RATING OF SPINNAKERS BASED ON AREA CALCULATION AGAINST CLASS STANDARD.

Transcription

1 RATING OF SPINNAKERS BASED ON AREA CALCULATION AGAINST CLASS STANDARD. Prepared for PHRF-LO 2012 January. Revised 2012 June 1. Spinnaker Rating Systems Our present method of spinnaker area calculation and rating of spinnakers with dimensions other than class standard is based on adjusted luff length (SLL, LL, AMG) and girth (G) dimensions, taking into account excessive luff length and J (JSP, SPL or BSL). Any dimension exceeding the PHRF standard is normalised to an adjusted G dimension for rating purposes. While we do handicap a smaller than standard spinnaker size if the SPL and girth are simultaneously reduced, we don t give credit for a shorter than maximum size luff length. The adjusted asymmetrical spinnaker luff length is based on a 60% weighted luff plus 40% leech to calculate the allowed luff length. This will allow a somewhat longer standard luff, but if used in area calculations it will over estimate the area. All other presently used rating systems use an arithmetic mean for AMG. Furthermore, we calculate the spinnaker area as SA=1.8*I*J, where ISP and JSP, AMG etc. are substituted for non- PHRF standard spinnakers. This is a crude estimate Other currently used rating systems that are based mostly on VPP which determines their handicaps using actual sail area. Such systems are ISAF, ORC, ORR (IMS) and IRC. These area calculations were compared in my Spinnaker Study of See the table below; areas in ft 2. Class / Boat PHRF IRC ORC UK max 85% ISAF Ben. F 10R Sona CC 115 Timorian J 105 ODR Casual Elegance J92 S MAXSAND Ultimate 20 ODR Nitro Compiled by Adam Farkas Page May-28

2 The ORR formula gives the same numerical result as ISAF and ORC. IRC estimates still slightly smaller spinnaker area. In comparison with these VPP based handicapping systems, the PHRF-LO method substantially over estimates the spinnaker size. This significantly impacts the estimated speed generating potential of a spinnaker in relation to the rest of the sail plan. Based on a decision by the Central Council in 2012 January the study and recommendations will use the IRC formulas for spinnaker area calculations. Adopting the IRC formula: SA = ((SLU+SLE)/2 * ((SF + (4 * SHW))/5)*0.83 and substituting the PHRF-LO definitions, we get: Symmetrical: Equation A SA = 0.83 * MLL * (FOOT + 4*MG)/5 Asymmetrical: Equation B SA = 0.83 * AML * (FOOT + 4*AMG)/5. Where AML = (MLL + MLE)/2 * The proposal is to adopt these formulas for class standard and actual spinnaker area calculations, leading to a handicap adjustment table in similar fashion to the present FS adjustment table, but based on actual sail area as opposed to an adjusted G. The advantage of converting to an accurate area assessment from an equivalent girth measurement of spinnakers is that it uses the sail area parameter in similar fashion to a VPP, and will allow a further improvement in handicapping downwind speed potential in relation to a realistic total sail area calculation. Furthermore, as this method is more accurate and agrees with other handicapping systems, it will also form a better basis for handicap comparisons as well as SA/D and SA/LWL parametric comparisons gleaned from these other sources of information. * Recommended change to the PHRF-LO rules. Compiled by Adam Farkas Page May-28

