Macro Half-Hull Model

Background


Since antiquity boat builders have had to design boats and then realise them in a three dimensional environment where very few lines are straight, very few surfaces flat and no angles constant over a distance. Hull models were used to make to-scale models which could then be used to review the design or communicate concepts to other persons. The laws of hydrodynamics demand that any vessel passing through a liquid be symmetrical in the cross-section if it is to track (move in a straight line) accurately. Consequently building one side of the hull in the model was all that was necessary - if the half-hull was perfect then its mirror image would also be perfect. In latter years half-hulls became used for symbolic purposes such as plaques, and nautical art.

This code came out of a project to build a model in FreeCAD of a 12.5 metre sailing boat. The builders no longer had drawings and the son of the original owner doubted there ever were full drawings. Consequently the hull dimensions had to be measured and then a model constructed within the software, Modern software packages like FreeCAD offer many beneficial features like symmetry constraints but when the final measurement is a model that is "easy on the eye", much "massaging" of models is required. This code was written to help automate that process.

The initial purpose of this code was to provide a symmetrical hull which was malleable so it could be fitted to the 3D model being constructed. As the software progressed it was generalised so hopefully it would be of some use to persons:
 * boat designers producing boat diagrams which can be built from
 * boat builders who are trying to model what is described on their diagrams
 * model hull builders.

Certainly commercial boat designers have all sorts of high end software to assist their work, this isn't supposed to replace that. Rather this is for hte hobbyist or home builder to tinker around with.

Nautical blueprints have a distinct history with respect to buildings and so are somewhat different in presentation. This is an example of a sailing boat which is more than a century old:



One of the end goals of this software is to use the Draft workbench to generate some of these diagrams using the model to generate the lines.

Description
For this macro the hull shapes are defined by a minimum of 3 FreeCAD Sketches: one in the YZ plane, one or more in the XZ plane, one in the XY plane. Here is the minimum hull supported by this macro, it has only 3 Sketches:



Note: in the above digram we are looking directly at the stern, the bow is pointing away from the viewpoint.

From front to back (bow to stern) the 3 Sketches are:

Perhaps it is easier to see with 7 Sketches (one in YZ plane, one in XY plane and 5 in the XZ plane):



With 5 Sketches in the XZ plane it is becoming easier to perceive the shape of the hull. The next 2 pictures show the Sketch lines superimposed on the model FreeCAD constructs,



the second is the same model rotated 90 degrees so the bow is in the foreground:



Some points to consider:


 * the Sketches are only for:
 * the stemline (or bow line) in the YZ plane (red in the above diagram);
 * the top of the transom in the XY plane (blue in the above diagram);
 * multiple cross-sections of the hull in the XZ plane (green in the above diagram)
 * only the starboard side of the hull is drawn in the Sketches, the port side will be generated as a mirror image
 * each multiple-segment line must be in a separate Sketch
 * each Sketch must have the same number of line segments (which is 3 in the previous examples)
 * the more line segments in each Sketch, the closer the FreeCAD generated model will approximate a curved hull
 * there is no limit to the number of line segment in each Sketch
 * there is no limit to the number of Sketches in the XZ plane (i.e. cross-sections)

With enough Sketches the model generated may even approach:



The HalfHull macro generates 4 models:


 * starboard half-hull
 * port half-hull
 * complete hull
 * bulkheads for the complete hull, either with a flush deck or with a coachhouse

These models are all output in the unified location space of FreeCAD so they can be fitted together - for example the bulkheads can be inserted into the complete hull seamlessly. This is a picture of bulkheads in a boat model during construction:



As well as the bulkheads generated by the macro (note that these bulkheads are for a coachhouse rather than a flush deck):



The composite image below shows the main outputs from this software (the port side half-hull is actually not shown but it is the mirror of the starboard half-hull which is shown). The outputs are in clockwise order from the upper left corner:
 * starboard half-hull
 * complete hull
 * bulkheads (for flush deck, the bulkheads in the previous image were for a coachhouse deck)
 * the complete hull with the bulkheads inserted



As novelty features, the macro will also optionally produce plaques for the half-hulls, and even a bottle for the complete hull:





Installation
All the code for halfHullModel.FCMacro is in one macro. So installation is comprised of copying the code to the appropriate Macro directory and invoking the Build Utility from the Macro menu, the Python console or a toolbar button (the preferred method).
 * see Macro_Install_HowTo for information on how to install this macro code
 * see Customize_ToolsBar for information how to install as a button on a toolbar

Usage
The

User Interface
There

Example
You

Options

 * starboard half hull, plaque, allowance for keel
 * port half hull, plaque, allowance for keel
 * complete hull, enclosing bottle, allowance for keel
 * bulkheads, either with a flush deck or with a coachhouse
 * canoe stern, sheer stern, sugar-scoop stern
 * trireme bow?

Remarks

 * doesn't do keels (yet)
 * doesn't do submarines (although it will do the lower half of a submarine)

Future Possibilities

 * option to automatically space the cross-sections
 * replace line segments of cross-sections with curved line
 * integrate with Draft workbench to produce drawings from models
 * handle keels

Glossary
As with any ancient and practiced trade, a rich and sometimes confusing vocabulary has developed around ships, boats and nautical practices. In describing this macro it is both awkward and inefficient to describe the process without using the correct and accurate terms. The obvious problem is that the average lay person will be unfamiliar with such terminology, hence this vocabulary:

Links

 * Half Hull Model Ship (Wikipedia)
 * Why Half Hulls (Maritime Half Hull Ship Models and Nautical Art website)
 * Traditional Model Yacht Design (US Vintage Model Yacht Group)