Precisely engineered CNC-cut metal kits simplify custom and

Tab A Into Slot B
Precisely engineered CNC-cut metal kits simplify custom
and production projects at aluminum sailing-yacht
builder K&M and steel-motoryacht builder Jetten, both
in The Netherlands.
Text and photographs
by Hans Buitelaar
Above—A 32.8' (10m) aluminum rescue
vessel in build at K&M Yachtbuilders
(Makkum, The Netherlands). Longi­
tudinal and transverse frames designed
and engineered to close tolerances and
cut by a computer numerically controlled
(CNC) laser cutter slide together
precisely and easily. This build method
enables the structural framework to be
erected and the hull shape verified
before any components are welded.
H
istorically, metal-boatbuilding
shops received raw plate or
rough-cut components directly from
their metal suppliers. A crew of
skilled builders cut and shaped the
metal to match patterns scaled off
the naval architects’ plans, and eventually welded those components to
create the finished hull shape. That
was then.
Today, at K&M Yachtbuilders, in
Makkum, The Netherlands (see the
sidebar on page 48), metal components delivered to the construction
hall are far more refined: Precut
stringers are the precise shape
required to yield the finished hull,
and they arrive from the computer
numerically controlled (CNC) laser
cutting facility with an intricate pattern of notches cut halfway through
their depth to interlock with the transverse frames, which in turn have a
corresponding pattern of narrow cuts
around their outer curves. These precut structural members with cutouts,
coupled with precut hull plating,
assure a stronger hull than a conventional build, and require fewer highly
skilled builders and much shorter production time on the shop floor.
About the same time carefully engineered CNC-cut metal parts for hull
construction were introduced at K&M
in 2007, Jetten Yachting and Vripack
Naval Architects (both of Sneek, The
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A yard worker at K&M confirms that the
numbered parts for the next section of
the Bestevaer 53ST (16.2m) are in the
correct order after they arrive from
the metal-cutting contractor Snijtech, in
Joure. Ideally, crews can then start building the section by picking parts from the
top of the pile.
Netherlands) joined a governmentfunded innovation project to develop
so-called lean manufacturing methods
for the boating industry (production
that minimizes inefficiencies in the
workforce and maximizes the return
on expensive equipment and shop
space). Twenty-five companies,
including boatyards, software developers, naval architects, and research
institutes, took part, helping spur the
development of precisely engineered
CNC-cut aluminum and steel kits for
boats. Building with kits saves much
of the expert but time-consuming
manual work, because engineered
Jetten Yachting
B
rothers Marcel and Sander Jetten
started their yard in the town of
Sneek in 2000. In the middle of the
Frisian lakes, the Royal Yacht Club
Sneek hosts legendary open-boat
regattas and is one of The Nether­
lands’ centers of powerboat cruising.
The brothers had built yachts at
several yards before deciding that
by combining their skills they could
do a better job than their respective
employers. The Jettens began building cabin cruisers under the brand
name Bully, producing more than a
hundred in lengths from 31.2' to 45.9'
(9.5m to 14m). Realizing that the
word Bully did not have positive
connotations in all languages, they
changed their brand to the yard name
Jetten to appeal to their growing
parts come precut from CNC-laser, or
CNC-plasma cutting machines and fit
together only one way—on basically
the same principle as simple wood or
plastic construction kits for model airplanes, cars, and boats. If a precut part
doesn’t fit, it’s in the wrong place, and
the builder must find the right part.
Sometimes it’s as easy as turning a
seemingly symmetrical part around.
Jetten Yachting (see the sidebar
below) now builds all its boats with
Vripack’s branded Smart Kit engineeringand-production system. CEO Marcel
Jetten says, “In the traditional construction method, it would happen
From metal cutting to
launching, the Gamma
20 (73'/22.1m) steel
motoryacht, built by
Jetten Yachting (Sneek,
The Netherlands) for
the Italian yacht company Gamma, was
completed in just eight
months thanks in large
part to Smart Kit technology from Dutch
design-and-engineering
firm Vripack.
international clientele.
Today, steel motoryachts from 30'
to 70' (9.1m to 21.3m) are Jetten
Yachting’s standard builds, with
more than 100 boats produced. At
the yard in December 2013, the next
Gamma 20 (73' /22.1m) for the Italian
yacht company was in advanced
stages of construction.
