Defining the Features of a Hull
Form Defined by Sections using X-Topology Curves |
Often the only information available when you want to recreate
a hull surface is section information. The foundations of any hull
surface based on X-Topology is the hull form feature curves, such
as the midship section, surfaces and flat of side curves. The following
example shows how to extract this information from a set of sections
and using the snapping tools build up the initial network of curves.
Alternatively, if you are starting with surface data this example
may also be useful.
The surface produced in this example can be downloaded here: xtopology_features.geo |
Starting with a set of sections and a few hull feature curves. The
first thing to do is to group the forward and aft portions of the
sections into two, to allow us to work with each end independantly.
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Selected the group of sections for the forward end... |
...and choose Right-Click -> Multple Selection -> Create Polyline
Group. Similarly, group the aft sections.
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The aftmost section for the forward curves should be set up to be
the forward end of the parallel middle body or the midship section
if the ship has no parallel middle body. Create a X-Topology curve
and using the snapping tools with "Snap to Points" and "Snap
to Polyline List" turned on, (see the snapping options in the
help), start at the base line and create the first point. |
Then, if the midship section only has points around the bilge radius
use the following approach to identify the beginning of the bilge
radius. Allow the next point to snap to the section curve, but before
clicking...
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...slide it along the curve toward the bilge radius until it meets
the first point snap. Left-Click to add a point. |
Now a point on the flat of side of the same section and... |
...slide it down to the bilge radius until it snaps to the first point.
Left-Click to add the next point. |
Snap the final point in at the top of the section.
|
As the curve starts out as a standard BSpline, we need to add a blend
around the bilge radius. Finish creating curves, select the new curve
and press F2 to edit. Select the control points adjacent to the bilge
radius... |
and add a Blended Segment constraint using Right-Click -> X-Topology
Curve -> Blended Segment. |
The completed midship section curve.
|
Now change the view to show the stem. Create a new X-Topology curve
and trace along the curve data.
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Snap some data point to the bow profile. |
It does not matter at this point that the initial BSpline representation
does not match the traced shape. These points can be converted to
interpolation points after the curve had been created. |
Zoom in to add specific features such as to accuratly place the knuckle
point at the top of the bulb. |
Complete the bow profile up to the deck. |
Now start detailing the features of the curve. Finish creating the
curve, select it and press F2 to begin editing. Select the point
representing the knuckle location...
|
...and Right-Click -> X-Topology Curve -> Knuckle Point to add
the constraint. |
There will now be a corner at the top of the bulb
|
To make the curve go through the points on the bulb itself, select
the points... |
...and Right-Click -> X-Topology Curve -> Interpolation Points
to change the way the curve is generated. If using Interpolation Points
or the Cubic Spline Curve Constraints it is good practice to also
enable "Cubic Spline Knot Vector based on Chord Length".
See the help for more details on this feature. |
The shape may not match the intended profile exactly so move the
points or if necessary...
|
...add further points by pressing the 'A' while moving the mouse. |
The reposition the points to improve the shape. |
The top part of the curve may have a linear shape which can be added
by selecting points...
|
and Right-Click -> X-Topology Curve -> Straight Segment to constraint
the shape of the curve to a line between the two selected points. |
The bow profile needs to be connected up to the midship section curve.
Zoom out and add a point on the midship section curve. |
Then add any additional constraints to make the curve the right shape. |
In this case, an additional knuckle was added at the bottom of the
Stem. |
...Two curves completed so far...
|
Now start adding some internal curve features such as the flat of
bottom. Start at the midship section and... |
...using the snap features, pick up the first points on the bilge
radius in the same way the midship section curve bilge radius was
generated. Try to avoid placing a control point on every section.
You want somewhere between 4-6 points at consistent x locations
all the way down the hull. But if you can get away with less points
it is better. Try not to use more but if you need to, then add more
points where necessary.
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Drag the curve along...
|
...and connect it up to the stem curve where the flat of bottom would
terminate at its forwards most position. As this curve has no internal
features itself the "cubic spline curve" constraint can
be used in conjunction with the "Cubic Spline Knot Vector based
on Chord Length" options |
Similarly, generate the flat of side curve. |
At this point, the surface tangent information can be specified
for the midship section. The "Force Curves along Surface Tangent"
constrains all attached curves so that their tangent vector is exactly
the same as the specified surface tangent vector rather than just
constrained to the plane of the surface tangent vector. In particular
this forces Flat of Side and Flat of Bottom curves tangents to be
perpendicular to the midship section curve.
|
Add in the deck curve, paying attension to consistent locations of
points in the x direction. Create a point at the end of the flat of
side curve but unlink the point as it is more desirable to attach
the flat of side curve to the deck curve. |
A straight segment constraint should be applied to the curve between
the midship section and flat of side curve.
|
To force the curve to interpolate the sections at the bow, select
the internal control points... |
...and apply interpolation point constraints. |
Generate any additional features such as the knuckle curve. |
If these curves have no internal features, the Cubic Spline Curve
Constraint can be used instead of applying Interpolation Point constraints
to individual points. |
The knuckle condition is assign by applying Surface Tangent Information
|
Smaller detailed features can then be added such as this Knuckle between
the main hull both and the bulb. |
This curve requires the Cubic Spline Constraint... |
...and the knuckle surface tangent information.
|
The completed set of curves for the bow. |
Similarly, the curves should be completed for the stern.
|
The complete set of curves after connecting up the forward and aft
sets of curves in way of the parallel middle body. |
Presently there is enough information to form the surface along the
parallel middle body. |
But not in the other areas of the hull. In these areas we need to
add more curves. This will be done in the following example. |
