IRC Rating - Dufour 34

Post Reply
Posts: 20
Joined: Wed May 11, 2011 6:10 am
Location: South Africa

IRC Rating - Dufour 34

Post by Bruce3477 » Sun May 15, 2011 12:39 pm

How is it possible that this yacht has the worst rating in the entire approx 34ft fleet, and is almost impossible in the normal course of events to sail to it’s rating #?
Average rating for 2003-2008 Dufour 34 Performance is IRC# 0.980.
(Not the later D-34e with T-keel and lighter mass etc which is about IRC # 0.992 approx).
A bit of history on this situation:
The prototype D-34 was based on a design specification of 4.7 ton empty mass. IRC methodology has always rated the yacht partly on formula and partly on the much debated HF (Hull Factor). At 4.7 tons, the HF should have been around 8.0 with a decent cruiser ‘bulb’ keel. At 5.5 ton and with same cruiser bulb, HF should be about 7.1 to 7.3 at maximum.
Holiday 34 DLR 191 HF 7.1
Dufour 34 DLR 197 HF 8.3
When the production build came out at 5.5 tons, it had already been ‘test certificate ’ rated with an ‘expensive’ HF based on 4700 kg, and the ‘accommodation factor/HF’ story was left at 8.3. This 8.3# was more in line with a 4.2 ton 34 ft cruiser/racer… with little relevance to the D-34 actual cruiser build.
X-332 DLR 179 Downwind SAD 0.0245 HF 8.3
L-34 DLR 164 Downwind SAD 0.0214 HF 8.4
D-34 DLR 197 Downwind SAD 0.0190 HF 8.3
Now, in the normal course of events, the rating would be ‘balanced*’with the HF increased and overhang factor decreased to equal out the boat performance against the target HF#, but in this case was charged with an ‘expensive’ HF and then ADDED the adjustment to the overhang factor instead of subtracting it.
Normal DLR for D-34 compared to H-34, about HF 7.3 based on DLR, Sail area, HF about 1% overstated
The HF would be ‘overstated’ here by approx 1.0% before the overhang ‘balance*’ factor.
With most c/r in this size with similar stern overhangs, the overhang factor would have been about 1.01 (first 5% is free, effective about 1.06 in real #), but D-34 rated at 1.04 ( effective 9% waterline overhang). The total stern overhang (SO) is only 6.4% of LWP, and with the DLR adjustment* based on approx #200, would have rated an overhang % of just over 6.0, an effective overhang factor of about 1.01.