—Hans Buitelaar
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The first plate of the hull skin is fitted to
the rescue boat at the K&M yard. The
precision-cut panels need no adjustment
or resizing to fit the underlying structural
members and consequently yield a hull
that requires little or no fairing.
that the shop-floor workers would
weld a part just a fraction aside the
marking line. Or sometimes the marking lines themselves were a bit away
from their drawn position. With Smart
Kit, the only possible way to build a
yacht is to follow the way the engineers planned it; otherwise it just
won’t fit. We have installed a DXF
viewer on the shop floor; the computer program shows the 3D model.
Any part number typed in will light
up on the screen, so we know where
the part goes in the boat. Sometimes
workers may be uncertain about
where parts of similar shape might
belong in the different sections of the
boat.”
Kits at K&M
In K&M’s kit system, CNC-cut
notched metal cutouts slide over and
mate with crossing components. With
each new precut piece fitted but not
bonded, the framework becomes
stronger and more stable. If the
demanding design work has been
done well, when all components are
in place every frame should be perfectly vertical, with virtually no movement in the structure. Welding can
then begin with confidence that no
adjustments or further cutting will be
needed.
“We started assembling precut
metal parts into boats with this construction method some seven years
ago,” says Klaas Kuperus, foreman at
K&M. “At first, we had to learn to
trust the engineers who had anticipated in their 3D design how everything would fit together perfectly. As
aluminum boatbuilders, we were used
to receiving a load of roughly cut
metal parts that we would adjust, fit,
and weld together on the spot.
Following a design plan, we were the
skilled craftsmen who could shape a
three-dimensional boat from a piece
of paper and a load of metal plates.
We still do that, but with the new cutmetal kits, we actually only need to
put them together, and the threedimensional form will take shape as
intended by the yacht designers and
naval architects who engineered the
metal-cutting pattern.
“It happened with the first kits that
at some point in the assembly the
wrong part was placed,” Kuperus
recalls. “From there on, every new
crossbeam would be a little more out
of vertical level than the one before.
So the builders started adjusting parts
to correct this—just like they used to
do. They were perplexed and at that
K&M Yachtbuilders
E
euwe Kooi, manager at K&M,
started building aluminum sailing yachts in the Dutch town of
Makkum in 1999. At the Frisian
shore, near the locks through the
Afsluitdijk (Enclosure Dike), which
separates the salty, tidal Waddenzee
from the freshwater IJsselmeer, the
yard has built a reputation for rugged long-distance cruising sailing
yachts of quality finish. Vessels like
the Stadtship 70 (68.9'/21m) by Van
de Stadt Design, the Axonite 69
(68.6' /20.9m) by Guido de Groot
Design, and the series of Bestevaer
sailing yachts from Dykstra Naval
Architects define the yard’s offerings. In early December 2013, construction started on the first MKII
Bestevaer 53ST (16.2m), inspired by
Gerard Dykstra’s own yacht. The
build was well under way during
my yard visit December 17.
Workers were nearly finished welding the structural members and
were preparing hull plating for
installation.
—H.B.
Naval architect Gerard Dykstra’s yacht, pictured here, inspired the Bestevaer
53ST, in build at K&M. A relatively fair hull right out of the construction hall is
particularly important for an unpainted aluminum boat that will not be touched
up with fairing compound and finish paint.
48 Professional BoatBuilder
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Marcel Jetten shows
how wooden panels
for the interior of the
Gamma 20 are also
CNC-cut and fit
together by sliding
slots over intersecting
parts.
point not quite convinced that this
new construction method was an
improvement.”
The engineers saw immediately that
parts had been incorrectly placed and
then unsuccessfully adjusted to make
them match the plans. When the misaligned structure was taken apart and
rebuilt with all parts in their proper
places, it fit perfectly and everything
was straight. It was a eureka moment
for the yard crew.
Kuperus, the son of a well-known
builder of Dutch sailing barges, has
been building boats since he was a
child. He acknowledges that with
adoption of the CNC-cut kits, boatbuilding is evolving from craftsmanship into a more industrial assembly
process.
“I like this new way of building
boats,” he says. Nevertheless, “I must
admit with the old method the welders on the shop floor had a very big
part in making the hull what it
should be. With the cleverly engineered metal parts that fit together to
become a firm structure without the
workers manipulating any part, construction follows the procedure
planned by the engineers. Still, it is a
fascinating process to see a yacht
take shape following the arrival of a
pile of metal parts. And we can build
much faster now.”
