Saturday, July 28, 2007

Design evolution

Early runabouts were built on the warped-plane shaped, semi-planing hull bottom. This is the form that interests me most. I am not interested in the high horsepower, blasting through the water experience, but would rather glide smoothly, quietly, at moderate, breeze-creating speed and with good fuel efficiency. Achieving a smoothly transitioning warped plane hull bottom is also more interesting for me to design than a constant deadrise planing hull would be.

I built a series of three models in developing the current design. This design method does not use a drafting table or a computer program/screen to create the hull. The hull is shaped by the use of mathematical equations and geometric projections of calculated (x,y,z) coordinates until an entire table of dimensions to define all surfaces and/or intersections is created. The hull only becomes visible as the coordinates are plotted in a connect-the-dots fashion. Similarly, the entire boat is created by plotting the dimensions of each piece (coordinates of many points) to produce a pattern for each part, model or full size. Many dimensions are mathematically exact; the remainder are accurate within about 1/32". The "connect the dots" dimensions are closely spaced so that a short 2'-3' batten can be used when connecting them for a full size pattern.

Mathematically almost any shape can be defined. In the real world each piece needs to be cut, bent or laminated to create the actual shape. Thus, I always try to design the final form using long, gentle curves rather than abrupt bends which are difficult to achieve during actual construction.

Monday, July 16, 2007

New direction

After I finished my model of a lobsterboat-style hull, I decided that , although fully functional, the boat looked drab and uninteresting. Rather slab-sided, not curvaceous enough, so I started over on the design. By changing co-efficients in some calculations, I gave the bow more flare, gave the stern more tumblehome, and flattened the run of the hull bottom. I also added a double cockpit and a fully cambered deck of classic runabout style. Then I started on building a new model. Drawings can't full visualize the three-dimensional form and proportions of such an object.

Lobsterboats are traditionally built in the range of 28 to 38 feet. Since I didn't want to exceed 20', the proper proportions weren't quite achievable on the reduced scale. Classic runabouts seem to be attractive to everybody, and I am no exception. I went to a wooden boat show and was disappointed in the selection of runabouts shown. Most were rather slab-sided, simple shapes; not a Riva among them. With the experience I have in creating developable shapes, I thought I could create something interesting.

I wanted fuel efficiency which generally means long and narrow. Twenty feet is the maximum length that will fit in most garages. The width had to be adequate for two people to sit abreast comfortably in the cockpit; that requires about a five foot width at the waterline. The result is a L/B ratio of 4:1 which is a reasonable compromise.

I am now finishing the 1/5 scale model of this new design and am extremely pleased with the result. This is the curvaceous and well proportioned shape I was hoping for. I hope to post pictures and more details in the near future. I don't plan to finish this model completely; I want to leave the deck open so that the structural details can be visualized- a guide for the future full sized hull. I do want to paint the outside of the hull and make it watertight so that I can check the hull buoyancy and loading.