n. 45 - Spring 2015 - Euro 8,00 - US $ 16,50 - £ 7,30
international
DOUBLE ISSUE
WITH:
Design
TECHNICAL
DSS A MOBILE FOIL
THAT COUNTERACTS HEEL
ART
VAN GOGH
ROYAL HUISMAN WISP
ADMIRAL MAXIMA 47 ENTOURAGE
DL YACHTS DREAMLINE 26 M
FERRETTI CUSTOM LINE NAVETTA 28
PRINCESS 40 M SOLARIS
3
technical
by Andrea Mancini
FROM RACING TO
CRUISING YACHTS:
A MOBILE FOIL THAT
COUNTERACTS HEEL
The Dynamic Stability System is simply an immersed horizontal foil that protrudes from the
side of the hull, to leeward, that contains heel
caused by wind pressure. Even if born to increase the performance of racing boats, it’s a
very simple device to use that can be applied to
all manner of sailboats, large or small that they
might be.
A modern sailing superyacht certainly needs to be beautiful, comfortable and functional, exclusive, easy to handle
- features which are a must when one’s spending a few
million dollars or euro for a sailboat. But, when you spend
that kind of money, you’re also entitled to expect some
great performance that however does not impair any of
the features listed above. That means working on hull design, the sails, but not only! A device has come onto the
market in recent years, that is both simple and clever, as
it makes it possible to improve a sailboat’s performance
without the complications of those delicate and problematic systems that are canting keels, and ballast, which have
been till today the only alternative available to those wanting to improve stability, and hence performance. This new
system is the DSS, acronym for Dynamic Stability System, a
system that’s revealing itself to be a true revolution for the
world of sailing. What exactly is a DSS? It’s quite simply an
immersed side foil that protrudes from the hull on the leeward side and generates lift capable of righting the boat in
a way that is directly proportional to its speed. The system
improves performance in terms of speed but not only. Said
like that it all sounds very simple but, obviously, one thing’s
the theory and another is the practice…
74
Fig.1 Keeping the boat
upright,
the
purpose
of a canting keel, ballast, and also the DSS,
means being able to exploit wind speed to the
full and be faster. As
everyone knows, when
a sailboat heels effective sail area is reduced
with
the
result
that
part of the propulsive
force
(FLAT)
unloads
downward, becoming a
vertical force (FZ) that
pushes the boat down
into the water. This increases
leeway
and
reduces speed as the
shape of the hull makes
a heeled boat less efficient. The result is
greatly reduced performance.
The well-known British racing yacht designer Hugh Welboum, who has designed boats for
the America’s Cup as well, started developing the idea 10 years ago, refining it over time
through research and testing using radio controlled models in open waters as well as in tank
tests, and with the aid of algorithms. But the idea only really took off in 2004 when prototype
DSS started being fitted to small racers, thanks also to Gordon Kay, a top class skipper and
project manager who became Welbourn’s sales partner. They started by putting
a foil just on one side of a
small 23 footer, so that they
could study the effect of the
foil on the starboard tack and
compare it to performance
without on the port tack, so
being able to immediately
assess whether the theory
stood up. Since then, a lot has
been done, including patenting the DSS so as to protect
the idea, then setting up a
company, called Infiniti Yacht,
in 2010, for the purpose of
building high performance
yachts equipped with DSS.
This because a number of
big yards, such as Wally and
Nautor, had shown interest
but without anything concrete coming of it, according to Stefano Gerardi, a
pro’ sailor and one of Infiniti
Yacht’s partners.
And so, after having tried out a number of prototypes on existing boats, today Infiniti Yachts
offers a range of Hugh Welbourn designed sailboats that are specifically built to best exploit
the DSS system. The Infiniti range are all top performance yachts recognisable for their minimalist and super modern design, the first of which, the Infiniti 36GT “Foiled!”, is getting itself
noted on race courses the world over thanks to the amazing speeds it’s capable of, which
allow it to compete with yachts that are designed to be top class racers. And this without 75
any
technical
From racing to cruising yachts:
A mobile foil that counteracts heel
of the complications typical of canting keels and ballasts, all systems that ensure performance
by simply keeping a boat more upright (figure 1).
But DSS is not just for small boats. As said, it can be fitted to boats that are considerably larger
than a 36 footer with surprising results, as shown with Wild Oats IX, the 100 footer that was
fitted with DSS in 2013 so that it could carry on winning, indeed dominating, having in the last
10 years notched up 9 wins at the Sydney to Hobart Race, the 600 mile blue-water race that
has been enthralling Australian and other international sailors for 70 years.
