May 2001
Study on the dynamic loads on offshore sailing yachts
By Paolo Manganelli - Department of Ship Science - University
of Southampton
In
the last two years we have been carrying out a study on the dynamic loads
that affect the structures of offshore sailing yachts. In particular,
our aim was to achieve a better knowledge of:
- the
hydrodynamic loads generated by slamming.
- the
inertial loads on the mast and on the keel.
As
part of this study, we developed a measurement system that was installed onboard
EBP-Gartmore and Voilà.fr for the last Vendée Globe. This system consists
mainly of an autonomous logging unit and of a number of accelerometers attached
to the structure of the boat (see figure below). This configuration allows
us to record the accelerations of the structure in six degrees of freedom.
By looking at these information we can:
- Estimate
the hydrodynamic loads on the hull due to slamming in waves, knowing that
the accelerations at the centre of gravity of the boat are a function of
her displacement, of the hydrodynamic pressures acting on the hull and of
the area they act upon.
- Evaluate
the inertial forces acting on different parts of the structure
- Analyse
the motions of the boat in a seaway (through double integration of the accelerations
in time). See animation.
The measurement system was designed to meet the constraints brought by the type of boat and by the environment in which the measures were to be taken. Thus, weight (~2kg) and power consumption were reduced to the minimum in order not to affect the performance of the boat. Besides, the system was made to be fully autonomous and it did not require any intervention by the skipper throughout the whole race.
Generally,
the logging unit is programmed before the start of the race to accomplish
the following tasks:
-
Recording of a « navigation log »
Data
like boat speed, wind speed and wind angle, heading, GPS position are stored
at regular intervals (typically every 20 seconds) during the whole race. They
give an accurate indication of the sailing conditions and of the performance
of the boat.
-
Time histories recording
When,
the acceleration at the centre of gravity or at the bow exceeds pre-set threshold
values following a « slam », a time history
of the signals from all the accelerometers is saved. In general, the 5 seconds
preceding the impact and the 5 following it are stored; as a result, it is
possible to calculate the attitude of the boat at the time of impact, the
“magnitude” of the impact (in terms of the linear and angular accelerations
it produces) and the structural response[1].
During the Vendée Globe we have collected about 4500 recordings of this kind,
each one corresponding to a significant impact.
- Statistical data recording
The
peak accelerations at every wave encounter are constantly stored into histograms
that also include boat speed and true wind angle. These data are particularly
useful with respect to the investigation of fatigue problems: they allow us
to have a full picture of all the loadings over a race.
In
order to assess the seakeeping characteristics of the boats and to develop
analytical models that can reproduce them accurately, it is very important
to know the sea state at the time of the measurements (i.e. significant wave
height, average period and main direction of propagation[2]).
These data cannot be measured from the boat (although devices exist that may
perform this kind of measurements, they are not suitable to be used on board
an Open’60). Hence, we make use of measurements taken from the Topex Poseidon
and ERS-2 satellites, which are kindly made available to us by the
« Southampton Oceanographic Centre »,
as well as of numerical model simulations produced by the « Ocean
Modelling Branch » of the NOAA. The later, in particular, appeared
to have a good degree of accuracy in most cases and were extremely useful
thanks to their high resolution.
Until
now, we have carried out five main series of measurements. Three were performed
over transatlantic passages (one of which during the 2000 edition of the « Europe1
NewMan Star ») and two over the Vendée Globe. This corresponds to 297
days of measurements over approximately 65000 nautical miles.
If
you wish to learn more about this study, you can read the paper entitled « An
experimental investigation of slamming on ocean racing yachts » which
was presented at the 15th Chesapeake Sailing Yacht Symposium (available
here in PDF format).