SUPERYACHT #505
May 2004
Article selected from our quarterly magazine dedicated to the largest
and most luxurious boats with information, interviews, technical
articles, images and yachting news
Summary
Subscription
Yachting catalogue
Navigation tests
Used boats
Boatshow
Video Nautica
Article by
Stefano Beltrando
Photos by Paolo Venanzangeli and Julian Hickman
Stefano Beltrando of Q.I. Composites carries out non-destructive analyses on
structures in composite to check quality and safety. In this context he has
personally taken part in many of the main projects, competitive and otherwise,
of the last 4 years
|
|
SAIL SUPERYACHTS
RIGGING SOLUTIONS
It seems that designs for increasingly larger boats are springing up everywhere
of late. Larger but I wouldn't say more technological inasmuch as they remain
"simply" large vessels.
In fact the trend we have been seeing recently aims at increasing vessel size
and consequently mast height, but without necessarily developing constructional
or material technology, as happens for example with open 60's or "tri" 60's. The
answer is simple, which is to say large dimensions involve great loads, great
risks and above all great costs. So these designs tend to develop more
conservative philosophies, since the dismasting of a 50 metre vessel involves
consequences that are certainly more catastrophic than in the case of a day-
boat. To give an example, consider that the new superyacht Mirabella has its
plating impregnated with vinylester resin, which is to say a resin that is no
longer used even on small boats where the better performing epoxy resin is
preferred. On the other hand it is much easier to find operators skilled in
working vinylester, just as there are less risks of error or polymerisation.
Going on to consider masts, we may say that when we think of mega/maxi yachts
today there is practically no longer the problem of choice of materials inasmuch
as carbonium is more or less the only material capable of taking enormous loads
without being excessively heavy. Aluminium is still used where tradition remains
to the fore, such as in Holland or Germany, but never for boats where
performance is not among the main objectives to be achieved.
It having been established that carbonium fibre is the main material, another
axiom must be stated, which is to say that practically no masts are now created
without the use of the vacuum, or better, of the autoclave. In fact the high
number of layers in a mast cannot absolutely accept simple manual compaction
inasmuch as there would be an unacceptable quantity of vacuums such as to render
vain the use of carbonium.
On this subject consider that the minimum thickness of a maxi mast is about 5
mm. while the maximum thickness for a superyacht may even be 80 mm; knowing that
the average thickness of a layer of pre-impregnated compacted with the vacuum is
about 0.4 mm, it may be deduced that in the two cases put forward the number of
layers will be from 12 to 200!
So the main problem to be faced by "mast-makers" is how to succeed in making all
these layers adhere perfectly to one other. Also because, to complicate matters,
there is the geometry of the mast itself which not only has on average an
elliptical profile but in certain cases has angles of up to 90°, with the
consequent difficulty of flexibility of fabrics that resist folding.
So though there are excellent vacuum technique products, the builders' dream is
almost unanimous: the autoclave. This instrument, which permits a pressure on
the laminate of 3 - 5 bars, guarantees the best possible compaction.
I would like to underline a fundamental aspect: excellent laminators, in
possession of the correct construction procedure and employing the vacuum will
certainly come up with an optimal product. But the same cannot be said of an
autoclave in the hands of inexpert builders. Inasmuch as if the laminate is well
set up beforehand the vacuum will be sufficient to give the right compaction,
whereas 50 layers with poor cohesion will be irremovable even with the most high
performance autoclave.
At this point we should take a general look at the polymerisation of the resin
phase, a step simultaneous with or in some cases subsequent to compaction of the
layers. Almost everyone nowadays takes the laminate to temperatures over
70°C, in certain cases up to 140°C. This is in order to ensure total
reticulation of the resin and, at the same time, push resin performances
upwards which go in step with the reticulation temperature.
There remains the problem of bringing a 30 to 70 metre mast to a temperature of
70°C, which requires something more than a heated cover. The problem is
resolved with three methods:
- mould heated by an oil circuit or electrical elements (rare);
- hot air oven that takes the whole mould (more widespread);
- sliding oven that "cooks" the mast a bit at a time (for masts over 50 metres).
A new method was experimented with in building Mirabella's mast: flexible panels
containing electrical coils were applied to the mould. The panels heat up when
current is passed through them.
The longest part of the job begins when the mast is removed from the mould: the
application of fittings and reinforcements and the drilling or cutting of
housings for shrouds, halyards and spreaders.
In many cases before this work is done the mast has to be assembled since masts
are often made not in one piece but may consist of a front and back, or a right
and left side and even an upper, middle and lower part. In all these cases it is
obviously necessary to join the pieces and ensure that these joints neither
contribute too much weight to the structure nor constitute weak points.
This operation may be carried out by gluing the parts, by gluing and bolting or
by gluing and subsequent lamination of reinforcements. It is superfluous to say
how delicate this operation is. It is often one of the causes of greatest
"preoccupation". But actually, none of the problems I've encountered with joints
have ever jeopardised the structure.
Currently the most important mast design solutions are found in the "millennium
rig", invented by the New Zealanders during the 2000 cup, in which two spreaders
are replaced by an equal number of intersections of the diagonals within the
mast, and the system known as AerorigT. The latter envisages the use of a wing-
profile mast without shrouds, rotating on its own access and equipped with a
boom for mainsail and for jib. Apart from these two solutions there is nothing
new inasmuch as the other cases are limited to building ever taller masts but
without any real design revolutions. Weight is kept down and certain elements
are simplified, chiefly the use of running backstays which have now practically
disappeared from maxis upwards.
Masts taller than 30 metres give us the chance to see daily what unexpected
consequences, but also what fascination, may derive from the desire for gigantism.
To give an example, just think that one of Mirabella's mast-head halyards weighs
more than the operator who carries out the normal rigging inspection.
Millennium rig diagram
The millennium rig, which started out as the "final" choice for every maxi, was
subsequently re-dimensioned since the reliability of a greater number of
spreaders is not replaced by the intersection of the diagonals. Moreover, there
remains the fact that proper adjustment of these masts certainly requires more
time and highly expert personnel.
Aerorig diagram
In practice the mast is rotated in accordance with wind direction while at the
same time mainsail and jib are set to the ideal angle. The advantages regard
both aerodynamics and practicality of manoeuvre: the only setting that remains
is the rotation of the mast, since main and jib sheets have been done away with.
Moreover, with the wind abaft the beam, the jib is in a privileged position in
comparison with the traditional rig because it is not in the lee of the mainsail
but equally exposed to the wind. I was personally involved in a project called
"Erika", in Holland, in which the rig chosen for a 47 metre aluminium vessel was
precisely a 51 metre aerorig. In this case the mast was created in two parts,
front and rear, and I should point out that during checks of the front part you
could actually walk inside the laminate: unlike the usual mast laid out on two
trestles, this seemed far more like the hull of a vessel. The owner's intention
was to create a yacht that could satisfy his desire to make long voyages with
only a family crew, so the deck plan was appropriately simplified.
|
