This page discusses two building methods available for the average skilled person who intends to build a metal boat. First, a look at building the hard chine method - then the constant radius chine. Last, an overview of how a Bézier Curve accomplishes a true round bottom hull.
BACKYARD BUILDERS AND HARD CHINE BOATS
All hull shapes, from hard chine to true round bottom, can be fabricated in steel or aluminum. The success of construction lies in the available equipment and skill set of the builder.
For the backyard builder, hard chine boats are the easiest to construct. They can be achieved with average construction skills and limited equipment. If, for example, you are experienced with a tape measure in your day job, you can build a boat from steel or aluminum with developable hard chine construction.
There are many design adaptations for hard chine constructed boats. The single chine is the simplest, requiring the least amount of work. Double chine designs are more appealing to the eye, and give the feel of a round bottom hull underway. Labor for the double chine increases slightly over a single chine, but the skill level is the same. Choosing between a single or double chine is simply preference.
Various methods are used to soften the appearance of the hull at the turn of the chine. On adaptation to accomplish this is to add a narrow strip of flat bar or section of pipe to help soften the appearance of the chine.
A hard chine constructed sailboat hull lacks the beauty of a true round bottom. No matter which variation is used, in the end they are all flat-sectioned hard chine hulls.
Round bottom hulls in steel or aluminum, are skill intensive. Understandably, this makes the hard chine hull building more attractive to amateurs. Although builders may prefer a round bottom hull, they are reluctant to take on the challenge.
BACKYARD BUILDERS AND CONSTANT RADIUS CHINES
An alternative method used to imitate a round bottom hull is a Constant Radius Chine design. This method uses a large radius, arc of a circle, for the mid-section of the hull between the upper and lower developable surfaces, For ease of construction, the radius determined for use in the mid-section carries along the entire length of the hull, therefore the name constant radius.
A constant radius design requires above average skill to construct a hull, similar to, but not an actual true round bottom.
To construct a constant radius chine hull, a fabrication shop rolls or breaks the constant radius mid sections used to plate the hull along its length. The roll or brakes form the girth dimension (the distance around the shape of the hull).
To illustrate this, imagine taking one of these pre-rolled pieces, cutting it as necessary to meet the curvature of the hull along its length. The builder places the pre-rolled sections on the framework and begins to work the material closer and closer to the framework of the hull. This is done using clamps, mallets, line heating (can not be used on aluminum), and other fabrication methods until the plating lies against the framework in a fair way. These methods require professional metal working skill and equipment.
If you're putting professional skill into building a constant radius chine, the end result should at least imitate a round bottom design. Does it? To look like a round bottom, the constant radius section of the hull needs to be seen at or above the water line.
Remember, if you cannot see the radius section when the boat is at rest, it may as well be a single or double hard chine design.
To sum it up backyard builders with average skill have been limited to hard chine designs. While persons with advanced metal working skill are capable of building a radius chine. It would be best that you leave the true round bottom design to the real professional. That is until now, with the onset of Bézier Curved design. The average hands on person can now build a true round bottom boat with this method.
BÉZIER CURVED CHINE VS CONSTANT RADIUS CHINE
For comparison lets take two sections from the sample boat, the "DS 28 Bézier" and try to overlay a constant radius chine.
Detail-B shows a typical transverse frame near the mid-section of the sample boat. The Bézier curved section of the frame is in red. Note that longitudinal frames are placed at the intersection of the developable and (until now) the undevelopable hull section the forms the curve of the chine.
In Detail-B1 two sections of the hull are shown. The top as indicated, is near the transom of the boat. The bottom section is near midship.
Indicated in red is the radius that would be required at this section if it were a Constant Radius Chine hull. This radius does not achieve the hull shape designed in black. This radius (arc of a circle) does not even fit to match the required curve. Why? Because the required curve is not the "arc of a circle", it is a Bézier curve! In addition, the "arc of a circle" will not be tangent to the upper surfaces and lower surface to which it attaches.
A smooth tangent transition between surfaces is a must. The same result is seen at the midship section, but to a lesser degree.
In all fairness, if the "DS 28 Bézier" was designed for a constant radius, the tangency problem would be remedied with a radius that would join the constant radius tangent to its adjoining surfaces. But do we really want to design a hull around a radius? Or do we want a esthetically pleasing true round bottom boat?
To define the hull of the "DS 28 Bézier" an infinite number of radius' would be needed for shape. The radius changes not only at every location along the length of the hull, but also along the girth of the hull.
Using a Bézier curve solves the problem of hard chine and constant radius chine limitations. It delivers the beauty of a true round bottom - seen above the water line.
About the 12.5 Design