Fig. 2 The mechanism that
makes the DSS move in
and out of its box, set
horizontally
under
the
floorboards. Also on large
yachts, the foil is moved
with a system of blocks
and tackle similar to those
used to adjust the seat on
racing canoes.
How the Dynamic Stability System works
We will now try and understand what a DSS foil is and how it works. As Stefano Gerardi,
someone who really does understand sailboats, points out, “the DSS system really is a simple
solution because, without too many complications, it makes it possible to exploit a force that
nature gives us for free”. The DSS foil is placed in a horizontal box set transversely under the
floor boards. In light air the foil is obviously left in but as the boat speeds up it’s pushed out
on the leeward side. Also on big boats the mechanism used to slide the foil in and out on
runners uses blocks and tackle, like with an oarsman’s seat on a canoe (figure 2). Should it
happen to collide with anything floating in the water, the foil simply bends towards the stern;
should the load become too great it will break before damaging the hull. As said, as wind increases and the boat starts to heel, the foil, which is opportunely angled, is slid out of the hull
to leeward and generates lift, exactly like the wing of a plane. In addition to the lift, there is also
the leverage effect caused by the fact that the fin protrudes quite a distance out of the hull
and thus also acts as a lever, adding its own righting motion (lift + leverage) and so increasing
the righting effect, as shown in (figure 3). Furthermore, as everyone knows, lift is a function of
speed (hence the use of the term dynamic in the name), thus, as wind speed increases and
the boat accelerates so does the lift, making the boat stay upright, and so exploiting to the full
the thrust provided by the wind. In practice, the DSS does the
same job that a crew on a small dinghy does when they shift
to windward, using their weight to hold the boat upright and
Fig. 3 On the right,
thus use the wind to best effect (figure 4). On the other hand,
How the DSS works:
Figure A, on the left,
a sailboat without DSS will heel more and more as the wind
shows how the foil
increases, as the righting effect is only provided by the boat’s
has no effect when
intrinsic stability, in other words basically the weight of the keel
fully retracted and
and the shape of the hull.The result will be that as wind increasthe boat is stationes and the boat heels, there will be a reduction in the amount
ary or moving slowly.
Figure B, in the midof sail that works effectively until such point as the boat finds
dle, shows how the
its equilibrium between the righting effect of its keel and hull
foil
continues
to
shape and the force of the wind.
have no effect also
But the DSS foil doesn’t just right the boat, as speed increases
when the boat bethe increased lift will be so strong as to raise the boat out of
gins to heel under
sail. As for any oththe water enough to reduce wetted area slightly and so reduce
er sailboat, stability
is in this case fundamentally given by the
keel and hull shape.
Figure
C,
on
the
right, shows the foil
at work on the leeward side. In these
conditions
the
foil
generates lift that
produces a righting
motion (lift x leverage) that adds to
the righting motion.
76
Dynamic Stability System and Wild Oats
XI: a sure success!
Wild Oats IX is a 100 footer that has become a legend: In
December 28th 2014, for the ninth time in the last ten
years, it took line honours at Sydney to Hobar t Race, the
600 mile offshore race that has for 70 years enthralled
Australian sailors and not only.
And this also thanks to the addition of a DSS (acronym
for Dynamic Stability System) fitted to Wild Oats XI for
the last two editions. The carbon fibre DSS foil on Wild
Oats XI is 55 centimetres wide and when fully extended
protrudes 2.75 metres to leeward (figure box 1a).
But a DSS is only the last ‘weapon’ to have been fitted to
Wild Oats XI, a pure racing machine that has already been
fitted with every possible underwater addition that can
improve performance, as can be seen in in the illustration
(figure box 1b). And what of its impressive set of figures:
30 metres long but only 5 wide, it draws an incredible 6
metres or nearly. And, of course, it’s super light, displacing a
mere32 tons, of which almost half is ballast, set against 900
m2 of sail area, which almost doubles with the spinnaker.
Above, Wild Oats XI’s hull with all its high-tech go-faster
kit. Starting forward, it has a retractable bow canard,
a vertically positioned foil that helps performance to
windward in light airs. At the centre it has two dagger boards and, just aft of that, a canting keel with
a bulb with winglets. The DSS retractable foil is also
visible amidships. The single rudder, a sharp blade,
completes the yacht’s underwater trappings.
drag (figures 5 and 6). In other words, a boat equipped with DSS will remain substantially
upright and raise itself slightly out of the water thereby improving performance thanks to
a reduction in overall drag, a reduction that is far greater than the drag caused by the foil
itself.
