In December 2000 we were asked by Hoek Design Naval Architects to assist in the optimization of the 82 ft cruiser/racer “Bharlin Blue”.
Bharlin BLue was built in Makkum, The Netherlands, by Bloemsma & Van Breemen and was launched in July 2002. Bharlin Blue sailed her first regatta in 2003, the St. Maarten Heineken Regatta. She won the regatta on line honours and on handicap. Amongst the rest of the fleet were very competitve yachts, like three Formula 1 Farr 65 designs.
Hoek Design had prepared a preliminary design of the yacht and it was our task to evaluate this design and define improvements.
We subjected the preliminary design to an exhaustive VPP study to establish the speed under sail of a large number of design variants. For each variant we determined the elapsed time on a windward-leeward, an olympic triangle and a circular random course. The IRM rating of each design variant was subsequently calculated and the corrected elapsed time on these courses determined.
The first analysis applied was the study of rig height and sail area, involving 9 configurations, where rig height was systemmatically varied to produce a matrix of designs to pin-point the effect of individual changes. From this analysis it could be determined that the greatest changes in performance were for a shortened rig with associated decrease in area, and a lengthened rig with associated increase in area, as could be expected..
The second analysis was the variation of J, E and SPL, involving a shortening and lengthening of the spinnaker pole, and an increased J with reduced E and a decreased J with increased E. Here relatively small differences in performance were noticed.
The third analysis was to assess the effect of keel fin chord modifications in combination with rig height. Two configurations involved a shortening and lengthening by 0.2 m of the keel chord while maintaining the basis rig, with the result that the chord of the base keel could be reduced. A further 2 variants involved the shortening and lengthening by 0.2 m of the keel chord for the shortened rig as used earlier. Another two configurations involved the shortening and lengthening by 0.2 m of the keel chord for the lengthened rig as used earlier. The results obtained showed the need for reducing the lateral area of the keel except for the tallest rig.
The fourth analysis looked at the effect of ballast increase. Two configurations used the lengthened rig as considered before and involved the addition of 2t and 4t ballast in the bulb. The results showed an increase in performance for both cases. Four additional cases were studied for the base rig involving the addition of 2t, 4t, 6t and 8t ballast in the bulb. Again the results obtained showed the need for increasing ballast.
The fifth analysis looked at a design which incorporated all of the performance improving characteristics pin-pointed in the previous four analyses, comprising the rig height increase, the desirable increase in J and SPL (requiring a move of the mast further aft), the shortening of the keel fin chord and an increase in the amount of ballast carried in the bulb by 4 tonne. The average delta over the base boat was found to be 27.1s/nm.
The sixth analysis showed the effect of decreasing keel chord in combination with an increase in thickness-chord (t/c) ratio of the keel fin with the rig and ballasting as considered before to study the trade-off between keel-chord reduction and t/c increase. It was found that the reduction of the keel chord was more important than the considered increase in t/c.
When the results obtained were analysed in conjunction with their individual ratings a different picture emerged. The IRM rule heavily penalises boats with a righting moment over a certain value, hence the series of configurations involving ballast increases which previously showed good speed have a very detrimental rating. The improvements over the basis design found after application of the rating were as follows:
- +3.5 s/nm Tall rig with area remaining constant;
- +6.3 s/nm Increased area rig;
- +9.8 s/nm Tall rig with increased area;
- +2.9 s/nm Keel fin chord shortened by 0.2 m, basis rig;
- +12.4 s/nm Keel fin chord shortened by 0.2 m, tall rig;
- +7.1s/nm Keel fin chord lengthened by 0.2m, tall rig.
These deltas are the averages across the wind strength interval of interest, for the Olympic Triangle Course.
We then assisted Hoek design in finalizing the design of keel fin, bulb and rudder.
Finally, we carried out helm angle calculations for different points of sail to check the location of keel and mast. (Our VPP also solves the equation obtained by equating the sum of the moments of all aero- and hydrodynamic forces in the horizontal plane to zero).