Overhang factors:
X-332 LWP 8.78actual SO 0.81 Y 0.18 SO/LWP% 9.22 Overhang factor 1.030
L-34 LWP 9.07 actual SO 0.56 Y 0.09 SO/LWP% 6.17 Overhang factor 1.000
D-34 LWP9.35actual SO 0.60 Y 0.17 SO/LWP% 6.41 Overhang factor 1.040
Both the X-332 and L-34 have 2.5 times (compared to D-34) bow overhang, and with raked bows, will have much more “bow” immersed waterline…
The L-34 with similar SO to D-34, but with ½ the “y” measure, will immerse its full waterline a lot quicker and in lighter conditions, but the D-34 only as approach its hull speed in much stronger conditions. i.a.w. The L-34 and D-34 should both have an overhang factor or 1.000!
The front overhang with “ims bow”, and where the ‘x’ and “h’ are measured suggests * defaults to greater of ‘h’ measure if ‘h’ > 50% of ‘x’, or 50% of ‘x’, onto the immersed waterline calculation. The bow immersion factor would be 0.9% in this instance.
The average immersion of a raked bow is about 15% of BO, and impact on waterline speed about 3% to windward.
The combined summary of stern and bow immersion will be about 7%, but where the Overhang factor (1.04) reflects a 9% overhang (4% plus 5% - 5% defaults to 0%).
This alone would have overstated the immersed waterline by about 2%/190mm in calculation of TCC#...
With HF requiring a linear (counter) relationship to square root of immersed waterline in this situation, the anticipated correction should have been : actual overhang 1.07 (7%) less HF adjustment 1% = approx 2% waterline, and which should have reduced corrected overhang actual to 1.05 (5%), and to net overhang factor 1.00.
The cumulative correction for all of above would have an approximate reduction of overhang factor from 1.04 to 1.00, and should have an impact of about (-) 2% on TCC...
Then the D-34, and in common with most post 2000 Performance Cruisers, have a very short ‘stub’ fractional rig and very high booms(to keep lawyers away when crew are ‘decked’). Now in the formula rating line, and as has been used for most of past century, the square root of LL/P is charged to the rating # to allow for off-wind and overlapping advantage of large overlapping headsails. Now in the normal course of events, a masthead rig would have a LL/P of about 1.10 and be charged with about 3% extra rating, and have a sail area relationship of mainsail/genoa of about 40:60. Proper fractional rigs with LL/P at 1.00 or less don’t have a cost (get credit as drop below 1.00) and have mainsail/genoa relationships at about 50:50. The average post 2000 Performance cruiser with ‘stub’ fractional rig and high boom is read by model in range 1.07 to 1.12 and charged with penalty for overlap like a masthead rig, but sail area relationship is often only 50:50 to 60:40 and does not perform anywhere like a masthead rig.
Fractional Rigs
X-332 “LL max” 12.92 “P”13.14 “LL/P” 0.98 Main m2 35.7 Genoa M2 32.95 MSA:JSA 1.08:1
L-34 “LL max” 11.33 “P”12.5 “LL/P” 0.90 Main m2 31.95 Genoa M2 30.17 MSA:JSA 1.06:1
D-34 “LL max” 13.30 “P”12.20 “LL/P” 1.09 Main m2 34.37 Genoa M2 32.80 MSA:JSA 1.05:1
i.e. The Dufour has the lowest overlap (by area), but 11% greater LL/P than X-332 and 22% greater LL/P than L-34. With default LL/P at 1.00, and based on sq.rt of LL/P as impact on rating line, the D-34 would be overstated 3% on rating and the L-34 3% understated on rating.
Accordingly, the Dufour (and similar) would have overstated rating here of 2.5-3.5% rating #
The impact of all the above would put the Dufour 34 at around 4.5 to 5.0 % above a ‘real world on-the-water’ rating #.
Being 1% on HF, 4% on immersed waterline, and 3% on LL/P measure!
In USA PHRF terms, the D-34 is scratch at 117 sec/mile and is usually rated at about 126 sec/mile with roller furler and internal cruiser build. This would equate to about IRC # 0.940 for a 2003-2009 D-34 Performance model.
Compared to X-34 and J-92’s with similar rating #’s 0.986 to 0.990, both these yachts are hugely faster than D-34 by about 3-5%+ on normal W/L course…with the J-92s on any form of off-wind leg an easy 10 to 15%+ faster! The J-92’s is also in bonus ** territory as with a downwind sail area/empty mass > 2.5% (metric M2 &Kg), receives extra 2% off rating...
D-34 DLR 197 Downwind SAD 0.0190 HF 8.3
J-92S DLR 153 Downwind SAD 0.0430 HF 9.1
And then when have to race in smaller fleets and/or same division, may compete against other larger yachts such a Farr 38’s with #3 jib ( 6 ton, 14% more sail area and 7% more waterline ) at similar IRC rating # 0.986. This would not make sense in linear rating systems, but get back 2% off rating ** if yacht’s LWP measure is 10.00m +
D-34 DLR 197 Downwind SAD 0.0190 HF 8.3 LWP 9.35 LWO+SO 9.910
Farr 38 DLR 150 Downwind SAD 0.0217 HF 8.0 LWP 10.09 LWO+SO 10.62
Farr 38 stern overhang 5.2% Overhang factor: 1.000
i.e. A Farr 38 with #3 jib has 75% of DLR (faster), 14% greater downwind area, 7% more waterline… and has the “same” rating # as D-34
And don’t forget the Archambault A-31, which rates at about similar 0.988, which has the extra trick of the “overhangs” ( (J-BO)/LWP) > 0.4…less 1.4% for this feature…. And with extra 1.4% for downwind SAD > 0.025, and with extra 1.4% for downwind SAD > 0.035… would be about 4.2% comparatively underrated. History would be made if a D-34 could stick with an A-31 downwind in any sort of breeze!
A-31 DLR 143 Downwind SAD 0.032
D-34 DLR 197 Downwind SAD 0.019

The Irish ECHO system, based on IRC # with PHRF adjustment, puts the D-34 at 0.948. This is about 4.2% under its IRC#, and about where it sits with normal c/r (such as X-332) fleet on the water. But said X-332 has little chance against the other specialized IRC boats detailed above…
Guess you could say the D-34 has been given an impossible task!

Post Reply