Key to the straightforward assembly of the metal parts is having them
arranged in the
sequence they will
be needed in construction, so the
builders are able to
pick up the next
part from the top of
the pile. In reality,
the kits are not always that tightly
planned, but at least the parts are
grouped by section, so the crew
knows where to look for the components in the section they’re working
on. In addition, this work routine
can accelerate the build even more,
as the first sections of the boat can
be assembled while other sections
are still being cut.
Another advantage of kitted construction is the ease of creating a
smooth hull skin. Kuperus: “As most
of our builds are aluminum sailboats,
a lot of owners like to leave the hull
unpainted. It gives a rugged look
and saves painting and repainting
many times during the boat’s life
span. This preference poses a challenge to us boatbuilders. We cannot
use fairing compound to even out
the inevitable bumps or dents in the
hull skin, so we carefully smooth out
the aluminum plating using a hammer after the yacht is finished. When
we build with metal kits, the hull
comes out smooth with a lot less
finishing work.”
Eeuwe Kooi, manager at K&M,
strongly believes in the benefits of kitting these components. “It eliminates
errors,” he says. “Every part will only
fit in one place. Yard workers know
that if it doesn’t fit, the part at hand
should fit somewhere else, and they
have to look on the shelf to find the
part that does fit. Although we have
the kind of skilled workers who can
build a yacht from a flat drawing on
their own knowledge, it saves time
and material if they can just work
steadily without having to think of
solutions to make non-fitting parts fit.
On top of that, the quality of the hulls
and the smoothness of the finish are
much better, thanks to refined CNC
cutting. And as we offer series of boats,
semicustom built, we really have a big
cost advantage when a boat of the
same design is ordered for a second
and third time. On such occasions
we already have the complete cutting
pattern and can start construction
immediately. A 50' [15.2m] boat can
be finished within months.”
In The Netherlands, as in most of
Europe, yards suffer from a shortage
of skilled construction workers, and
every year it’s harder to find metalworkers who can build a smooth hull.
That, Kooi admits, is also a compelling
reason to invest in foolproof engineering. It allows welders and yard
professionals with less experience to
Intricate curved T-section hull-to-deck
reinforcements were assembled before
fitting them to the Bestevaer hull.
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build a strong, lightweight hull exactly
to specification in a predictably short
time.
But it’s not all about savings in
build time and costs. “Especially in the
case of one-offs, the whole process of
design, engineering, and construction
does not in all aspects become
cheaper,” Kooi says. “We gain time
on the shop floor, but we have to
invest more in engineering and laser
cutting. For the typical boat we build
here, we would spend some 80 to 100
hours of engineering to make a traditional cutting plan. Work­ing out a
refined kit cutting plan, we invest 250
hours of engineering.”
The upside is that no material is
lost when the process is being refined
on the computer; the downside is that
trained engineers and designers are
paid higher hourly rates than most
construction workers.
“Fellow yard managers often ask
me, ‘Eeuwe, why do you invest so
much in these cutting plans? You
have the yard workers that can build
a hull from drawings. Why don’t you
let them go with their experience?
You put a lot of money in the engineering and take the work out of
experienced people’s hands.’ Well,
my answer is that I see progress in
the quality of our boats. We can build
more of them to more competitive
prices. Even though third parties can
present us a load of traditionally cut
plates and we will be able to build a
yacht from it, all our own boats are
now built from refined CNC-cutting
plans. I want to make this step toward
a more industrial approach because
in the end, if we can build boats at
better prices, we are able to sell more
of them.”
Boat designs come to K&M from
offices such as Dykstra Naval
Architects (see “Sailing Master,”
Professional BoatBuilder No. 89, and
“Dykstra Revisited,” PBB No. 141) or
Van de Stadt Design. “They provide
the lines and look of a yacht,” Kooi
says, “and, of course, the required
structural strength at critical points as
well as estimated weight of the hull,
keel, and rig. After the engineering
office has translated the design into
an aluminum-cutting plan, we send
this back to the initial designers for
them to confirm the resulting
strengths that should be achieved in
our construction: height of frames,
distance between longitudinal stringers and crossbeams. Even with our
experience we think it is good to have
the naval architects cross-check it.”
