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Major news -115' Super Maxi
Building the fastest possible sailboats for oceanic record-breaking is the
newest and best game in town. Juan Kouyoumdjian explains the emergence of
his remarkable new design for Frank Pong.
The wake up call
When somebody comes to you and requests a lightweight, high performance
monohull capable of sailing offshore and achieving speeds sufficient to
break existing oceanic records, combining innovative solutions and without
the restriction of any rule or qualification society, a combined feeling of
excitement and fear runs through you. On the one hand the excitement of not
having to think about rules and regulations but simply speed. On the other,
fear for the size, loads and stresses that such a sailboat will have to
endure safely. This philosophy immediately drives the design process towards
engineering technology that can accurately and reliably account for such
stresses. The fundamental criteria is to combine and structure such
innovative solutions within a seaworthy, ocean-going sailboat, that allows
the crew to rely upon and also exploit the boat's capabilities in extreme
wind and sea states. Early discussions quickly help to identify the general
specifications desired by the client, and also to better understand the
influence in performance of both length and the ability to control sail
power by specific hardware. Simply put, above a certain length, loads become
so high that it is practically impossible to sail the boat to its full
potential - rather performance is limited by the useable power dictated by
the available hardware.
During these early evolutionary stages it also became evident that it is
necessary to divide the project into two distinct phases. The first consists
of defining the boundaries, i.e.: defining a box around the project that
represents the limits we believed we should not exceed. The second phase is
the construction, i.e.: scantlings, specifications and drawings that
accurately express what has been designed in order to ensure an accurate
construction. It is also pointless to concentrate so much on designing for
such a high performance and accuracy if one cannot communicate properly to
the builder what has been conceived. At the end of the day yacht design is
very much about the creation of accurate drawings (ask any good boat
builder), provided the good ideas are there in the first place. The
objectives of this first phase were: To define the general aspects of the
sailboat in terms of sail area, displacement, righting moment, materials to
be used, structures required and general specifications. To create a bidding
package that would allow candidate yards to quote an accurate price. To
simulate the possible performance of the boat as a function of different
performance oriented solutions such as canting mast, canting keel, etc. To
choose a building yard and formulate a fixed price contract agreement. What
turns out is that, in most cases, solutions that significantly improve the
boat's performance also help to reduce the loads and stresses imposed onto
the structure. Unfortunately, most of the subsequent solutions, and
canting/rotating masts in particular, are forbidden or restricted by ALL
current monohull racing rules.
The concept
The starting point was the engine, i.e.: the rig and sail plan. It had to be
such as to create enough power while only imposing loads within the limits
of standard deck hardware and the necessarily lightweight internal
structure. The combination of soft sails and a rotating wing mast allow us
to achieve a very wide range of power without requiring a massive rig
compression with a simple, non-spreader rig. Such a rig also allows the mast
to be able to cant to windward. By doing so the gains are two-fold: Improved
stability - by shifting the CofG to windward. Thrust - by turning forward
the lift component of the sails, i.e.: in similarly reducing heeling moment.
But the optimal arrangement of such a rig requires it to be attached to the
boat in such a way as to minimize the compression loads when canting - or
simply when upright. This obviously means a wide shroud base. Within a
monohull concept, this translates into a wide boat, which in turn increases
the hydrodynamic drag, so reducing the speed potential It therefore became
apparent that to go forward we must combine a narrow hull with a wide shroud
base, therefore we find ourselves with giant wings. Once the wings have been
acknowledged, the next step, of filling them with water to further increase
the righting moment, becomes self-evident.
Going fast - Power vs. Resistance
The speed range of this sailboat is so large that it requires radically new
blends of power and resistance. Since the boat will never sail 'downwind',
due to its speed (the apparent wind angle during sailing will range from 17
deg upwind to a maximum of 92 deg downwind) the sail combinations go from a
maximum power full mainsail and gennaker to a 3-reef mainsail with staysail.
These aerodynamic combinations have to be properly balanced under the water
throughout the speed range. Here it gets tricky, as we ask the same
underwater configuration to work in equilibrium for a full mainsail and
gennaker at 10deg heel and 12 to 15 knots boatspeed, and also with a keel
canted 30 deg, a boat heeled 20 deg and a 3-reef mainsail and staysail.
Although a combination of canting keel and asymmetric daggerboards has been
analysed, such solution is left for future developments of the boat.
Instead, a more standard configuration of a lifting keel and rudder will see
the day initially. We are lucky to be able to use CFD extensively inside the
office to quantify correctly the lift required by the appendages and the
distribution of such lift to minimize drag. This has a direct influence in
the final performance as to achieve a good balance forces to increase drag.
We were interested in quantifying the influence in drag of reducing the
distance between the bulb and the free surface. This is true in every boat
when heeling but even more so for canting keels. We studied a bulb without
fin or even hull, at four different depths : 2, 4 and 6 meters and infinite
depth (i.e. without free surface). The bulb is 4.6 meters long and has a
maximum diameter of 83 cm. The speed of the runs is 15 knots, which means a
Reynolds Number of about 30.106 (based on the bulb length). The model for
the free surface is 25 meters long for 15 meters large and the calculated
drag is a sum of a pressure drag, a friction drag and a free surface
interaction drag (unless for the infinite depth case). We assume that the
friction drag is the same for all cases. So we can consider that the
interaction drag due to the presence of the free surface is the difference
between the total drag in the infinite depth case and the total drag with
the free surface.
