A Complex Boat Hull
---------------------------------------------------------------------Making a boat hull might not seem very easy. If we want to use the Form editor, shouldn't there be some sort of near-radial symmetry? It turns out that you can really push the Forms Editor around in ways Impulse hoped we'd discover.
A boat hull is a pretty simple object, right? Well, sorta. If you wanted to build one in the Detail editor, you'd make an outline the shape of an iron (as in for pressing clothes), then you'd extrude it and use slice to make the bottom.
Well, this would work, but it's a pretty cheesy boat hull, no matter how good your iron outline was. Even if you were a good modeler, made a series of outlines and used skin to blend them, you are going to get a hull that is boxy as opposed to a nice graceful curve.
Think of a big ocean-going vessel, not a cheeseball rowboat. The prow is sharp, to cut into the water, and it angles down and back. The body of the hull is fairly straight, and the stern rounds off smoothly with a flat face as opposed to the prow's sharp point. At no point, however, does the hull look like it was constructed of different sections. To reduce drag, the shape smoothly changes both from the top view (a teardrop with a squashed bottom?) and the side view. It has one axis of symmetry (left/right).
How could we ever model this in the Detail Editor? Not easily, and certainly not in one piece.
Well, fine. But how could we ever model this in the Forms Editor, either? It certainly is not very obvious.
A big hint of how we would design a form for the hull lies in where we place the center, the radial point, of our cross sections. We also have to decide whether the slices should be coming out horizontally (like the axis was a vertical mast) or at right angles. The choice is not obvious.
If the axis is horizontal, then the radial sections would tend to form a dome over the hull. If you made the radii of the overhead portion negative (There is no problem doing this!) we could just make a double-thickness of hull. This is messy, but workable.
The second option is to use a vertical axis, which gives us the benefit of a simpler object since there is no "dome" to add extra needless points. We want to make a new form object with an XY cross section. Select 3 slices and 7 points and we'll make a very simple version of the hull and work from there.
The question is where to but the center of our (vertical) axis. There are three places on the hull where there is a sudden change in cross-section -- the bow, and the two stern corners. If we want to make these changes fairly sudden, we probably want to define each one of them as one of our 4 cross sections. The interpolated cross sections by definition are interpolated, so there's no major change in shape. Thus, we want the bow to be in line with one of the four cross sections, as well as in line with the two stern corners. This makes our choice easy- the only place we can do this is the very back of the boat, along the (port-starboard) centerline.
Now that we've decided where to put the axis, how do we want to define the cross sections? Well, we want something that is left-right symmetric, and NOT front-back symmetric, so we should turn on left-right symmetry.
We want to change the default nearly-closed spherical cross section to something more resembling an open gravy dish. Move the very top point(s) way out and down some. The front cross section point (the left one in the Right view) should be moved the most. The back cross section point should be moved down, but not out very much. Remember that the stern is very close to the axis, and does not have much detail.
The Front view should look somewhat like a big "U", and the Right view should look like a sideways stretched "U" with one end (the bow) sloping back and the other pretty vertical.
The horizontal cross section (top view) should look (reasonably) like a boat viewed from above. The front should be pointed, the back should be fairly blunt, but rounding off to the sides.
Describing the shape of these forms is harder than describing a Coke can. Look at the picture hull_one to see what my crude shape looks like.
You can see in the perspective view that this basic form has a little hull-like character. It is sharp in front, and has a blunt rear. You can add extra points to each of the views to make a smoother form with more details. My final boat hull is shown in picture hull_two. To get an idea of how complex the real object is, there is a picture of the same hull shown in the detail editor in picture hull_three.
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