Sail Technology

The ability of sail designers to create efficient aerofoils has improved beyond all recognition over the last 20 years mainly due to the advent of ever more powerful computer processors and design software along with the development of sail membranes and manufacturing techniques.

Ultimate Sails design software is a fully integrated suite of programmes that give our designers unlimited flexibility of boat, rig and sail analysis. We can test every element of the boat, rig and sails in a variety of conditions “on the screen” before manufacture.  This combined with the ability to accurately compare finished results after sailing has reduced development time and costs significantly.

We can even export the entire rig and sail design in a “viewer” format and email to the customer for analysis and comment before production.

Here is an example of the design process from start to sailing.

Boat and rig:

We start with either the sail plan or accurate measurements taken from an existing boat to create a 3D model of the boat and rig. We take into account the size of the boat, its displacement, righting moment, yaw and designers VPPs. On the rig we specify sheeting angles, spreader angles, track positions, mast properties including surface area, rake, initial mast bend, maximum mast bend and mast twist.  We can also take into account the mechanical properties of the rig’s structure and allow for stretch of head stay, shrouds, halyards and mast tip fade.

Fig 1. below shows a hull and rig ready to accept sails with sheeting positions and halyards for a mainsail, Code 2 Jib, Code 0, asymmetric spinnaker on a pole and a spinnaker staysail.
 

Sails:
The next stage is to define the 3D surface of each sail and apply it to the rig.  All sail dimensions are controlled in 3D and any type of sail can be placed on the rig including, mainsails, Genoas, Jibs, Code 0s, Asymmetrics, staysails etc.

Fig 2. shows the 3D boat and rig with the mainsail and code 2 Jib sheeted for upwind sailing in 13kts true. The 3D modelling is so accurate that in this instance the onboard actual sheeting position of the clew and position of leech on the spreader were within 10mm of the design calculation.
 

Fig 3. This shows the leech of the Code 2 Jib with a 200mm hollow between the battens sheeting around the spreader.  Note the increased hollow between the bottom batten and clew to help the sail sheet around the D1 diagonal.
 

Fig 4. The same Code 2 Jib when sailing.
 

SAILOPT
“Sailopt” is a revolutionary aerodynamic simulation software that computes the shapes and dimensions of sail designs. This software was developed at the Swiss Federal Institute of Technology of Lausanne within a research program for the America’s Cup, where it has been extensively used for sail plan and sail design, and is now offered by Ultimate Sails as service for the designers of its customer’s sail boats. As you know, upwind sails should function as efficient wings, developing the maximum lift with the minimum drag and heeling moment. This goal is achieved when the optimum spanwise circulation distribution is chosen and when the separation of the flow is minimized. These choices reflect the two main objectives of sail computations: the calculation of precise sail coefficients to feed the Velocity Prediction Program (VPP), and the determination of the optimum sail dimension and shape to aid the sailmaker in the design of the sails.
 

Optimisation of Upwind Sails
To determine the sail shape that maximizes the thrust for a given condition of wind speed and angle, an inverse 3D Vortex Lattice Method has been developed. The optimisation procedure is based on a genetic algorithm (developed, as the whole software, by Ing. Mario Caponnetto), where the optimum is obtained adding successively random disturbances to an initially arbitrary shape. With this approach several different constraints can be added very easily. Sails can be optimized with or without the constraint of maximum heeling moment. Limiting values of the sectional lift coefficient can be included to prevent stall. The viscous drag (taking into account the separation of the flow behind the mast) is added at each section using 2D drag coefficients that have been previously calculated using Fluent (a Navier-Stokes flow solver) for different combinations of camber ratio and mast dimension. During the iteration procedure, unrealistic sail shapes can be discarded. Typical is the case of mainsails having an inverse camber in the upper part; theoretically this shape maximises the thrust in strong wind conditions, but is obviously very difficult to trim.
Using this code it is possible to obtain the complete polar diagram of the “best” sails in upwind conditions and their corresponding shapes.

Fig. 1 shows a sample case of how the optimum circulation distributions on the genoa and the mainsail change varying the wind speed. In both cases a heeling moment of 30 t*m, corresponding to a heel angle of about 30 degrees for an IACC, has been imposed. In medium wind conditions (Aws=18 Kn) the main is very loaded, especially toward the head. As the apparent wind speed increases (Aws=30 Kn), the main must be unloaded at the tip, and in theory the circulation should be zero from the headboard to the top of the Genoa. For the same sample case the corresponding twist angle and camber ratio are plotted in Fig. 2 and Fig. 3.

These results are in good agreement with the experience. In medium wind both the
Genoa and the main have relatively highly cambered sections. The twist of the main is large, but the boom is close to the centerline of the boat. In stronger wind the sails must be flattened, especially the main in the upper part, to lower the center of pressure. The head of the genoa must be still relatively fat to avoid separation. The twist of the main is reduced, but the angle of the boom is increased.

Fig. 1 – Spanwise circulation distribution
Fig. 2 – Spanwise twist distribution
Fig. 3 – Spanwise camber distribution
 

Given as input the dimensions of the sail plan (P, E, four main girths, headboard width,
BAS, I, J, headsail foot length, mast longitudinal diameter, rake), the sailing conditions
(heel angle, boat speed, true wind speed, true wind angle) and the heeling moment that must be developed (equal to the righting moment of the boat for the given heel angle),
“Sailopt”, considering also the vertical wind shear and the water surface symmetry effect, gives as output:
- Aerodynamic resulting forces and moments (driving force, heeling force, vertical force, heeling moment, yawing moment, pitching moment), extremely useful for the boat designer to design and position the appendages, to feed the VPP with very precise upwind aerodynamic coefficients and to compare the performance of different possible sail plans.
- 3D optimum sail flying shape and sail trim, very useful for the boat designer to position the headsail tracks and the standing rigging, and for the sail designer to evaluate optimum camber and twist of each section of the upwind sails.

A lot of effort has been put in the validation of the software, and the good results achieved in real testing allow mean we can trust the values given by “Sailopt” computations.
Here is an example of the comparison between “Sailopt” aerodynamic driving force and tank testing based IACC hydrodynamic drag, which should be equal in the VPP computed sailing points. As you can see in Fig. 4 the two values are very close.
 
Fig. 4 – Comparison between aerodynamic driving force and hydrodynamic drag.