**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.
T**he 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. |