BAMA Rating Analysis for 1999
by Thom Davis

Below is the rationale for changes to ratings based on 1999 race results. Following are some assumptions used in the rationale and a discussion of the assumptions' merits.

Assumption 1: PHRF (Time on Distance) works well for similar designs with similar characteristics. Consequently, PHRF can be effectively used to rate non planing keelboats. When used to rate different designs or boats that behave much differently depending upon wind direction and speed (for example planing), then PHRF is not as accurate as we would wish.

Assumption 2: Time on Time corrections provides a better estimate of the relationship between any two different designs; despite differences in wind speed or course direction.

Assumption 3: Skippers and crew can make a difference of up to 10% in overall boat speed through the course.

Assumption 4: This year's racing has been in conditions that are not typical Bay Area Conditions (Beaufort 5).

Assumption 5: Any rating changes that appear to be needed will be made by half of what is calculated.

Discussion of Assumptions:

Assumption 1 is actually more of a fact than an assumption. PHRF relationships between boats is a 1 over v relationship--that is, the seconds per mile needed by one boat to beat another with a PHRF handicap is inversely proportional to the boat speed for that race. Therefore, PHRF relationships between boats creates a situation where in light winds, boats with lower PHRF handicaps are likely to win while in high winds, the boats with higher PHRF handicaps are more likely to win. As an illustration of this point, on the last page look at the PHRF rating sailed for Beowulf in the Delta Ditch Run and Jazz Cup (both downwind races). The DDR had winds of 20+ while the Jazz Cup had winds of 5 kts or less. Beowulf sailed in high winds to a rating of +18 while in low winds sailed to a rating of -291.

Assumption 2 is not accurate. You can consider multihulls as belonging to three classes. 1) Boats that do not exceed hull speed (or do so very rarely). 2) Boats that exceed hull speed in certain specific conditions (such as wind speed or direction). and 3) Boats that exceed hull speed in most conditions. It is obvious, given these classifications, that a single relationship between these three different types of boats is impossible in all conditions. However, with most races run in the 8-20 kt wind speed range and on three legged courses with equal windward, leeward and reaching legs, TOT can be more accurate over a wider range of boat classes than PHRF.

Assumption 3 has a corollary. Since skippers and crew make a difference (and if they don't why do we race?), then rating is not subject to rigorous statistical analysis.

Assumption 4 is somewhat true in my experience this year. There were numerous wind holes with wind filling in from odd directions. As a result of this assumption, however, we must recognize that PHRF sailed numbers will average above the current assigned PHRF.

Assumption 5 is to reduce the likelihood of overcorrecting. Particularly since there may have been few races for comparison, this seems a prudent precaution.

1999 Races and Results

We had excellent participation in races this year by three specific different boat designs. F25C, Seawind and F27. Lets look at how they did compared with one another before we go further in other designs. These three can then become the basis for any needed changes to other multihulls.

The F25C and F27 raced against one another in 13 different races. Because of assumption 2, we are interested in knowing how the predicted speed ratio (expressed as a ratio of TOT CFs) of the F25C compared to the F27 and the difference between the actual speeds over the course. The predicted speed ratio is taken from the Farrier National TOT numbers and is the ratio of 1.2218 for F25C and 1.0656 for F27 (stock). We predict, therefore, that the F25C should be about 15% faster over a course than the F27. The speed ratio tables on the following pages shows the difference from predicted. The F25C was a high of 23% faster than predicted (that is, it was actually 23 + 15 or 38% faster) when compared to the F27 in the Vallejo 1 race but was 14% slower than predicted in the IC2 race (that is, they raced nearly head to head). Overall, the average difference between the F27 and F25C is 1%. Because of Assumption 3, no rating change is required.

The F25C and Seawind raced against one another in 8 races. The TOT CF for Seawind is 0.8765 so we predict that the F25C should be about 40% faster over a course. As seen in the speed ratio chart, the Seawind appears to be on average about 1% faster than predicted, but because of assumption 3, no rating change is required.

The F27 and Seawind raced against one another in 10 races. The predicted speed difference is that the F27 should be about 22% faster than the Seawind over a course. On the average over the year, however, the F27 was in fact 5% faster than predicted. However, because of assumption 3, no rating change is required.

Finally, let's look at these three boats as compared to keelboats on the same course. The Rating Sailed Summary shows that the PHRF rating for the F25C on 8 races is within 3 seconds per mile of the current assigned PHRF. The PHRF rating for the F27 over 9 races is off by a whopping 48 seconds per mile. However, looking into greater detail shows that three of those races were downwind (Vjo 1, DDR and Jazz Cup). Averaging the remaining 5 races gives a PHRF difference of 25 seconds per mile. It appears, that at least this year, the F27 was a slow boat downwind. Seawind sailed on the same course as keelboats in only 4 races; and 3 of those were downwind races. One point of interest-Seawind consistently outperformed her predicted speed ratio in the three downwind races.

Given that the average rating sailed this year has been 19 seconds per mile slower than the current assigned PHRFs, and given the fact that multihulls did not fly spinnakers in the IC race series, and given assumption 4, no changes to PHRF ratings are warranted to the above three boats. However, a couple of things bear further investigation for the coming years. The F27 and Seawind both perform differently than predicted in downwind races. The F27 more poorly and the Seawind better. This might be something to consider before sending in your entry fees (or appeal for a rating change before specific downwind races).