Pressure Repartition along the bulb (No angle of attack)
So we can note that this interaction drag can be important. It can reach
more than 10 % of the total drag for depths less than 2 meters.
Nevertheless, we must absolutely avoid interpolating this curve for depths
less than 2 meters, more especially as this kind of situation will probably
never happened. Finally, we can note the asymptotic behavior with the amount
of depth. The interaction drag reaches less than 2 % of the total drag under
about 5 meters deep immersion.
Visualization of the interaction between the bulb and the free surface (Cp
picture) The hole just behind the bulb's wake has a 9 cm amplitude Then we
concentrated on the hull shape. Due to the slenderness of the hull and the
speeds she will sail at, the principal aspect of resistance is wave drag.
This is the drag due to deforming the free surface of the water when going
through it. Volume distribution along the boatlength is the key here, so we
analysed different hull shapes, even including chines at the back and front
which did not pay off due to the high heel angles in comparison to a skiff
dinghy. Another interesting thing coming out of the CFD computations is the
difference in wave between low speed and high speed. This implies a hull to
behave very differently depending on which end of the speed range. This
dynamic behaviour is very important to understand as it influences tha
capacity of a hull to stay at high speeds longer. The key is not a hull that
peaks at 35 knots sometimes but one that steadily holds 30 knots. The shape
of the resistance curve shows this and the goal here is to make such curve
as flat as possible in the planning regimes. An example of resistance curve
is shown in the graph. This curve is comparable to that one of an IMS 50!!!
Holding everything together: the structure Since the beginning, it was
obvious to me that construction would need to be in composite as weight has
such an overriding bearing on performance. I was however sufficiently
interested in a lightweight aluminium solution to also pursue a comparison
for weight and cost. It was therefore decided to analyse two models, one in
aluminium and the other in carbon-Nomex composite, of the same hull and deck
geometry, and targeted displacement. This was calculated from an estimation
of rig, deck and interior hardware and a performance calculation (VPP) as
per the desired target speeds, which are obviously related to the weight.
Both models were loaded the same way, ie: same rigging and hydrostatic
loads, and then scantlings were designed for an equivalent deformation, or
at least we tried to as it fast became evident that the aluminium structure
rapidly gained in weight to achieve the same deformations. Doing such a task
takes forever, not to mention that in order to compare the results
correctly, all of the geometries have to be equally treated and the
mathematics behind each analysis compatible. We used Pro/Engineer and
Pro/Mechanica software to model the geometry and analyse both the aluminium
and composite solutions under the same loading case. Reassuringly, the
results converged very quickly, due the good compatibility between both
programs. Common sense is important at every design stage, so for the
aluminium solution the hull and deck plating was fixed at 4mm thickness in
order to allow good welding without distorting the panels. Then it became a
question of finding the most lightweight internal structure accounting for
the keel and mast attachment. It did not take long to find out what
experienced aluminium builders have known for years, that we would require a
lot of internal framing in order to be able to create hull and deck panels
as a function of their local stresses and deformation. The next step was to
minimise the weight of such internal framing by minimising their size and
welding and making lightening holes at the least stressed places without
over stressing the adjacent area. Finite Element Analysis (FEA) helps
enormously here, as results clearly show the stress and strain paths in the
analysed structure. As the process is very graphical, common sense and trial
and error is, however, very important, so this optimisation process remains
very time consuming. The problem was that the weight calculation of such
optimised structure in aluminium soon exceed the targeted displacement for
the geometry used, with minimum weight around 13 tonnes for the hull, deck
and structure. At this point we had no comparison, so I decided to switch to
the composite solution to come up with an initial comparison and more
importantly, evaluate whether the targeted displacement was realistic enough
to pursue. In the same manner as for the aluminium, an initial thickness of
the core and skins for the hull and deck were defined. Our general Americaís
Cup experience and testing over the last two IACC campaigns were of great
help here, not only providing a very accurate starting point but also
enabling us to input real numbers into the FEA model, as most tests had been
done on cured pre-preg samples and not only on the fibres. Although a
completely different philosophy, the composite optimisation was done
similarly to the aluminium one. Starting from fixing the keel, rigging and
mast and then engineering the rest of the structure spreading the loads
evenly and reinforcing for local deformations. Using our IACC data as a
start point, the initial results for our new boat were incredible, with a
hull weight very close to 2,000kg, a deck of 1,350kg and the internal
structure at less than 500kg. This gives a total composite weight for the
primary structure of under 4,000kg. For the same deformation under constant
static loading, this is less than one third the weight of our best aluminium
solution. These results called for a different aluminium philosophy
otherwise there is no point in even considering it. We therefore came up
with what we called an aluminium-sandwich mix. This consists on using
aluminium honeycomb, of similar properties to the Nomex, combined with thin
aluminium skins rather than carbon. The aluminium skins are not welded but
glued. They are 'laid'up on the hull plug as two plies of 1mm thickness per
skin at +/- 45* in the same way as pre-preg. Interesting though it was,
economics dictated that we stopped looking at this solution very quickly, as
we realised that to build properly would require at least the same amount of
hours and cost as the composite solution. At this stage we had clearly
validated the construction to be composite, and we also had a good idea of
the laminates of the hull, deck and internal structure required for a static
loading case. Now we needed to study further the local reinforcements,
internal structure and how the individual components (hull, deck, wings and
structure) were going to be built and assembled together.
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