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55
NOVEMBER 2015
TECHNOLOGY
In particular, this methodology is aimed at the various
phases of the product development cycle, proposing
changes and innovation at various levels: starting from
CAD project drawings, to operations in fi ve axis CNC
work centres and, fi nally, using the Raked Strip Planking
technique in construction.
The project
The project stems from the need to relaunch the wooden
sailing boat sector addressing a larger slice of potential
buyers, thanks to lower costs and production times. In fact,
the yacht sector is suffering, especially in Italy, a signifi cant
downturn. One of the reasons is linked to a need for innovation
Massimo Mele, Cristiano Fragassa, Ilaria Broussard
INDUSTRIALISING A WOODEN BOAT
THIS ARTICLE DESCRIBES AND ILLUSTRATES A NEW DESIGN PROCEDURE AIMED
AT THE CONCEPTUAL AND CONSTRUCTIONAL DEVELOPMENT OF MODULAR
COMPONENTS WITH SEVERAL PARAMETERS THAT ARE PARTICULARLY SUITABLE
FOR BEING USED, IN AN EFFICIENT AND FLEXIBLE WAY, TO BUILD WOODEN BOATS.
in production processes, which are too costly and do not yet use
modern design techniques. Wood was chosen as the material
not just because of its aesthetic peculiarities and mechanical
characteristics, but above all for its eco-sustainability, a key
element in the modern approach to technological innovation
where it is the market that counts. At the same time, procedural
innovations can bring greater benefi ts to construction in wood,
which is still based prevalently on manual work. In general terms,
the design and development of a boat that will be appreciated
by the market demand the ability to combine high-performance,
low costs and maximum eco-sustainability, both as regards the
end product and the processes used to build it. The relaunch
of wood as a material for boat construction makes it possible to
I
56
NOVEMBER 2015
TECHNOLOGY
guarantee maximum eco-sustainability of the product, thanks
to its biocompatibility and complete recycling at the end of its
useful life, a problem which is still a dramatic one for those
in composites. Also as regards performance, wood offers
several advantages, thanks above all to its excellent mechanical
properties such as resistance, reliability, ease of working and
pleasing aesthetics. However, to use wood in a more correct way
it is necessary to change yacht design and production methods,
so as to make it possible to compete, also in the phase of
industrialisation, with “poor” materials such, for example,
as fi breglass.
This article describes the integrated use of assisted design tools
used to guide the production, through numerical control, of the
entire structural part of a wooden boat. In particular, it describes
the efforts that were needed to obtain great simplifi cation and
modularisation of the components, aimed at making the cutting
and assembly of components competitive.
The logic was to steer design towards components that could
be mass produced, rather than shaped by the patient art of
master carpenters. The result was an interesting experience of
complete engineering and industrialisation of a wooden boat that
was innovative, fl exible and eco-sustainable, suitable for modern
yachting, and that lays the foundations for making a wooden boat
from a “made-to-measure kit” (fi gure 1).
The phases
The research phases can be subdivided into three steps:
the design phase: development of a conceptual and operational
methodology for 3-D CAD design of the components (in this case,
the structural components of the boat) with several parameters
that make it possible to modify them in size and proportions in a
quick and fl exible way.
machining phase: the implementation of innovative machining
techniques and dedicated control algorithms for the use of
CNC machine tools in cutting, modelling and fi nishing these
components with an industrial process logic (for example
geometries developed on partly fi nished 2D components)
construction phase: development of “innovative carpentry”
solutions to make it possible to replicate construction procedures,
limiting human intervention to hold down costs and production
times while maintaining the properties, performance and
technical qualities of the boat.
Innovation, fl exibility and eco-sustainability thus become
the characteristics of a product aimed at a broader slice
of purchasers, thanks to the reduction of costs due to
industrialisation processes.
Figura 1: CAD 3-D modelling during the design phase. Figura 2: Point cloud acquired by laser scansion.
Figura 3: The polygonal mesh.
Figura 4: Points of intersection of water lines and ordinates.
57
NOVEMBER 2015
The boat design
Traditionally, a hull is designed using the naval construction plan,
with three views obtained by sectioning the hull with three families
of planes parallel to the principal ones, the horizontal, vertical and
transverse planes. Thus three families of curves are identified
for each plane, called the water lines, ordinates and longitudinal
curves. The latter are used to verify the contemporaneous
shaping of the water lines and the ordinates. This kind of
procedure, apart from obviously demanding experience from the
designer, takes a very long time. The proposed methodology can
be applied both to a construction plan derived from hydrostatic
and hydrodynamic calculations, with the relevant offset table,
and to the reproduction of an existing vessel, built in wood
or other material, reading the external geometry with a laser
scanner. Reverse engineering offers important advantages in
terms of quality, time and costs if entire boats or parts of them
are scanned. In particular, in the yachting sector, its performance
makes it widely used in reconstructing surface models starting
from an existing boat, and then generating precise 3-D drawings
to use in the design phase. The boat chosen for implementation
of the methodology is a sailing boat about 10 m overall; obviously
Figura 5: Curvature analysis.
