The Portsmouth Yardstick, Explained

The Portsmouth Yardstick is what dinghy and small multihull clubs use instead of PHRF. If you have ever sailed a handicap start on a lake, in a frostbite series, or at a club with more boat types than fleets, you have been scored on it.

Here is what the number actually means, the one formula that uses it, the wind-band feature that PHRF has no equivalent for, how much to trust any given rating, and how to run it at a club without inheriting an argument.

Where the number comes from

The system started at Langstone Sailing Club in England in 1947, where Stanley Milledge needed a way to race dissimilar boats and pegged everything to an Island One Design set at 100. The Royal Yachting Association adopted it in 1960. In North America the Dixie Inland Yacht Racing Association built its own version on the RYA scheme, which is why you will see the ratings called D-PN, for Dixie Portsmouth Number, and administration passed to what is now US Sailing in 1973.

The important word in the official handbook is empirical. Portsmouth is not a measurement rule. Nobody measures your sail and computes a number. The Portsmouth Numbers Committee collects finish times from real races at real clubs across the country, regresses them against the boat's principal dimensions, and publishes what the fleet actually did. A PHRF rating is a regional handicapper's judgement about your boat. A D-PN is a national observation about your class.

The reference boat is the Thistle, at exactly 83.0, chosen in 1961 for its strong class organisation, wide geographic spread and varied racing. Everything else in the tables is calibrated against it.

The arithmetic is one line

A Portsmouth number is a time. The handbook puts it about as plainly as it can be put: a boat rated 80 will sail the same distance in 80 minutes that a boat rated 90 sails in 90. So a higher number is a slower boat, and correcting a race is one multiplication:

corrected time = elapsed time × 100 ÷ D-PN

That is the whole system. A boat rating exactly 100 sails its own elapsed time. Everything faster than that has time added, everything slower has time taken off. Send four classes round a course that a notional 100 boat would sail in an hour, and if each one sails exactly to its rating, they all correct to the same second:

Notice what is missing from that formula: the course length. Portsmouth is a pure time-on-time system and never asks how far anybody sailed. If you have read our guide to PHRF scoring, you will recognise the advantage immediately. There is no distance to measure, no distance to get wrong, and shortening course costs you nothing but a shorter race. The same multiplier applies whatever length the race turns out to be.

It rates the class, not your boat

This is the difference that matters most in practice, and the one that catches PHRF sailors out. Your D-PN is a property of the class. Every Laser in the world carries 91.1. You do not apply for it, you do not pay for it, and you cannot be re-rated for sailing well. You look your boat up in a table.

That makes the system extraordinarily cheap to run, and it is why it suits club dinghy racing so well. It also carries an obligation that clubs routinely ignore: the published number assumes the boat is rigged and sailed in class configuration. The handbook is explicit that competitors should declare non-class equipment, and it publishes standard modification factors for the common deviations, with the sensible caution that no more than two adjustments should be applied and the combined effect should stay inside roughly plus or minus 4.5%.

In club terms: if someone turns up with a bigger rig, no spinnaker, or one crew where the class expects two, that is a rating question, not a shrug.

The wind bands, which PHRF has no answer to

Here is the part of Portsmouth that deserves to be better known.

A planing dinghy and a heavy displacement boat do not keep the same relative speed as the breeze builds. A catamaran that is pedestrian in three knots is transformed in eighteen. One number cannot describe that, so alongside each class's all-purpose D-PN the tables publish up to four wind numbers, DPN1 to DPN4, keyed to the Beaufort scale and used when conditions are settled enough to name. The handbook recommends carrying an anemometer and assessing the average condition for the race.

Look at what happens to three classes across the range:

Put numbers on it. Take the Hobie 16 and the Laser round a course where the Hobie sails an hour, and let each boat sail exactly to its rating for the conditions. Score it correctly, and they tie. Score it on the all-purpose number instead, and here is what the committee hands out:

This is the same shape of problem as the time-on-time versus time-on-distance argument in keelboat racing, and it has the same moral: a handicap that was right for one set of conditions is quietly wrong for every other. The difference is that Portsmouth already ships the fix, and most clubs never use it.

How much to trust the number in front of you

Portsmouth is honest about its own uncertainty in a way few rating systems are, and almost nobody reads the notation.

In the published tables, a figure in (parentheses) is based on limited race data, between five and fifteen data points, and the handbook tells race committees to regard it as suspect. A figure in [brackets] is an estimate from fewer than five data points and should be treated as highly suspect. Clubs are explicitly urged to verify and modify bracketed ratings using their own race data.

That matters because the tables are long and the popular classes are a small part of them. The Laser, the Sunfish, the Thistle and the Hobie 16 are backed by thousands of races. The obscure fifteen-footer that one member owns may be carrying a number somebody estimated from four results in 1994.

The practical rule: if a boat in your fleet is consistently winning or consistently last by a wide margin, look at whether its rating is bracketed before you conclude anything about the sailor.