Jetten and Vripack
“We had a lot of demand for the
kind of quality-built steel motoryachts
we build here,” Marcel Jetten recalls
of the booming market in 2007. “To
maintain the competitive edge with
our national industry, ways were
sought to improve on building speed,
while maintaining the high quality
standards. That is why the innovation
program was started [the governmentfunded project mentioned earlier;
Vripack’s Smart Kit was one outcome—Ed.]. We saw increasing international interest in our yachts at that
time, with wealthy owners willing to
order ever-bigger yachts but not willing
to wait for delivery maybe two years.
This was our reason to join.”
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boats built by the Smart Kit method
was a 40' (12.2m) steel-hulled cruising
yacht. The yard had built hulls to this
design previously but always by tradi­
tional methods. The same boat built
from a Smart Kit was finished in just
eight months, cutting build time in half.
Soon after that Smart Kit build, the
Italian motoryacht company Gamma
Yachts approached Vripack to design a
73' (22.1m) steel luxury yacht. The
Italians chose a Dutch yard, as steel
motoryachts from The Netherlands
enjoy a reputation for being well built,
and Gamma also required fast delivery.
“That is why Vripack directed Gamma
and their client to us, in January 2010,”
Jetten says. “We had just successfully
finished the pilot. We were now ready
to prove the concept for a third party,
with a client that was anxious to get
delivery as soon as possible.”
This second Smart Kit build evolved
into a model project. The yacht was
also finished in just eight months,
launching in Sneek in August 2010.
Since then, Vripack has provided
Smart Kit services to many boatbuilders
and shipbuilders, creating designs and
In Vripack’s Smart Kit, parts
for the Gamma 20 hull
under construction at
Jetten Yachting are indelibly numbered. In addition
to the assembly order of
structural metal parts,
installation of systems and
equipment is engineered
and specified beforehand.
improving the engineer­
ing for its ready-to-build
packages.
Vripack’s commercial
manager, Marnix Hoekstra,
points out other advantages
of Smart Kit engineering.
“Yachts become lighter, as
no extra parts are made
on the yard floor to even
out fluctuations in the hull construc­
tion,” he said. In traditional metal
boatbuilding, welders often have
extra pieces fitted to ensure strength.
Smart Kits eliminate this extra metal.
Hoekstra continues, “It thereby saves
lots of metal parts that need to be
protected against corrosion and that
need to be welded in place; it
saves working hours and thus saves
the yard a lot of money. An extra
benefit—one I must admit we did
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not at first foresee—is that hulls
constructed with Smart Kits have
extra sound-dampening qualities. A
vessel will be quieter if assembled
this way. This saves on extensive
sound isolation.”
In Build
K&M crews working as if it made no
difference which construction method
they used. They just worked fast.
From a shop-floor perspective, the
two methods are very similar. One
difference mentioned by Kuperus is
that while in both methods engineers
design slotted metal plates that slide
together and lock at their intersections, Vripack’s Smart Kit employs an
additional pin-in-hole assembly method
to key parts to one another.
Another advantage made possible
by the integration of 3D design
software with precise laser cutting
and sharing design drawings over
My visit to K&M in December 2013
was a great opportunity to compare
the two types of kits in the two new
builds rapidly progressing there. One,
a Dykstra-designed Bestevaer 53ST
(16.2m) sailing yacht, was being built
with the K&M-developed CNC-cutting
and assembly process. The other, a
32.8' (10m) aluminum rescue boat for
a merchant vessel, was designed by
Vripack as a Smart-Kit build. In the
production hall the two aluminum
boats stood side by side with the
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computer networks is the ease of outsourcing or subcontracting components, systems, or whole sections of
the boat. Thanks to digital design,
suppliers such as interior builders,
equipment suppliers, and electronics
installers can preassemble their components to close tolerances with confidence in the fit (see the sidebar
“The Cutter”). Later, when equipment
is installed, the benefits of a kit engineered to tight tolerances are obvious:
cutouts in crossbeams and structures
allow flawless placement of cables,
pipes, and hoses.
CNC laser cutting of steel and aluminum, with margins of 0.1mm over
large pieces of metal, allows for tight
assembly, so Jetten can build different
sections of the yacht at different sites
and assemble them later. Such is the
case of the Gamma 20 hull, under
construction in its assembly hall during my visit.