Summarizing the three "anchor" boats. Seawind is faster than predicted by 1% when compared to F25C. Seawind is slower than predicted by 5% when compared to F27. Therefore, no change to Seawind is warranted (assumption 3). F27 is faster than predicted by 1% when compared to F25C. F27 is faster than predicted by 5% when compared to Seawind. Therefore, no change to F27 is warranted (assumption 3). F25C is slower than predicted when compared to Seawind (1%) and slower than predicted (1%) when compared to F27. Therefore, no change to F25C is warranted (assumption 3).

Now that we've looked at our most prolific participants, let's look at the remainder and use our prolific participants as the benchmarks.

Antrim is probably faster than we've assigned. She shared the course with an F27 and Seawind in 6 races. However, her average speed difference is 2% over F27 and 8% over Seawind, so because of assumption 3, no rating change is warranted; but she is probably about 5% faster than the rating we've assigned.

Indigo (Dragonfly 800) competed with Ja Mon in 4 races, with an F27 in 5 races, an F28R in 1 race and PDQ 32 in one race. She was overall 2% faster than predicted compared to F25C. She was 11% faster than predicted compared to F27. No rating change is warranted at this time because of assumption 3 and because the bulk of the difference is due to two races (IC1 and 8/1 HDA). However, current data indicate that Indigo may be as much as 6.5% faster than our ratings allow for.

F28R shared courses with an F27 in 2 races, Seawind in 2 races, F24MKII in 1 race. She was 2% faster than predicted compared to F27 and 1% faster than predicted compared to Seawind. No rating change is warranted (Assumption 3).

F24 MK I shared a course with an F27 in 3 races and F25C in 2 races and Seawind for 1 race. She was slower than predicted by 8% compared to F27 and slower than predicted by 5% compared to F25C and 18% faster than predicted compared with Seawind. Since the Seawind comparison is from only one race (where Seawind did not do well) we will ignore that race. F24 MK I is slower than our two anchor boats by an average of 6.5%. Because of Assumption 3, no rating change is warranted at this time. Randy and Roger, I will continue to review race results with a rating change in mind for your boats.

PDQ 32 shared a course with F25C in 2 races, Dgnfly in 1 race and F27 in 2 races. She is 12% slower than predicted compared with F25C and 8% slower than predicted compared with F27. The PDQ32 was slower than predicted an average of 10%. Consequently, her TOT CF and PHRF ratings will be adjusted.

Calculation: PDQ32 and Seawind are the same "class" of boat (that is, boats that do not normally exceed hull speed). Her current assigned TOT CF is 0.8099, Seawind's is 0.8765. The ratio of the two boats is predicted to be 1.082. However, PDQ32 is 10% slower than predicted (but we will only adjust for half of that (Assumption 5)). Consequently, the speed ratio is 1.136. Working backwards, then the TOT CF of PDQ32 is 0.8765/1.136 or 0.7716. The new PHRF for Concerto in Sea, therefore (corresponding to TOT CF of 0.7716) is 231 seconds per mile (from 195 seconds per mile, which is her current assigned PHRF).

APPEALS

YRA provides the following process for appealing a rating. BAMA will follow this process as well. A yacht’s rating or a competitor's rating may be appealed. There shall be a fee for filing an appeal to cover the administrative costs. When the rating of another yacht is being appealed, a copy of the appeal will be sent to the owner of that yacht, who will not pay a fee. The owner of the yacht whose rating is being appealed will be asked to rebut the appeal prior to the meeting at which the appeal will be heard. All appeals and rebuttals shall be in writing and provide evidence and/or rationale for and against the appeal, as appropriate. All documents shall be provided to the BAMA Ratings Chair at least two weeks prior to any appeal meeting.

Summary:
 

	Average
Antrim/F27	2%
Antrim/Seawind	8%
F25C/F27	-1%
F25C/Seawind	-1%
F25C/DGNFLY	-2%
DGNFLY/F27	11%
DGNFLY/F28R	-3%
F28R/F27	2%
F28R/Seawind	1%
F27/F241	8%
F27/Seawind	5%
F25C/PDQ32	12%
DGNFLY/PDQ32	0%
F27/PDQ32	8%
F25C/F241	5%
F241/Seawind	18%
F28R/F27F	12%
F28R/F242	6%
F27F/F27	0%
F27F/F242	-5%
F27F/Seawind	-11%
F27/F242	-5%
F242/Seawind	-4%

PHRF Rating Sailed Compared to Keelboat when On the Same Course
 

Class	JK Frst	VJO 1	VJO 2	ENC TI	PT Bonita	DHF	DDR	IC3	Silver Eagle	Jazz Cup	8/1 HDA	Knox
F25C		-57	-51	-72	18	-56		-15	-33			-57
Antrim 30+		-57	-51			-99
Seawind 1000		168				102	174			222
DGNFLY 800				33	9			60			-39	-36
F27	8	120		93		30	126	54	36	120	52
24 MK 1	90	144
24 MK 2										87
Beowulf							18			-291

SUMMARY:
 

Class	Average	Current PHRF	Difference
F25C	-42	-45	3
Antrim 30+	-69	-45	-24
Seawind 1000	167	141	26
DGNFLY 800	2	12	-10
F27	69	21	48
24 MK 1	117	45	72
24 MK 2	87	33	54
Beowulf	-137	-120	-17
		Average	19

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