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Dufour 34 Performance Cruiser - The IRC Rating LL/P anomaly.
Following on from my study of Dufour Performance Cruisers vs. various regression rating systems (including IRC), with the sometimes huge variances between Dufour 34 Performance Cruiser and similar yachts such at X-34, and also compared to the the data IMS type VMG models. Most single number regression formula?s use a combined sail area in model (with various weightings such as main at 100% and foresail at 60% to allow for upwind tacking, and 40% for downwind with spinnaker). But, these regression models have one or more extra tweaks to allow for genoa overlap advantage and offwind angles too close for spinnaker to be factored into general regression #.

This is usually a measure (multiplied) of LL/P or FL/P in the regression equation. With a masthead rig, and the P measure always being substantially lower measure than LL or FL, and with a net LL/P relationship of about 1.1, this ratio will always increase the regression formula rating of masthead yachts. In the case of fractionally rigged yachts, the normal ratio (IRC optimized) would be about LL/P of close to 1:1, so very little to no adjustment in this case. But some of earlier fractionally rigged yachts from the mid 80?s to early 90?s have below parity relationships (0.875 to 0.95)with big P measure?s relative to LL measure and sometimes show a very nice IRC Rating in spite of a mix of other not so advantaged features such as poor spinnaker ratio bias etc. This feature was very common to said Lavranos, Simonis, smaller J-Boat designs, Laser 28 and classic?s (such as Swede 55 style) fractional rigs over this period.

When we get the post 2000+ Performance Cruiser fractionally rigged cruisers, with style of IRC racers (but with raised boom to prevent cruiser?s crew getting decked) and with short measures of mast above hounds, we end up with masthead style measures of LL/P or FL/P of about 1.05 to 1.1. This regression model would then ?read? this LL/P measure as a high overlap bias and give it same rating penalty as a conventional masthead rig. The only difference here is that a masthead rig would have a genoa /mainsail relationship of about 60:40 (with big drive in slot and increased close wind power), while the Performance Cruiser would be about 50:50, the same as optimised IRC fractional yachts?.. At the displacement speed region of the fleet, said rating # anomaly?s become very visible.

Obviously a LL/P factor of 5 to 10% above or below median point is a high number, but all sail area ?power? models work on numbers in regression equation of somewhere between the square to cube root of sail area in said regression equation. In the real world of IRC Rating #?s, a masthead rig with normal 60:40 genoa /mainsail bias would rate about 2.5% to 3.3% above a fractional rig.

Perhaps racing fleets will see the IRC Ratings of Performance Cruisers with bad LL/P #?s in a new light?