How is a base PHRF calculated? - SailNet Community
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post #1 of 9 Old 09-17-2013 Thread Starter
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How is a base PHRF calculated?

Just curious if there is a method or formula used to calculate a base PHRF for a boat that does not have any racing history.

s/v Siren Song 1994 Precision 28
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post #2 of 9 Old 09-17-2013
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Re: How is a base PHRF calculated?

Found this:

Quote:
R' = 610-8.36*(SA/Disp^.333)+0.0000511*(SA^2)-55*(P/(J+E)) -30.8*(LWL^.5)-602*(DR^2/SA)

where SA= .5*(I*J)+.5*(P*E)

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1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
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post #3 of 9 Old 09-17-2013 Thread Starter
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Re: How is a base PHRF calculated?

Cool thanks. Where did you find it and what is DR?

s/v Siren Song 1994 Precision 28
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post #4 of 9 Old 09-17-2013
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Re: How is a base PHRF calculated?

"DR" is draft. PHRF committees will use the formula pretty religiously when calculating the base rating to production boats. They will get out their yardsticks for custom, one off type boats. They will not change your rating after a season unless 1) You have extensively modified your boat necessitating them to break out their yardsticks again, or 2) You have a highly political committee who wants to favor someone else’s boat. Your local YRA will often "tweak" the formula to match their prevailing wind conditions. For example, my boat base calculates out at a 144, I race as a 147 on San Francisco Bay, if I were to race in Boston I would rate something like 154. I am assuming your boat is a Precision 28? Where do you sail?

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post #5 of 9 Old 09-17-2013
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Re: How is a base PHRF calculated?

Quote:
Originally Posted by mmazour View Post
Cool thanks. Where did you find it and what is DR?
http://www.well.com/~pk/YRAphrf-sf.html

Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
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post #6 of 9 Old 09-17-2013
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Re: How is a base PHRF calculated?

Take the weight of a unicorn and devide it by the combined weight of two leprechauns.

Then say fock it all and make something up. Usually higher numbers for old sailing buddies of the committee members.
night0wl, FarCry and Cap'n Russ like this.

Merit 25 # 764 "Audrey"
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post #7 of 9 Old 09-17-2013 Thread Starter
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Re: How is a base PHRF calculated?

Shoot, I am fresh out unicorns BBQ'd the last one Saturday after the race in celebration of my 4th and 5th pre-PHRF finishes, which wound up 3rd and 3rd after PHRF correction.

My boat Precision 28 is PHRF'd by our fleet at 198 but there is a 32 Hake Seaward that has a PHRF of 208. This seemed odd to me just based on the old theoretical hull length calc so I wanted see a formula that would give me an idea of how this works. According to the formula provided by Faster my boat is a 208 and the Hake should be 163.

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post #8 of 9 Old 09-18-2013
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Re: How is a base PHRF calculated?

Then there is the local how do you sail it after a period of time. Hence why the Hake is probably rated slower. The person racing it, is a bit heavy, bad sails, eventually the rating gets slower! That has happened with a dufour 34 locally! A rating in the mid 190 range! Yeah it has smallish sails, shoal draft....but still, a deep draft with proper race sails would be in the 100-120 range! I know I beat it on skill, not due to it being slow!

Marty

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I drives me dinghy!
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post #9 of 9 Old 09-19-2013
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Re: How is a base PHRF calculated?

Hi guys there is a base rating for Precision 28s nationally. The lowest is 192 in West Florida and the highest is 203. Average is 195 to 201.

The Schell regression formula can be a starting point for determining a rating. What it doesn't take into effect is sail area/wetted surface, foil efficiency, and pitching gyradius. Remember that quoted waterline lengths are deceiving and depend a lot on aft overhangs. To get a lot better idea of what the effective waterline length of a boat is add the length on deck and waterline length and divide by 2. That is a rough rule of thumb but works out.

Looking at the Hake, it is all waterline with a transom hung rudder and no overhangs. I highly doubt its 8300 lb all up weight. Most likely it is way heavier. The hull shape looks pretty beamy with max beam forward. Lots of wetted surface and small small area make for a slow boat. On the other hand the Precision is a Jim Taylor design who has designed several IMS race boats and also the late model Sabre Yachts line. Way better design pedigree and with a SA/D of 17 and light weight it should be a pretty fair rating at 198.

My take is that the Precision is an all around performer and the Hake does well on a reach when the wind speed approaches 15 and above. 9 times out 10 I'd rather race on the Precision at that rating.
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