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Author Topic: Understanding and practicing half-hull boat modelling  (Read 12982 times)

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Understanding and practicing half-hull boat modelling
« on: May 09, 2007, 09:45:09 PM »
By Colin Sheehan

An Old Art Turned Full Circle.
Many people who are unfamiliar with the early days of boat and ship design & building often wonder, when they see the half-hull models of boats on yacht club and museum walls, "Why only half a boat?" or "Where are all the details like portholes and rigging?".

In fact originally, the vast majority of boatbuilders were also the "designers", although the term had not then been coined as the design of a hull was considered just a natural part of the building process. In the 18th & 19th centuries, the average shipwright was usually a man with little need for his "letters" so drawing a detailed plan with associated specifications would have been of little value to him. All building had, for centuries, been done by eye and this ability has been carried into the 20th century, although these traditional skills are sadly, fast dying out. So, as the shipwrights searched for a more efficient means to plan a new design,  the half-hull model was developed.

An experienced shipwright would also be a man with much knowledge of the sea and the way a boat performs under varied conditions and loads. This knowledge would allow him to sit down and carve the shapes he felt appropriate in a suitable hull to do the job demanded of it, whether it be carrying cargo, fishing or pleasure cruising. Therefore, he would trim a little off the forefoot if he wanted to allow the boat to tack better; produce a long straight keel if the boat was to track straight and hold a course; full, firm bilges if the main criteria was load carrying capacity and so on. When this was done and he was happy with the result, he would then take the model to the workshop and transfer the lines and dimensions (scaled suitably) to the real thing.

"Why Only Half a Hull?"
The major reason for building only half of the hull is the fact that boats throughout history share one factor which is inescapable. The laws of hydrodynamics demand that for a boat to track through the water straight & true, the hull must be perfectly symmetrical in cross section. If a shipwright's eye was not 100% sharp, the resulting craft might be a real pig to steer or worse.

So it became obvious that if you designed half a boat to perfection and then took the lines off the model to make the patterns for one side of the boat, then reversed the lines for the other side, it would be identical on both sides. Quite simple really and not a computer in sight.

There are various methods used for lifting the lines from a model for the lofting of the full sized vessel, but these are outside the scope of this article. The interesting thing to observe about the art of half-hull modelling and the building of boats or ships is that nowadays, it is becoming more and more common and in some cases mandatory to design the vessel on a computer powered CAD system, build the boat and then almost as an afterthought, knock up a nice little half-hull model for the owner's wall from these plans. Quite the opposite to it's original purpose.

The Model and it's Planning
There are basically two methods I use to plan & construct my models. Both of them rely on the part of a boat's plans known as the "Lines Plan". It is this set of curved lines which dictate the form and fairness of the finished hull. They give the viewer a very clear picture of the plan, profile and sectional shapes (All to the same scale) of the boat. At first sighting, the lines appear a jumbled mess of often overlapping curves, but after some small amount of study and/or experience, the boat's true shape becomes quite apparent and more easily worked with.

Understanding the Lines Plan:
Understanding the Lines Plan:
With minimal explanation and a bit of careful observation, the myriad lines drawn here will start to make some sense to the new modeller. Starting with the Sheer Plan (Profile) view, you will notice that there are many parallel horizontal lines drawn at regular intervals throughout the length of the boat. These are accurately reflected in the Body Plan (sectional) view to the right. These lines are the various waterlines (lifts to the modeller) and the longer one which extends beyond the hull is called the Load WaterLine (LWL). This last is the actual waterline as we know it when the craft is sitting in the water and loaded as it was designed. The waterlines are also represented, again accurately, in the Half Breadth Plan view at the bottom of the plan sheet in the form of the curved lines rising up from the stern area centreline and slowly merging together at the bows. This gives you an accurate impression of the shape of the hull at that waterline. An appreciation of this particular aspect of the lines is of major importance when modelling in the "Lift Method".

Also in the sheer plan view will be seen a series of regularly spaced vertical lines. These stations, as they are called, are duplicated in the half breadth plan view at bottom. If you study the body plan view at right, you will see that each of the stations is represented by one of the curved lines flowing down from the sheerline at the top of the hull to the keel. Also, you will notice that one side of this Kelpie, Australia's oldest yacht still sailingview is quite different to the other. The right hand side of the hull shows the stations from the bow to midships whilst the left hand side displays the stations from midships to the stern or transom if one is fitted.. The stations in this particular view give a very accurate impression of the shape of the hull at each station. Thus, by comparing all three views, you can get a very clear picture of the various shapes and curves which make up the graceful lines of the craft in question. In the "Station Method" of modelling, the station lines become the focus.

« Last Edit: May 10, 2010, 01:04:45 AM by magus »
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