Figura 6: 3-D hull model.
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NOVEMBER 2015
TECHNOLOGY
it is possible to construct several standard models for boats of
different lengths and with a defi ned number of planes.
The result of the scanning of the geometry is a “point cloud”
(fi gure 2) which then has the noise, due basically to the
divergences of the laser beam and the standard deviation of
the instrument, removed. Then triangulation is carried out and
a polygonal mesh constructed (fi gure 3), correcting anomalies,
followed by 3-D modelling. At this point, the 3-D model obtained
in this way can be subdivided using the water lines (horizontal)
and the ordinates plane (vertical), in line with the traditional
concept of design. The points of intersection of the curves lying
on these planes are those that constitute the offset table (fi gure
4). Thanks to CAD software, we now have a model of the hull
where we can perfect the design in accordance with project
specifi cations. The most evolved modelling software also gives
the user immediate control of the geometric characteristics of the
curves (surfaces) used in the design, such as, for example, the
curvature of the lines of the hull which play a fundamental part in
its hydrodynamic behaviour.
Curvature analysis
Most CAD modelling software for boat design uses NURBS
(NonUniform Rational B-Splines) to defi ne curves and surfaces,
which adapt well to describing a free form like the hull of a boat.
The NURBS, from a mathematical point of view, derived directly
from the B-splines, which are polynomial curves of a given
degree, but cannot describe curves such as circles or ellipses
which need to be represented using rational functions.
Let us look briefl y at the analytic formulation of B-spline curves:
p(u) =(u)P
1
i=0
n
∑
For a given set of n+1 control points P we can defi ne a curve p(u)
as a linear combination of these and of certain functions which
are called mixing function M, with suitable degree in continuity.
These functions have the important property of local support:
they are different from zero only in a certain sub-interval, the
extension of which depends on their degree. Moving a control
point infl uences only a part of the curve and leaves the rest
unaltered.
The B-splines permit intuitive control of the curve; in fact it is not
diffi cult to get a general idea of the shape of the curve simply by
looking at the control polygon.
A NURBS curve is defi ned by the following equation:
p(u) =
wi
i=0
n
∑Mi(u)P
i
wi
Mi(u)
i=0
n
∑
Where the only difference compared to (1) is due to the
presence of the weights w. If these are all set equal to 1, the two
formulations coincide. To modify a portion of the curve it can
be suffi cient to act on the weights of the nodes controlling it.
The effect of this modifi cation is to move the curve towards the
control point (if the weight increases). Note that the weights have
an effect similar to the many control points in Bezier curves, but
unlike these they give greater fl exibility because their degree is
separate from the number of control points. We can thus use
Figura 9: 3-D detail of the fi nal design of the boat.
Figura 7: Planking construction curves. Figura 8: Curved plank, left, and fl at plank, right.
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59
NOVEMBER 2015
curves of lower degrees and still have several control points. In
addition, it is possible to change the position of the control point
without changing the shape of the overall curve.
Equivalent relationships can be obtained for the surfaces:
p(u,v) =
wi
Mi(u)w j(v)P
i,j
j=0
nv
∑
i=0
nu
∑
wiMi(u)wj
Mj(v)
j=0
nv
∑
i=0
nu
∑
Analysis of the curvature of the water lines in the ordinates is
carried out using the control points, correcting any imperfections,
such as excessive variation of the radius or the flexion points.
In our case, since we had laser scansion, it was sufficient to
(figure 5), but it is possible to use iterative calculation using the
behaviour modelling functions that can automatically correct the
points so as to obtain the desired curve, with considerable time
saving.
Planking design
On the general surface of the hull (figure 6) are the construction
curves of the planking and frames. (Figure 7). The planking
was modelled on developable surfaces to obtain the flat
development of the planks. This function makes it possible to
obtain automatically the geometries of the planks to send to the
CNC work centre, which will carry out shaping, produce the
male and female joints on the long side and the comb milling for
the joining of adjacent planks. Then the planks of the chosen
thickness were subdivided into the required number. The
following images (figure 8) show the curved planking, modelled
on the surface of the whole, and the flat plank. The shapes, and
so the ribs (or ordinates) that make up the skeleton of the boat
were modelled so as to have the correct shape for assembling
the planking. To associate the geometries of the part with the
work that the numerically controlled machine must carry out on
it, it is necessary to adapt the structure of the design to precise
specifications on the basis of the CAM software used: in fact,
according to the decodification system the software uses, it is
necessary to follow specific rules in which sequences of letters,
symbols and numbers have a preestablished significance. For
example, the creation of the geometries must be conventional
and adapted to the characteristics of the machining and tool path
so as to take into account the direction of the tool on the open
and close geometries, the use of tool compensation and any
geometries that cannot be imported into the CAM software used.
So positions, references and fields are fundamental in the final
design. This means establishing coordination between the design
phase and the manufacturing phases, which means the designer
needs to know the characteristics of the machine tool. The final
3-D project, where in addition to planking and ordinates the water
planes are also shown (figure 9).
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