The graph shows the performance of the 100 footer Infiniti 100 with and without DSS. It
shows how off the wind the system can improve speed as much as 30 to 40% whilst when
going to windward the system is less effective. When sailing downwind in light air, the foil
has no effect and is thus kept stowed. Looking instead at performance to windward, when
boat speed (and hence lift) is less, the drag it produces can make the DSS less effective, but
there is significant improvement in VMG (Velocity Made Good is the actual speed at which
you get closer to where you want to go when a direct course is impossible, as is usually the
case when going to windward) , of up to 10 and even 15%. This because the boat points
up better and, above all, shows greater stability in the presence of waves, which means less
FIG. 4 above, the DSS
has the same effect as
the crew of a dinghy when
they lean out to windward
to counteract the heeling thrust of the wind, so
keeping the boat upright
and better exploiting the
wind.
Fig. 5 and 6 - The Infiniti 36GT under sail with its Dynamic Stability System extended
to leeward (Figure 7 on left). Looking at the boat from the windward side (Figure 8
on right) you can clearly see how much the hull has risen out of the water thanks to
the lifting effect of the foil.
77
technical
From racing to cruising yachts:
A mobile foil that counteracts heel
slowing down and more constant speed. The reason for this is that such a large and extended foil dampens all vertical motion and also reduces pitching and yawing. This improves the
stability of air flow over the sails and the appendices that increase the boat’s efficiency, but
not only. The damping effect makes the boat easier to steer and, obviously, makes for a far more comfortable ride, giving the impression
of being on a far larger and more stable yacht. And this is the sort
of added bonus that the cruising yachtsmen could well appreciate,
as it’s comfort more than speed that they seek. Indeed, as Stefano
Gerardi points out, “the DSS system is so simple that, unlike similar
systems based on canting keels and movable ballast, it can be used
by one and all. And it is in any case something you can choose
to use or not as it works in synergy with the traditional keel that
is still there to do its
job. And if by mistake
Fig. A - The DSS has been developed
you push the foil out
and refined through lengthy R&D
the wrong side, to
and experiments on models, both
windward, no harm
with computer modelling and tank
done, as it simply has
tests.
no effect at all”.
In a nutshell this is an
ideal system with which also to improve the performance of cruising yachts in complete safety and great simplicity, given also that
the system has no impact to speak off on the inside of a yacht, the
whole thing being under the floor boards, where it does not occupy
usable space. But does the gain in performance justify retrofitting a
DSS to an existing yacht? Construction problems aside, which need
to be looked at case for case, from a hydrodynamic point of view
the DSS system improves performance on any boat, also existing
ones. But for sure, a purpose designed hull, as is the case with the Infiniti range, will mean
that you’re exploiting the system’s potential to the full.
So what can the future of this system be? Potentially it’s an idea that could be revolutionary
in the traditionally minded world of sailing, where it’s not always the case that good ideas
make headway, at least not right away, and where the big players who cannot come up
with such an explosive innovation of their own will tend, as they’re already starting to do,
to defend their own market. So as to learn something more and have the latest update on
the development and application of the DSS system, we decided to quiz Gordon Kay, the
man who has developed the DSS and one of the partners of Infiniti Yacht.
Are the benefits of the DSS the same whatever the wind speed and a boat’s heading? No.
the benefits vary with wind speed and direction. The simplest way of explaining such differences is as follows:
- in light air a DSS equipped boat will perform better all-round, as it’ll be lighter than a
similar boat. And so less weight means less resistance to forward motion.
- when sailing to windward in moderate winds the benefits are: less pitch, less heel, greater
Fig. B - A model fitted with DSS undergoing tank tests at the
Wolfson Unit MTIA at
the University of Southampton
78
stability, with improved performance as the wind gets stronger (better VMG). Off the wind,
in moderate winds, stability in significantly improved and there will be less heel and resistance. With the wind astern one instead has greater stability and greater safety, so jibing will
be easier and safer.
- with stronger winds, 20 knots
and over, when going to windward the benefits remain the
same, whereas off the wind speed
will greatly increase thanks to the
foil’s increased dynamic effect.
Stability will also improve downwind. Furthermore, jibing will be
much safer as the foil can be first
moved to the other side so improving stability on the side we
want to jibe towards.
What are the disadvantages?
If you use the foil when speed’s
too low, say 6 knots, you could
end up with more drag than lift.
But this is in effect simply a case
of using the system wrongly.
Is it possible to quantify these
benefits and draw a comparison
with other boats?
