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:
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:
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. 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). 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).
 Here we shall note that the instruments we use do not provide us with a direct measure of the hydrodynamic pressures. The alternative solution would be to use pressure gauges fitted through the bottom of the hull. However, this would be very unpractical, as pressure gauges need to be regularly recalibrated (something which is not compatible with the duration of our measurements) and they potentially affect the watertightness of the hull.
 Significant wave height (H1/3) is defined as the mean of the highest third of all encountered waves. In practice, it often corresponds to the wave height reported by an experienced sailor,