3 2. PHRF Standard Spinnaker Area Calculate the PHRF standard spinnaker for both symmetrical and asymmetrical based on rig and boat dimensions as follows: Rig: I, J, class ISP and BSL measurements are known Luff: LL = 0.95*(I^2 + J^2) ^0.5 Max/Mid Girth Gmax = 1.8*J Foot length Foot = 1.8*J After substitutions in the above formula and simplifications for both configurations: Equation C SAstd = 1.8*0.79*J*(I^2 + J^2) ^0.5 Or for assessing sprit boats: Equation D SAsprit= 1.8*0.79*BSL*(ISP^2+BSL^2) ^0.5 Sprit boats can be reverse engineered to PHRF standard on sail area basis by: Equation E Asym factor = ASF = SAsprit / SAstd = (BSL/J)*((ISP^2+BSL^2)^0.5 / (I^2 + J^2)) ^0.5 and SAstd = SAsprit / ASF as a class standard for PHRF comparison purposes. The allowed max. luff is based on class standard ISP and BSL; AML = 0.95*(ISP^2 + BSL^2) ^0.5 and, Gmid = 1.8*BSL Over size pole and higher than I class ISP cases for symmetrical spinnakers can also be assessed in this manner, such as Farr 30 and 40 ODR. For Asymmetrical spinnaker only JT notes that the foot usually measures % of max girth. Compiled by Adam Farkas Page May-28

4 3. Measured / Non-Standard Spinnaker Area The actual sail areas are bases on sail measurements, not on rig measurements: Symmetrical: Equation F SAsym = 0.83 * MLL * (FOOT + 4*MG) / 5 MLL = measured luff length FOOT = measured foot length MG = the greater of measured Gmax or 1.8*SPL Asymmetrical: Equation G SAasym = 0.83 * AML * (FOOT + 4*AMG) / 5 AML = (MLE + MLL)/2 FOOT = measured foot length AMG = the greater of measured mid-girth or 1.8*BSL (or JSL) 4. Sail area Adjustments for ASP Since the standard sail area is based on 180% spinnaker area, the symmetrical SA% can be calculated as follows: SA% = 1.8*SAsym / SAstd For asymmetrical spinnaker SA%: SA% = 1.8*SAasym * ASF / SAstd For the time being use the existing table for adjustments, except substitute SA% for G%. Some refinement of terminology and definitions will be needed for use of these new algorithms in a revised manual, but they can be used for sample calculations for validation in the PHRF-LO database. Use the spinnaker area % for adjustments as previously was done on the basis of Gadj. This keeps it simple and easier comprehensible on the basis of previous understanding, except now it is based on area. Recommended change to the AML formula Compiled by Adam Farkas Page May-28

5 5. Assumptions and Issues A. Standard spinnaker dimensions are based on the SP list boat characteristics: I, J, ISP, JSP/JSL/BSL are used. The ASP calculator and ASP list are conflicting with this assumption, where ISP > I and JSP/JSL/BSL > J What is the PHRF-LO standard spinnaker definition? B. In the study the standard spinnaker is established on the basis of spars and boat dimensions, not on sail measurements or class rules for LL, MLL, MLE, Girth, AML, AMG, FOOT not used. C. Spinnaker size handicap adjustments for ASP are based on actual sail area measurements and calculations and rig and boat dimensions are not used. The resulting figures are areas and not adjusted girth sizes. D. Upon implementation of this new rule, any grandfathering decisions are up to the Central Council. E. Asymmetrical spinnakers fixed on the boat symmetry plane should get a built in credit of 6 s/m or an alternate size adjustment table. For example, the nominal 180% standard for symmetrical spinnaker and the related ASP scale can be 200% for asymmetrical spinnakers on a fixed BSL in similar size and T/A steps. F. The existing ASP adjustment table is ignorant of the fact that a small 200% spinnaker deserves a smaller penalty than a large 200% spinnaker, relative to the size of the mainsail. This is the reverse of the NFS credit, as the loss of a small spinnaker is less significant than the loss of a large spinnaker for a fair ASP adjustment. The difference could be significant, as the mainsail area in relation to the downwind sail area varies from about 13% to over 50%, depending on the individual class sail plan. This illustrates the dominance of the actual spinnaker sail area. This method is more accurate than assessing the spinnaker alone, but not easily transparent to owners and sailmakers, because of the relatively complicated calculations. BC sailing is using such method for all their ASP adjustments for jib, main and spinnaker penalties. I agree with their methods, but the actual adjustment table is far more punitive than PHRF-LO is using for spinnakers. Compiled by Adam Farkas Page May-28