Advanced Search provides additional search options providing the ability to narrow your search by combining multiple search variables.

Note that by default, the Date Range set to 2017 will return all results from a text search. You can select both Date Range settings to narrow the returned results.

In recent years a number of Dynamic Velocity
Prediction Programs (DVPPs), which allow studying the
behaviour of a yacht while tacking, have been developed.
The aerodynamic models used in DVPPs usually suffer
from a lack of available data on the behaviour of the sail
forces at very low apparent wind angles where the sails are
flogging. In this paper measured aerodynamic force and
moment coefficients for apparent wind angles between 0°
and 30° are presented. Tests were carried out in the
University of Auckland’s Twisted Flow Wind Tunnel in a
quasi-steady manner for stepwise changes of the apparent
wind angle. Test results for different tacking scenarios
(genoa flogging or backed) are presented and discussed
and it is found that a backed headsail does not necessarily
produce more drag than a flogging headsail but increases
the beneficial yawing moment significantly. The quasisteady
approach used in the wind tunnel tests does not
account for unsteady effects like the aerodynamic inertia in
roll due to the “added mass” of the sails. In the second part
of paper the added mass moment of inertia of a mainsail is
estimated by “strip theory” and found to be significant.
Using expressions from the literature the order of
magnitude of three-dimensional effects neglected in strip
theory is then assessed. To further quantify the added
inertia experiments with a mainsail model were carried out.
Results from those tests are presented at the end of the
paper and indicate that the added inertia is about 76 % of
what strip theory predicts.

Optimisation of Span-Wise Lift Distributions for Upwind Sails

Wind tunnel experiments using a Real-Time Velocity Prediction Programme to investigate the optimal trim of a VO70
model under various simulated true wind speeds are reported. The results illustrate that the decision made depend upon the particular
apparent wind direction and true wind speed. It is suggested that these can be sub-divided into three broad bands: low wind speeds
where the total drag is minimised and the trim that provides the maximum thrust coefficient is chosen, moderate wind speeds where
the heel angle has a strong effect and the optimum choice includes a reduction in lift coefficient and centre of effort height and strong
winds where the heel angle and hence heeling moment is limited to the maximum acceptable value and the optimum loading
distribution is strongly constrained by this limit. Extended Lifting Line Theory is used to further investigate the detailed loading
distribution on an AC90 mainsail. The result illustrate the way in which the optimal distribution changes with varying conditions.

Unsteady Aerodynamic Phenomena Associated with Sailing Upwind in Waves

Velocity Prediction Programs (VPPs) based on a steady-state equilibrium between aero- and hydrodynamic forces continue to be important tools when assessing the performance of yachts during the design process. Over the last decade a number of Dynamic Velocity Prediction Programs (DVPPs), which also allow study of the dynamic characteristics of the boat, have been developed. Most DVPPs are based on numerically solving the equations of motion of the yacht according to Newton’s second law with the aerodynamic forces being calculated from quasi-steady theory. This paper discusses whether this assumption of quasi-steady aerodynamics can be justified and also analyses the error introduced by such a quasi-steady analysis. Unsteady potential flow theory is used to predict the pressure distribution on an aerofoil-like, two-dimensional “slice“ of a mainsail carrying out harmonic oscillations both perpendicular to, and along the direction of the incident flow. Such types of motion occur when a yacht pitches or rolls in waves. Theoretical pressure distributions are compared to wind tunnel measurements on an oscillating, rigid mainsail model of 3.2 metre span and 0.447 metre chord length. Experiments were carried out at reduced frequencies ranging from k = 0 to k = 0.8, as the mainsail of an International America’s Cup Class yacht sailing upwind in waves typically encounters reduced frequencies in this range. It is found that predictions based on unsteady theory match the measured pressure distributions much better then quasi-steady predictions. This leads to the conclusion that, if the performance of the yacht is to be predicted on a time-scale shorter then the pitching period, this can be achieved best with an unsteady aerodynamic model. In the paper no attempt is made to investigate the influence of the flexibility of the sails, sail interaction, three-dimensional effects or phenomena related to dynamic stall.

Development of a Three Dimensional Inverse Sail Design Method

A code which generates the camber shape of a sail from a desired sail plan-form, sail twist distribution and surface pressure map has been written. This is an iterative 3D inverse sail design code. The method initially uses inverse thin aerofoil theory, applies this to the desired pressure map and creates an initial sail shape. A theory which gives a relationship between the change in the pressure map and the change in the sail camber was developed and is described. The code applies that theory to the difference between the desired pressure map and the pressure map of the initial sail shape. The calculated camber difference is added to the initial shape to give an improved shape with a pressure distribution closer to the desired one. This process is repeated until the generated sail produces the desired pressure map. Validation tests were performed by generating a pressure distribution from a known sail shape using a VLM code, and then the method described in the paper was used to find the shape from the pressure distribution. The sail shape was successfully obtained in as few as five iterations, with a maximum error of only about 0.2 % of the sail chord, which is acceptable in sail design practice.