit for fabric skinned boat, where the cost of the extremely heavy/ multiple layers of fabric make it financially impractical.

Most all the large SoF boats use leather for the skin, which makes even solid fiberglass look cheap. Or they use hundreds to thousands of layers of tarred paper, which is not really cheaper than plywood and epoxy.

 

The problem with skin-on-frame is that all of the Primary loads have to be carried through the longitudinals (or space frame) rather than the skin. Except for very light construction, this is structurally inefficient. See this old post.
Hull Scantling https://www.boatdesign.net/threads/hull-scantling.65114/#post-900048

 

That makes total sense.
Thanks

Off to research plywood

 

In order to build such a craft you would have to incorporate some sort of diagonal stiffener into the structure. There are two main ways of doing it, using diagonal bracing on top of a conventional stringer frame structure, or by using a geodetic (geodesic) frame, with or without longitudinal stringers.
Both can be done in wood only, either by steam bending or laminating. The connections can be mechanically fastened or glued.
For the skin there is PVC and hypalon coated fabric available, as used on inflatable boats or trailer covers.

Since we're already discussing the topic, let me tell you that light and flexible are actually myths.
Leather weighs somewhere in the range of 800-1000+ kg/m3, it is heavier then meranti or birch plywood. Kayaks that were used in the arctic didn't skimp on leather thickness, they had to be able to go trough thin ice. The 36ft currach Brendan used 1/4" cow hides, doubled on the stern and quadrupled on the bow.
Traditional lashed boats have always gone to great lengths to ensure the lashing is as thight as possible and the joint as rigid as possible. Leather/rawhide lashings are done wet, the material shrinks when dry. Coir rope feels and grips like sandpaper and the knots used tighten themselves when wiggled. Also, a lot of the joints where also pegged in addition to beeing lashed. More modern currachs have copper riveted frames.

Where do the myths come from then?
Flexibility comes from the scantlings used. Thin wood is flexible, that's all. Viking ships are also said to flex on the waves like an inflatable, and they are riveted and pegged wood. Flexibility is a relative thing, fiberglass and carbon laminates are actually very flexible, just look at what a fishing pole can do. A car tire is flexible, but that doesn't mean you can fold it like a T-shirt. And after coating the modern ballistic nylon skin, the result can't be either, at least not without cracking the coating and loosing waterproofing.
Lightness comes from comparison to carvel boats, wich need a minimum skin thickness in order to hold caulking (about 5/8"). Caulking also madated the planks edges beeing kept in alignment, wich means tight frame/rib spacing, and why you don't see longitudinally framed carvel boats.

 

Great post.

Thanks

 

Is there an extremely rough rule of thumb about scaling scantlings ie.:
A similar boat that is 3/4 the size of the one I want to build has x dimension, so mine should have 2x dimension for the same part.

I know it is not that simple, but if you were designing a boat a little bigger than one that you had already done all the calculations for would you start at the beginning for the new one or try scaling it up and check a few calculations and see how it checks out?

 

There is a large space between the traditional approach to construction and the possibilities offered by modern materials used to a limited extent in boat construction.
What if we use compressed stringers construction or pneumatic e.g. dropstitch.

 

What would happen if you apply one layer of the skin on the diagonal?
Not leather but thick ballistic nylon or polyester.
Would it act and diagonal tension members and take some of the torsion loads?

 

It does not scale up linearly. That would increase the volume and weight by 2.4. The stress of the parts would increase by the same ratio. However, the strength of materials remains the same. It requires all new calculations.