Flapdoodle Science

Mechanics of the Design

(perhaps more than you really want to know)

The classic dinghy shape dates back to the 1800's as a time-tested design and is responsible for the Flapdoodle's ability to glide through the water with hardly a ripple, even when heavily loaded.

First, the curves of the panels not only define this shape, but give it the necessary rigidity.

Secondly,  shear forces were employed where possible instead of putting the materials in compression, or tension. 

Forces in Shear

The PVC cloth used to make the waterproof panel hinges is laminated to Dacron. When it is glued to the hull panels and the covers glued on, #4 x 3/8" screws hold everything firmly.  When the boat is loaded, shear force (mostly) tries to pull the cloth laminates sideways, out from under the layers of glue and wood. It takes considerable force to do this.

Naturally...

The entire dinghy design is a result of natural curves.  The red curve (below) defines the keel, and the black curve that makes the gracefully hull shape.  Both curves are designed to follow the natural bend of the wood just as classic dinghy did more than a century ago.

As a result of this, any two of the spreading components, center seat, front deck, or rear seat will produce the final shape:
(not to scale)
There is no need for a hinged toggle seat or transom to open the boat. It opens quite easily. During assembly the center seat will spread the sides with only a gentle force. The transom and bow plate drop into place smoothly.

Trying to spread the sides apart only forces the ends tighter together making it even more rigid. Included as side forces are the mast and daggerboard.


A load on the bottom of the boat (including the seat which is supported via a pedestal to the keel) forces the sides inward against the three spreading components:



Locking up

The transom tries to move in the directions indicated below when boating. This is dynamic, and changes constantly with wave action, wind, and movement of the pilot and passenger. In addition, a trolling motor or small outboard can cause considerable force to the rear (drawing on the right).


It was a challenge to find a way to handle these forces and keep the weight and complexity of design within reason, but locking T slots provided a solution. As a result, the transom slides smoothly into place during assembly.

 

The peculiar inner curve of the stern lock post was dictated by accessibility to the large brass screws (red) that attach it through the keel and into the skeg (not shown). More screws come up from the bottom through the keel. The transom has a T slot that slips onto a mating T on the post. At the top is a similar T that mates with the rear seat, locking the entire stern into a rigid unit.

NEW  The Flapdoodle III options give additional locking for movement of the keel in these places:

Copyright Bill Weller 2006-2007