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      A Few Simple Rules   05/22/2017

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n.deroulhac

Portsmith rating system

16 posts in this topic

Hey,

 

Just looking at the portsmith rating system, and I am assuming that the numbers do not translate to seconds. How are boats rated and scored?

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From USSailing:

 

Portsmouth Yardstick Handicapping System - The Portsmouth system is a time-on-time system that uses the Thistle as the standard boat in the United States. While PHRF generally addresses what we term as "offshore boats", Portsmouth addresses up to medium-sized production cruiser-racers as well as multihulls and centerboard classes. Each boat is assigned a series of numbers that are applied based on the average wind conditions of the race. Using the Portsmouth system corrected time (CT) is calculated using: CT = ( ET / HC ) x 100

 

ET = Elapsed Time

HC = Applicable Portsmouth Handicap number

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RYA UK Portsmouth, the formula is similar but x1000 instead.

 

Handicaps are results derived. In the UK, clubs are encouraged to send in detailed results for all handicap races they run (and digital systems are now being set up to help this). The UK doesn't have a specific "reference" boat, but there's a subset of numbers termed "Primary"- these are classes with large amounts of historic data, and the numbers rarely change.

New classes get a suggested number, usually from the manufacturer/designer, which is a priority for review as soon as data is received.

Clubs are encouraged to change any numbers that they feel are wrong for them.

 

The RYA has more than you could want to know on their site

 

It is most definitely NOT a measurment rating system.

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In trivia, the reason why corrected times are divided by the handicap, not multiplied as would be easier, is that the guy who devised the system (long before calculators) wanted to sell books of tables which could be used to work out corrected times. He knew if he used a multiplier everyone would do the calcs with mental arithmetic or a piece of scrap paper and he wouldn't sell any books. Division is that that much harder, so he sold lots of copies!

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You got any good pot?

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I think the Thistle was used as the base boat. Not that really helps in understanding the rule but there you go.

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Put simply, Portsmouth is a ratio.

 

As a "time on time" (time-based) handicap system it gives a base handicap that gives an idea of how long a boat would take to complete a course compared to a baseline "scratch boat" (a bit oversimplified).

 

So if your boat rates "80" it would take 48 minutes to cover the course that the scratch boat (Thistle, for example) covers in an hour.

 

For most boats (except small dinghies and 4-knot SB's) the calculated time is bigger (longer) than the elapsed time.

 

Portsmouth can be used as the basic handicap number (D-PN) or with handicaps that are modified for how well boats do in different wind ranges (referenced by Beaufort wind force range numbers; i.e. BN 2-3 = Force 2, pleasant mild breezes with wavelets on the water).

 

Portsmouth is convenient for many clubs because it rates small craft, which aren't covered by many "big boat" handicap systems. It's also fairly easy to manage since it's based on data tables maintained in the USA by US Sailing; there are no local handicap boards and normally no measurements involved.

 

Different boats and crews may do better with different handicap systems, depending upon wind, water, course length, course composition (windward leeward vs. offwind), the competitiveness of different fleets that report their results to handicap boards), and even how well one can politic the handicap boards or measurers in some systems.

 

And, it's spelled "Portsmouth".

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Its the original formula for incorrectly comparing the speed of different boats.

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Its the original formula for incorrectly comparing the speed of different boats.

 

And, ironically, still one of best.

 

 

Put simply, Portsmouth is a ratio.

 

As a "time on time" (time-based) handicap system it gives a base handicap that gives an idea of how long a boat would take to complete a course compared to a baseline "scratch boat" (a bit oversimplified).

 

So if your boat rates "80" it would take 48 minutes to cover the course that the scratch boat (Thistle, for example) covers in an hour.

 

 

Having raced under Portsmouth the last 3 years, the thing I find annoying is I can't get my head around a 'sense' of the relationship of the handicap number to boat speed. For example in PHRF, a boat with a 100 rating and a boat with a 150 rating, I can easily understand the 50 seconds per mile difference. I get a sense of how fast the boats are compared to each other, and can figure in my head (roughly) how much time I need to win.

 

Can't do that with dpn, the numbers just don't correlate to anything useful. But, the guys who have raced Ports their whole lives "think" in dpn numbers naturaly and don't understand my confusion. (I don't even think they understand what I'm talking about, they just 'know' what the numbers will mean in terms of boatspeed. ) So I always convert the ratings to phrf so I can 'see' relative ratings...

 

An example, my boat rates 81.4 and a competitor 61.9. His boat is 115 seconds per mile faster!

 

I'd love to come up with a quick mental calc method of 'grasping' the dpn numbers. Anybody got such a thing?

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When I was a kid in the UK it was in 100s and I think the osprey class was the reference with a py of 100

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You should check out the IRC web site.... This is also a time on time rating system and they publish calculators that will give you the time deltas that you want for two boats.... just use the PN ratings.

 

When your elapsed time is converted into portsmouth time and compared to other boats in the fleet...The rankings are determined but the time differences are a bit meaningless to you.

 

What you want to know is how much time you needed to save per hour on your competitor in real minutes/secs. Your results sheet should publish the back cacluated elapsed time which tells you how far back of the winner in real time your particular boat finished. this equates to time spent going in the wrong direction, slow tacks, and the second tier start! (Sailwave is the Scoring Program the beach cat world (not to mention the Olympic pre regattas) recommends))

 

Here is a link to tables for single handed catamarans. 3 measurement rating systems are published as well... DPN is the single number used for any windspeed. It's not as accurate as using the rating that applies to the actual wind speed of the race. For beach cats... the performance differences are huge when you can fly a hull or not... (This is similar to a boat that can plane and dinghies also have wind speed PN ratings.) DPN and Wind adjustment columns are included in the tables.