“The hull and other parts of the
yacht were built at three different
locations,” Marcel Jetten says. “We
have seven different modules: the hull
itself, sections of the motor mount
Building with kitted CNC-cut components is so efficient that just two weeks after
construction began, the hull of this Bestevaer at K&M is ready for plating, which has
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The Cutter
V
ripack, Jetten Yachting, and K&M Yachtbuilders all
go to the Frisian company Snijtech, in Joure, for laser
cutting. Founded in 1995 this full-service metal-cutting
facility was early to invest in computerized laser cutters.
Today it provides engineering, and CNC-laser, CNCplasma, and CNC-waterjet cutting of kits for components
in aluminum, steel, and stainless steel.
“We get involved with our clients in the yacht-building
industry,” says sales director Peter Aukema. “With our
experience in metal cutting and marking the different
parts, we try to advise yacht builders on efficient ways to
get their processes organized. We are one in a chain of
companies that are all involved in the making of a yacht.
If all of these companies work closely together on a
project basis, a lot of overhead cost may be eliminated. Even if it is just by arranging the numbered
parts or organizing transport directly to the different
locations where parts of the yacht are being built.”
Eeuwe Kooi, manager at K&M, agrees. “We try to
do all the steps of the process at the work floor
that is best laid out for that specific task,” he says.
“So the deburring of the parts after cutting can be
done most efficiently at Snijtech. We also ask them
to do the arranging of the different parts by number, so we get them at our shop floor in the correct
order. It just saves us all time to do it like that.”
—H.B.
Clearly labeled and sequentially stacked aluminum
parts precut at Snijtec are ready for assembly on the
shop floor at K&M.
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The refined engineering necessary for effective kitting of metal
hulls also allows larger intricate components to be prefabricated with certainty that they will fit the hull as planned. That
includes elements as large as the keel framework for the
Bestevaer (right).
and the engineroom, the tank section
in the yacht, the cabin, stern sections,
the staircases from the flybridge to the
aft deck, and the flybridge itself.
When starting construction at different
locations we try to choose locations
close to each other. We also carefully
think, before starting production of
the different sections, about how we
are going to transport them to the
assembly hall. The bigger sections
like the main part of the hull and the
flybridge we therefore build at our
main facility.”
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As with any new technology there
have been instances when perceived
advantages eluded builders, and others
where unintended advantages emerged.
“We have gone through some learning with this type of pre-engineered
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assembly,” Jetten says. “With the first
kits we were engineering too much. For
example, we had the laser cutter carve
a piece of steel to be formed into a joint
between deck and hull plating. This
piece was then placed in a folding
brake. Following all these actions we
had the same thing we could have
bought from stock in the metal shop: a
steel angle profile. In following versions of the Smart Kit we eliminated
this bit of over-engineering.
Accurate design files also facilitate outsourcing large structures such as this
staircase from the aft deck up to the
flying bridge on the Gamma 20.
Built to spec by another shop, it fit
perfectly into the time-sensitive build at
Jetten Yachting.
“After the first five boats built we
have come to the point where Vripack
engineers, in co-operation with our
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shop-floor professionals, work out
about 10% fewer parts, as we can easily use standard parts at these places
in the boats. Other aspects of thorough engineering save time. The
metal-cutting machine can make holes
for fittings like the cabin window
frames a lot faster and more precisely
than any yard worker with a drill
could do it.”
Another cost savings of Smart Kits
for Jetten is that not all plates must be
cut by laser. Laser metal cutting allows
no more than 0.1mm deviation from
computer drawings, but less expensive
plasma cutting is now as precise as
0.5mm to 0.8mm maximum deviation.
“So we have the main construction
of transverse frames and stringers
laser-cut, but the hull plating plasmacut,” Jetten says. “The process of optimizing cost and build time for our
yachts continues. The Smart Kit gets
smarter all the time, while we also try
to improve on yard logistics and cost
efficiency. In the end, motoryachts are
luxury items that not everyone can
afford. The lower the cost of our
yachts, the bigger our market. Short
delivery times can make the difference
if another yard might be able to offer a
yacht with dimensions, performance,
and finish comparable to ours. If we
have the same yacht built within eight
months, while a year and a half is the
common building time, we hope to
get the order.”
About the Author: Hans Buitelaar is
a freelance marine journalist specializing in the Dutch and North European
boatbuilding industry.
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