Portsmouth and PHRF are the same shape, different scales

The handbook publishes a conversion table between D-PN and PHRF, and it is a straight line: D-PN = PHRF ÷ 6 + 55. Which lets you calibrate an unfamiliar number against one you already have a feel for:

Useful for intuition, and useful for a club that runs both. What you must not do is score one race with a mixture of the two, or convert a boat's PHRF into a D-PN and race it against classes carrying real Portsmouth numbers. The two systems are calibrated from different data against different reference boats. The conversion tells you roughly where a boat sits. It does not give you a rating.

North American numbers are not RYA numbers

This one bites clubs with international members, and it is worth ten seconds of care.

The RYA list, used across the UK, runs on a scale centred near 1000, and corrects with elapsed × 1000 ÷ PY. The North American list runs on a scale of 100. Same family, same idea, completely different magnitudes:

Pick one list, name it in the notice of race, and never take a number off a British club's results sheet to fill a gap in an American one.

Running it at a club

The handbook's guidance is short and mostly administrative, which is a good sign for a system you have to run on a Sunday morning.

• Name the scoring method in the notice of race. Basic D-PN, wind numbers, or modified. When the notice of race names the Portsmouth Yardstick, a boat's assigned handicap counts as her rating certificate under the racing rules.
• Post every boat's handicap before racing begins. Not after the finish, and not after the complaint.
• Split fleets by similar D-PN if you are running more than one start. A fleet spanning a Hobie at 76 and an Optimist at 123 is a scoring exercise, not a race.
• Set a balanced course. The handbook asks for fairly equal amounts of beating, reaching and running, because that is the shape of racing the numbers were derived from.
• Take the wind reading yourself if you are scoring on bands, and record it. The band is a committee decision and it should be defensible.

The version with no arithmetic at all

There is one more option, and for a mixed dinghy fleet it is arguably the best format going: spend the handicap on the start line instead of after the finish.

In a Portsmouth pursuit, the slowest boat starts first and every other boat is held back by its rating, so the Optimist is away while the Hobie is still waiting. If the ratings are right, the fleet converges at the finish and the first boat across the line has won. No corrected times, no multiplication, no explaining to a ten-year-old why they finished first and placed fourth.

The trade is that the handicap gets committed at the start, against a nominal race length, so a race that runs much longer or shorter than planned drifts. What you buy is a finish that everybody watching can understand, which for a junior or mixed fleet is worth a great deal more than theoretical precision.

So should your club use it?

• Do your boats have PHRF ratings? If they are dinghies, sailboards or small multihulls, they do not and will not. Portsmouth is the answer by default.
• Is your fleet one design? Then score it scratch and do not handicap anything. First boat home wins.
• Do you have a reliable course distance? Irrelevant here, which is the quiet luxury of the system. Portsmouth never asks.
• Are your conditions consistent day to day? If yes, the all-purpose numbers will serve you all season. If your venue does drifters and gales in the same month, learn the wind bands.
• Is anyone's rating bracketed? Check before the season, not after the protest.
• Do you want the finish to be self-explanatory? Run a pursuit.

The Portsmouth Yardstick is not a precision instrument and has never claimed to be. It is an eighty-year-old averaging exercise that lets four completely different boats have a real race, which is exactly what a club with fifteen sailors and eleven boat types needs. Publish the number, publish the method, and go sailing.

Sources

1. North American Portsmouth Yardstick Handbook (PDF), US Sailing, 2017 revision (the empirical basis and DIYRA origin; the 1973 transfer of administration; a boat rated 80 sails in 80 minutes what a boat rated 90 sails in 90; the Thistle as primary yardstick at 83.0, chosen in 1961 and held level across the Beaufort scale; corrected time = elapsed ÷ D-PN × 100; the wind handicap system and its Beaufort basis; the parentheses and brackets confidence notation; standard modification factors and the 1.045 / 0.955 limit; naming the method in the notice of race; posting handicaps before racing; balanced courses; the warning about rapidly changing conditions; Table V, the D-PN to PHRF relationship).
2. North American Portsmouth Yardstick: Table of Pre-Calculated Classes (PDF), US Sailing, 2017 revision (the all-purpose and DPN1 to DPN4 figures used throughout: Thistle 83.0, Hobie 16, Laser, Sunfish, Optimist Pram, and the five recognised class groups).
3. DPN, US Sailing (the D-PN programme: statistical analysis of national race results, ratings reported as time-on-time correction factors, and the centreboard, small keelboat and multihull scope).
4. Portsmouth Number List 2025 (PDF), RYA (the current British numbers, including ILCA 7 at 1104, ILCA 6 at 1154 and ILCA 4 at 1216).
5. Portsmouth Yardstick, Wikipedia (the 1947 origin at Langstone Sailing Club under Stanley Milledge, the Island One Design pegged at 100, the RYA taking over in 1960, and the scale difference between the RYA and North American schemes).
6. Time-on-Time or Time-on-Distance? A Club Racer's Guide to PHRF Scoring, Committee Boat HQ (the keelboat counterpart, and why a time-on-time system does not need a course distance).