DPN is the single number used when wind speeds are not measured.

 

Single handed cat tables

 

 

For the USPN table... the Hobie 17 owes the Dart 18 (1 up) 19 secs an hour of racing in DPN racing. (the times are all relative to the slow boat in the table)

 

The F16 (1up) owes an A cat about 51 seconds per hour of racing in moderate breeze (Beufort 2-3)

 

Its the original formula for incorrectly comparing the speed of different boats.

 

And, ironically, still one of best.

 

 

Put simply, Portsmouth is a ratio.

 

As a "time on time" (time-based) handicap system it gives a base handicap that gives an idea of how long a boat would take to complete a course compared to a baseline "scratch boat" (a bit oversimplified).

 

So if your boat rates "80" it would take 48 minutes to cover the course that the scratch boat (Thistle, for example) covers in an hour.

 

 

Having raced under Portsmouth the last 3 years, the thing I find annoying is I can't get my head around a 'sense' of the relationship of the handicap number to boat speed. For example in PHRF, a boat with a 100 rating and a boat with a 150 rating, I can easily understand the 50 seconds per mile difference. I get a sense of how fast the boats are compared to each other, and can figure in my head (roughly) how much time I need to win.

 

Can't do that with dpn, the numbers just don't correlate to anything useful. But, the guys who have raced Ports their whole lives "think" in dpn numbers naturaly and don't understand my confusion. (I don't even think they understand what I'm talking about, they just 'know' what the numbers will mean in terms of boatspeed. ) So I always convert the ratings to phrf so I can 'see' relative ratings...

 

An example, my boat rates 81.4 and a competitor 61.9. His boat is 115 seconds per mile faster!

 

I'd love to come up with a quick mental calc method of 'grasping' the dpn numbers. Anybody got such a thing?

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OK...this thing makes my head hurt.

 

Two Questions for the brethren.

 

- does anyone have the conversion from RYA Portsmouth Yardstick to US Sailing Portsmouth Yardstick?

 

- if you do the math for your boat (assuming there's a portsmouth number) converting from PHRF to Portsmouth, does it seem the numbers are wildly out of place?

 

Viper rates 70.4 US Sail and 894 UK

 

The conversion (PHRF = 6 * DPN - 330) leaves the Viper with a PHRF of 92.4. The worst Viper PHRF rating in the US is SF Bay with a 96. Most areas have it at 102.

 

Mixed sportboat fleet racing is pretty damn hard to find an equitable rating scheme.

 

SMS really doesn't work well for bigger spritboats. PHRF sucks all the way around (though I'm much more familiar with it) and Portsmouth is just a fucking mystery to me.

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Its the original formula for incorrectly comparing the speed of different boats.

 

And, ironically, still one of best.

 

 

Put simply, Portsmouth is a ratio.

 

As a "time on time" (time-based) handicap system it gives a base handicap that gives an idea of how long a boat would take to complete a course compared to a baseline "scratch boat" (a bit oversimplified).

 

So if your boat rates "80" it would take 48 minutes to cover the course that the scratch boat (Thistle, for example) covers in an hour.

 

 

Having raced under Portsmouth the last 3 years, the thing I find annoying is I can't get my head around a 'sense' of the relationship of the handicap number to boat speed. For example in PHRF, a boat with a 100 rating and a boat with a 150 rating, I can easily understand the 50 seconds per mile difference. I get a sense of how fast the boats are compared to each other, and can figure in my head (roughly) how much time I need to win.

 

Can't do that with dpn, the numbers just don't correlate to anything useful. But, the guys who have raced Ports their whole lives "think" in dpn numbers naturaly and don't understand my confusion. (I don't even think they understand what I'm talking about, they just 'know' what the numbers will mean in terms of boatspeed. ) So I always convert the ratings to phrf so I can 'see' relative ratings...

 

An example, my boat rates 81.4 and a competitor 61.9. His boat is 115 seconds per mile faster!

 

I'd love to come up with a quick mental calc method of 'grasping' the dpn numbers. Anybody got such a thing?

 

You seem to be trying to use a time on time system as a time on distance system. Excuse my ignorance of your systems but I don't think that will work.

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If you make assumptions about some sort of supposedly average conditions you can use the rough conversions between TOT and TOD handicap systems to compare the "feel" of the different kinds of ratings. Since each handicap system has built-in assumptions, and since the systems already don't do well at comparing boats that aren't similar, you could wind up piling assumptions on top of assumptions. I suppose you could attempt to compare boat ratings between TOD and TOT systems by making multiple comparisons of systems while changing the assumptions about wind speed, course type (random leg vs upwind vs offwind), sail inventory, etc. Spend enough time at it and you'll never have to worry about having time to go sailing.

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I know it would be tough for most local race committees, but if the scorers would use the correct rating for the wind conditions PY should easily be the fairest and most accurate of the handicap systems. I would really like to encourage it's use and I honestly believe a lot more people would come out and race small boats if more clubs offered Portsmouth fleets as a part of their open regattas.

 

In the US the base boat WAS the Thistle. If you are confused on how the scores work just consider how you would do in a given wind condition against someone sailing a Thistle.

 

RD

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Does a conversation from RYA py to USsail PY even exist?

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