Fluid Structure Interaction (FSI) is a scientific simulation method to optimize the sail design and consequently the performance. The analysis is made in a comprehensive sail design solution. The methodology is very often applied to projects within the super yacht and grand prix racing segments.

However, the FSI tools  can be used on smaller scale projects as well - as this case illustrates. In this specific case the FSI analysis has been used to optimize rig and sails of an open 30’ racer, a modified Ultimate 30 racing on the northern Italy lakes.

Piero ReFraschini, owner of BandaLarga:

” The boat was back to sail 8 months after a refit – an  amazing work. Performance was totally changed. Now we are between 0.5 and 1 knot faster upwind, at least 1 knot faster down-wind. Easier to maneuver, faster, safer, 6 trapezes to give more adrenalin.  We can only say we are very happy with the result. And the beer, cheese, salami and mortadella after sailing is just as good, if not even better.”

Background

When BandaLarga was purchased the team sailed for one year with the original set-up. However, the team decided to change the very soft aluminum mast with a new much stiffer and lighter carbon rig. The sail area was with a bigger square head mainsail and a bigger jib, and trapezes were added for all the crew except helmsman and main trimmer. 

The mast designer and the boat designer overlooking the optimization work needed reliable load data to be able to optimize the mast design, for such a high load type of rig. Thus we used FSI analysis to optimize and illustrate the new sails. We received VPP data for the most significant sailing points for the mast designer and related load conditions. The load conditions were simulated 24/7 for three days, comparing existing set-up with new alternatives.

After running our FSI model, the resulting loaded mast curve was straighter than expected, therefore we designed a straighter luff curve, reducing the mainsail girths, and a more roached jib leech, increasing the jib girths. And the real sailing proved correct, no recut was needed and the customer got a sail that makes his boat much faster.

Thanks to the use of the FSI analysis software we were able to design a mainsail with the luff curve perfectly matching the mast bend even if the mast didn’t exist yet. In addition to that we were able to design a jib and a code 0 with the leech “kissing” the rig under load with no need of any modification afterwards.

Before and after

In below illustrations you can very well see the 1) before and 2) after situation.

1) Before                                                                2) After

In this example you see the main has been narrowed because of the very stiff mast and the jib has been widened to have the leech just at the limit of touching diagonals and spreaders:

FSI studies make sure that theory and predictions match reality perfectly

Any structural analysis starts with a field of forces applied on the sails surface. In a simulated sailing condition with a predicted boat speed, True Wind Angle (TWA) and True Wind Speed (TWS), the characteristics of the boat and the initial sail trim, the FSI calculation delivers a vast amount of essential and critical information in order to obtain the most efficient flying shape of the sail. A very efficient and at the same time extremely accurate way of designing new sails.  

> Check out our gallery of nice shots from the Lake!