| The GAETA is a small traditional
fisher boat from Croatia. It is made of wood, approximately 7
to 12 meters long and was used in the Mediterranean Sea along
the Adriatic coast for fishing and transportation between the
islands. The boat excels by its rounded bow and stern, its high
carrying capacity and its good navigability, especially on the
rough sea. However, when less expensive plastic boats appeared
that were mostly produced by foreign manufacturers, the number
of authentic boats drastically reduced.
One of the few still existing Croatian shipyards,
specialized in the construction and repair of wooden boats, is
located in the small town of Nerezine on the island Lošinj. Enthusiastic
about the beauty and navigability of its 8 m long GAETA, the
Lekić family decided to restart the production of these wooden
boats. They own a boat with excellent characteristics and have
the infrastructure, the knowledge and the necessary experience
in building wooden boats. Now, with the help of modern technology,
they want to revive traditional boat building.
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Fig. 1: Structure of a wooden boat (drawing: V. Salamon).
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Making wooden boats is a big challenge. The boat's
frame (fig.
1) consisting of ribs, beam and keel defines the shape, the
stability and the navigability of a boat. In the past, experienced
craftsmen produced these wooden frames by guess and experience
but with varying success. A reliable method to create a good new boat was
to copy a boat that proved to be successful. The ribs of the original
boat were copied by stencils and their shape was transferred to
the wood out of which the ribs for a new boat were produced.
Modern technology simplifies and quickens the
manufacturing of products having reproducible characteristics.
In order to make it possible to create a faithful copy of the
Nerezine boat, the Croatian company TOPOMATIKA performed a 3D
measuring of the boat's shape using the photogrammetry system
TRITOP of GOM. Figure 2 shows the equipment used for measuring:
A professional digital photogrammetric camera, a notebook with
TRITOP software for calculating the measuring points from the
images, scale bars and additional equipment.
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Fig. 2: Photogrammetric system TRITOP of GOM mbH.
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Fig. 3: Preparation of the bow for photogrammetric measurement.
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Approximately 300 reference point markers (adhesive
paper with white points on a black background) were applied to
the boat in order to exactly define the shape of the hull (fig.
3). Then, several images from various directions were recorded
with the digital camera and loaded into the notebook PC. The TRITOP
software automatically identified all reference point markers
in the digital images with high accuracy.
After the reference point positions in all images
are defined, the system automatically allocates the individual
markers for the pre-orientation of the images and exactly calculates
the 3D position of the reference points. Scale bars, the gauge
length of which was ultra-precisely certified by an approved calibration
office, provide for proper scaling of the measurement. The TRITOP
computation results in numerous precise 3D measuring point coordinates
in a global coordinate system.
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Fig. 4: Reference points (red) and camera positions
(yellow) shown in the TRITOP software.
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Fig. 5: Result of the photogrammetric measurement
- the measuring points define the shape of the boat's
hull.
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Fig. 4 shows the position of the measuring points
on the boat's hull (red points) and the camera positions during
recording of the images (yellow). The yellow lines represent the
lines of sight from various camera positions to one measuring
point. The intersection of these lines of sight defines the point's
exact position in space. Such lines were determined for all measuring
points but are not all shown here for reasons of clarity. The
exact positions of the measuring points in space (coordinates
X, Y and Z) define the 3D shape of the boat. Thus, the dimensions
of the boat (e.g. its length fig. 5) are determined as well.
A software for surface reconstruction then defined mathematical
surfaces spanning between the measuring points. These surfaces
completely define the shape of the hull as CAD model.
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Fig. 6: Calculated CAD model
of the boat based on photogrammetric measurement.
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Fig. 6 shows the calculated 3D model of the boat
based on photogrammetrically measured reference points. Fig. 7
illustrates the characteristic lines of the hull (parallel sections
in all three planes).
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Fig.
7: Characteristic lines (parallel sections) of the boat's
hull. |
Now, it is easy to adapt the CAD model to customer-specific
modifications. Based on the CAD data and the sections lines, the
designer can easily and precisely define the ribs and other supporting
elements of the hull. Fig. 8 shows the shape of the ribs and the
position of the top hull edge. The green area (right side) shows
the bow ribs, the stern ribs are displayed in yellow. A CNC milling
machine may use the CAD data to directly cut the shape of the
ribs into the wood. For small lots, the rib shapes defined in
CAD are printed on a plotter, manually transferred to the wood
and sawn out.
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Fig. 8: Rib shape, based on the calculated boat
model.
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Measuring the hull using TRITOP is fast, flexible
and precise. The created mathematical model of the boat is a faithful
copy of the original. Customer wishes can easily and correctly
be implemented and optimal planning and execution of production
is possible.
We would like to thank the Nerezine shipyard
for their trust and for the permission to publish this report.
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