
SUPERYACHT #501 January 2004
Article selected from our quarterly magazine dedicated to the largest
and most luxurious boats with information, interviews, technical
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Fabio Petrone interviews Mario Grasso, engineer with Navirex System
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THE NAVIREX SYSTEM TRANSOM PLATE
Some time ago (Editor's Note: Superyacht June 2003)
we presented new systems for controlling stability - Intruders
and Interceptors - specifically designed and produced to be
installed on fast motor boats.
As an alternative to the classic flaps, this apparatus
consists of a kind of metal blade, hinged parallel to the
transom and moved by a one or two piston system, thus sliding
up or down. When it moves down even a little below the bottom
lines, this "blade" creates friction, a resistance to the
progress of the hull that increases with speed and generates a
significant thrust right under the stern sections such as to
lift the stern and consequently lower the bow. But the
benefits of this recent innovation - constant adjustment of
trim both longitudinally and transversally with considerable
advantages in terms of reducing roll and pitch - do not end here.
New studies are being carried out in various quarters and new
apparatus is being fine-tuned which, in the wake of what has
already been achieved in applications on fast boats, may also
bring benefits to other kinds of vessel. One of the companies
that seems to have already developed new uses for this type of
device is Navirex of Genoa. Engineer Mario Grasso gave us this
interesting interview.
What is Navirex and what are "Transom Plates"?
Navirex is a Genoese company set up in 1992 which to date
has completed various designs for owners and yards both in
Italy and abroad. The company is organised in two divisions,
Navirex Design and Navirex System, which respectively handle
boat design, especially superyachts, and computerised
systems in the boat field, as well as newly created systems
for trim control called "Transom Plates". (Editor's Note:
one of the most recent designs developed by this Genoese
company is the 24 metre semi-displacement navetta "Enneffe"
presented by the Mondomarine yard at the last Genoa Boat
Show). These plates, created and manufactured by Navirex
System, were designed specifically for the semi-displacement
hull market, following numerous studies and tank testing
where, in effect, we were able to ascertain that Interceptor
gives good performance not only on planing but also on semi-
displacement hulls.
Can you tell me about this application on the Mondomarine navetta?
The "Enneffe" is 24 metres long overall, with a 6.40 m beam
and a full load displacement of 80 tonnes. She has two 1.300
HP MAN D 2842 LE 404 engines. This means that with Transom
Plates installed she can reach a maximum speed of 22 knots,
practically double the performance she should have as a
round bottomed boat.
Overall Length (m) 12 24 35 45 60 70 |
Waterline length (m) 10 20 30 40 50 60 |
Maximum speed (knots) 8 11 14 16 18 19 |
That's a really considerable increase
There's a table, drawn up from statistical data and
therefore approximate, which however gives a quick check on
what the actual performance levels of a round bottomed hull
of this kind should be .
Well, if these are the results, it looks very interesting to me
The owner of the "Enneffe" loves steering the boat himself,
keeping her at a cruising speed of 12/16 knots. When he
wants to accelerate quickly all he does is incline the
Transom Plate by 30/60 degrees and, even before throttling
up, the speed increases by 4 knots in three/four seconds,
reaching 20 knots without any throttle at all and with the
engines at a steady 2.000 rpm. This is still below the top
speed of 22 knots, achieved effortlessly at 2.300 rpm,
whereas without the Transom Plate the boat won't do more
than 16/17 knots. This is because the Transom Plate, with
minimum friction, creates a lift similar to that created by
the Transom Wedge. (Editor's Note: a sort of spoiler or
fixed dihedral with a variable inclination of 3 to 9
degrees, part of the hull and made of the same material).
The result is a decidedly more bow-heavy trim. If necessary
however, which is to say with a big sea running when a trim
with the bows very low in the water could make for heavy
going, its effect can be eliminated. In practice it can be
disabled, bringing the hull back to its original trim.
Interceptors on fast boats also act as trim adjusters
against roll. Do Transom Plates do the same?
The Transom Plate effect is also similar to what you get
from the well known flaps which also create lift and "raise"
the stern, in effect reducing the vessel's weight.
The Transom Plate idea is by no means new. It was created in
the eighties in the Soviet Union but was never really
developed, apart from a few military applications. It was
then taken up by some European companies who used the
"shutter" or "guillotine" system, that is, a blade that
descends vertically on the transom, usually under the
runner, projecting a few centimetres below the hull. Other
companies make it with a pin at the end and with a "chopping
board" action.
The Navirex System "Transom Plate" is just a shorter flap,
which may be inclined up to 90 degrees instead of the normal
10/15, thus bringing it into a vertical position, though it
already starts functioning at 20/25 degrees. So it takes up
less space than a flap and eliminates, among other things,
the problem of bumping it against the quay. It is in
stainless steel with suitably sized pistons and can be
applied to hulls in composite, steel and aluminium. There is
a control unit, usually positioned in the stern. On the
bridge, on the flying bridge and in the steering areas there
is a joystick with which the two parts of the Transom Plate
can be raised either together or separately. An analogue
indicator clearly supplies the datum with the angle at which
the Transom Plate is positioned.
Any further developments of the system?
Navirex System's next step is to make the apparatus fully
automatic by linking it to the GPS, to engine revs and,
lastly, to the stabilisers in order to help them do their job.
However it will always be possible to disconnect it from the
automatic system or actually zero its inclination, for
safety reasons that will be obvious to any sailor.
This system should therefore be envisaged not only for
planing boats, on which it has already been widely tested,
but also for semi-displacement craft, pilot boats and round
bottomed yachts with fairly high speeds: vessels on which it
is possible to obtain advantages of trim and speed (an
increase of several knots) with a low initial cost that is
moreover paid for by the great reduction in consumption.
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