The Rabl Method

This post will be my attempt to explain the theory behind designing a plywood planked boat.  As I tend to like to understand how things work, I did some reading to understand the process and boil it down to a basic description.  This certainly won't help you design a boat, but it does help explain the Rabl method of fairing and why it works.  You can ignore this post if this type of things doesn't interest you.  I won't mind.  If you decide to read this, I suggest you read my previous post which shows how I used the Rabl method to fair the forward part of the hull.

When a boat is developed for sheet material such as aluminum or steel plate or, as in my boat's case, plywood, there are some restrictions on the hull shape that can be designed due to the fact that plate material, such as plywood, can only lay flat, be curved into a cylinder or into a cone. Plywood cannot take on a compound curve, one that bends in two directions. This can be visualized with a piece of paper.  You can smoothly roll it into a cylinder or cone, but if you want to bend it in the other direction at the same time, you cannot do it with a smooth curve, because you get a nasty crease.

Now, metal can take on compound curves as evidenced by a car body, but it requires huge presses and dies to deform the metal.  This is not possible when building with plywood, so the Rabl method is needed to design a hull that can be made from sheets of material.

In 1941, Samuel S. Rabl wrote a book called Ship and Aircraft Fairing and Development, which describes the method to design boats (and airplanes) with sheet materials. Naval architects like Glenn L. Witt, who designed the boat I am building, had to use this method to convert his conceptual design into a build-able plywood boat. This process might require changing shapes and curves slightly to make sure the plan is "developable."    To be developable, the design must ensure the surfaces are conical or cylindrical in shape.  In most cases, conical shapes are used. Rabl identified that mulitple cones could be used to develop a hull shape.  The only requirement is that the cones have a radian in common as shown here:

A radian is any straight line that starts at the apex and runs down the surface of the cone.   So a hull shape can be developed that varies over its curvature as long as it stays on these common conical surfaces. There is an example of part of a hull shape on the diagram above. 

I have a .pdf file of an article by Rabl, which shows how a boat concept is developed into a buildable design. Here is a link to that file.

https://docs.google.com/viewer?a=v&pid=explorer&chrome=true&srcid=0B4wD-Wh1mk4VZDBmZjZkYmItNmE3Yi00NGQwLTk1YjgtNzUzMjNjOWJmNGNh&hl=en&authkey=CPekg_kJ

So, how does this relate to fairing the hull, in particular the complex area near the stem.  Since the design is based on cones, the measured points as described in my previous post, such as A-A on the stem and chine are actually the points where the stem and chine intersect the same radian.  Since a cone keeps getting larger as you get further from the apex, the distance between radians get further apart.  So, by measuring equidistant points on the stem we are defining one set of points on arbitrary radians.  By also measuring equidistant points on the longer distance of the chine, we define another set of points on the same radians further down the cone.  They are the same radians because we started at a common point which is the point where the stem and chine meet.  Since the two points are on the same radian, we can connect them with a straight line, such as the batten I used to measure the fairing.