Although it is possible to quantify the increase in stability and
the reduction in resistance, you
can’t draw any kind of comparison with a given yacht because
with DSS the entire boat will be
different. For example, the Infinity 36 is something like a 1000
kg lighter than similar racers and
behaves completely differently. It’s
difficult to quantify improved performance when the differences
are so great.
The DSS stops a boat from heeling much like a canting keel or
ballast. What are the advantages
and disadvantages compared to
these systems?
Fig. C on the left, The Infiniti
36GT “Foiled!”, the first ever purpose built yacht for the Dynamic
Stability System that is currently
drawing lots of attention on the
race circuits for its impressive
performance, that allows it to
compete on an even field with far
racier yachts.
Fig. D, The Infinity 60, a yacht
specifically designed for the DSS.
Construction may well start within the year.
technical
The graph shows what
speeds the Infiniti 100
can reach with a 16 knot
wind at various headings
with and without using the
DSS. If off the wind the
DSS can improve performance as much as 30,
40%. To windward the
DSS is less advantageous
due to the lower speeds at
which the boat progresses, hence reducing lift and
with greater drag. How-
ever, the angle at which a
boat can sail to windward
is improved and stability
on waves is also better,
which means less slowing
effect produced by waves
and thus more constant
speed.
All
this
greatly
improves
VMG
(Velocity
Made Good, the speed at
which you head for a place
you can’t go to on a direct
course, as is mostly the
case when going to windward).
Fig. E Close to starting
construction, the Infiniti
100S, is a 30 metre
yacht designed specifically for the Dynamic Stability System,
showing
how
this
technology can be
used on big boats.
80
From racing to cruising yachts:
A mobile foil that counteracts heel
When it functions optimally the DSS has a greater righting effect than a canting keel. And
also, unlike a canting keel, should there be problems, like a malfunction of the system, there
are absolutely no negative consequences, like for example the boat capsizing. Furthermore
there is less risk than with a canting keel should you manoeuvre incorrectly, like an accidental jibe when the keel is on the wrong side. And what’s more, the DSS doesn’t need the
heavy and complex mechanisms needed to move a keel under load. Should the DSS give
problems one simply carries on sailing, even in regattas, whereas with a canting keel failure
this would be impossible, you’d just have to stop; the DSS reduces hydrodynamic resistance
whilst a canting keel does not; the whole of a DSS’s mechanism occupies a very small volume under the floorboards in the middle of the boat, whilst the one for a canting keel takes
up a lot more room; the DSS improves comfort, whilst a canting keel will not reduce pitch;
the DSS is easy to use, whilst a canting keel is far more difficult and dangerous (a boat can
suddenly heel to windward and disorient the crew). In other words, unlike a
canting keel, the DSS is for one and all. And the same can be said for water
ballast that adds weight, takes up space inside and makes accidental tacks and
jibes even more dangerous, when all the water is on the wrong side of the
boat.
Boats fitted with DSS have already raced, with what result?
Victory in its class at the Bol d’Or, the prestigious regatta on Lake Léman, for
“Quant ‘28”; “Brace! Brace!! Brace!!!”, a 25 foot custom, has won in Australia
and Hong Kong; an Infiniti 36, “Foiled!”, came third in the Palermo to Monte
Carlo and second at the Grimaldi trophy with the ORC rating system. It’s
here important to point out that the rating of our Infiniti 36 is very high due
to certain of the boat’s characteristics, that have nothing to do with the presence of the DSS. And of course there are Wild Oats IX’s wins at the Sydney
to Hobart Race.
At least in theory it’s possible to retrofit a DSS on an existing yacht, but
would you recommend it on your best friend’s boat?
It depends on the boat. We’ve had some positive results with existing boats but there’s no
doubt that we’ve had the best results with purpose designed new boats.
To date the biggest purpose built boat for the DSS is the Infinity 36. What does the future
hold in store? What boats with DSS are there in the pipeline?
As one can imagine the projects that turn into something concrete are for the moment
linked to the world of racing. It is no chance that there’s a new Infinity 46R being built in
Istanbul, whilst we’re hoping to start building the new Infinity 60 for the IMOCA inside this
year. Indeed, last October the IMOCA 60 class assembly decided to include the possibility
of the DSS in their regulations. It’s thus reasonable to assume that in future all IMOCA 60
class boats will have DSS. But there are also less extreme projects, like the Infinity 100S, a
100 footer that already came close to being built last year, and also a new 110 footer that
we’re currently working on.
For further information: - www.infinitiyachts.com - www.dynamicstabilitysystems.com