Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


REEF AND FLAT OR TWIST AND EASE: AN IMPROVED TRIM PARAMETER MODEL FOR YACHT SAILS


H Hansen, FutureShip GmbH, Germany P J Richards, Yacht Research Unit, University of Auckland, New Zealand P S Jackson, College of Engineering, University of Canterbury, New Zealand (DOI No: 10.3940/rina.ijsct.2010.b1.91)


SUMMARY


Velocity prediction programs (VPPs) are commonly used to assess the performance of a yacht during the design process. The ‘reef and flat’ trim parameter model has been used to describe the changes in aerodynamic forces acting on the yacht during de-powering of the sails since the first VPP was developed about 30 years ago. Based on the concept of the ‘reef and flat’ model this paper introduces the extended ‘twist and ease’ model. Wind tunnel tests with the Real-Time VPP have been conducted to obtain realistically de-powered sail shapes and the results are compared to the trim parameter models. The comparison shows that the ‘twist and ease’ model describes the changes in lift, drag and centre of effort height due to de-powering more accurately and the predicted boat speed is within ± 1% compared to the under prediction of up to 4% of the ‘reef and flat’ model.


NOMENCLATURE α


βA, βeff ε, εt φ


ρair a


AS, AR


zCoEminDi Centre of effort height for minimum induced drag [m]


Angle of attack [°] Apparent and effective wind angle [°]


Trim parameter ease and ease due to twist [-] Heel angle [°]


Density of air [kg m-3]


CD, CDi Total and induced drag coefficient [-] CDp


Force coefficient [N]


CL, CLopt Lift and optimum lift coefficient [-] Cl


Sectional lift coefficient [-] ClminDi


Sectional lift coefficient for minimum induced drag [-]


Clopt Optimum sectional lift coefficient [-] ct, ct′ ctL cs


Twist weight constants [-] Di


e, e′ f


F F


M


Twist weight constant for lift [-] Separation constant [-] Induced drag [N]


Efficiency factors [-] Trim parameter flat [-] Force [N]


Force vector (FX, FY, FZ) [N]


MX Heeling moment [Nm] qeff RL r t


t0, t′ VS


xCoE zboom zCoE


Moment vector (MX, MY, MZ) [N m] Effective dynamic pressure [N m-2]


‘Power’ ratio of lift [-] Trim parameter reef [-] Trim parameter twist [-] Base twist and total twist [-] Boat speed [m/s]


VT, VA, Veff True, apparent and effective wind speed [m s-1]


Longitudinal centre of effort position [m] Boom height above design waterline [m] Centre of effort height [m]


©2010: The Royal Institution of Naval Architects


CDpOpt Optimum parasitic drag coefficient [-] CDs CF


Reference sail area [m2] and aspect ratio [-] Parasitic drag coefficient [-] Separation drag coefficient [-]


Point on central axis (ax, ay, az) [m] zCoEopt Optimum centre of effort height [m]


zmast Mast height above design waterline [m] Dyna Name of the Berlin sail-force dynamometer IMS International Measurement System VPP Velocity prediction program


1. INTRODUCTION


During the design process of performance oriented and competitive yachts it is important to investigate different design options and evaluate them based on the best resulting performance. Velocity Prediction Programs (VPPs) have been developed and used to calculate the boat speeds for given sailing conditions based on semi- empirical mathematical descriptions of the forces acting on the sailing yacht. The semi-empirical modelling of the forces acting on the yacht sails is crucial for an accurate performance prediction and two aspects are of particular importance when modelling the sail forces. Firstly, the flying shape of the sail is not the same as the design shape and it can be changed by trimming the sails. Secondly, the aerodynamically most efficient shape of the sail might not always be the desired flying shape. The aerodynamically most efficient shape can be defined as the shape that produces the most driving force (i.e. the optimum lift and drag combination) for a given apparent wind angle. However, in many sailing conditions the maximum drive force will no necessarily result in the maximum boat speed because other force or moment components, such as the side force and heeling moment produced by the sails, also influence the performance of the yacht. In particular, the heeling moment (MX) should not be ignored for the majority of sailing conditions. The heeling moment generated by the sails will result in a heel angle, which increases the hydrodynamic resistance and reduces the appendage and aerodynamic efficiency. This could result in a slower boat speed even though the drive force generated by the sails is larger than for a


B-9


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60