How the Mac is Won

The Chicago Yacht Club's Race to Mackinac runs 289 nautical miles from Navy Pier to Mackinac Island. Bayview starts off Port Huron and sends divisions onto two courses: 204 miles up the Michigan shore, or 259 miles around Cove Island and back through the Straits. All three finish in the same narrow water below Mackinac Island.

Chicago crosses Lake Michigan from Navy Pier. Bayview's two courses start within about an hour over the same water, giving us two course geometries in nearly the same weather.

Weather varies between editions

Across the thirteen Chicago editions with lake-scale wind, the racing fleet's share of hours unable to lay Mackinac ranged from 97 % in 2018 to 1 % in 2024. In 2024 the fleet ran for 80 % of its hours. Six years earlier it beat for almost the entire race.

Later comparisons account for this variation by grouping races by wind regime.

Retirements and archive coverage

Across forty tracked fleets the median retirement rate is 7.0 %. The highest rates were 47.2 % on the 2026 Bayview Cove Island course, 42.7 % on its Shore course and 32.4 % in the 2017 Chicago race. High retirement rates can follow either a violent squall or a long beat with no strong convective signature. Section 5 keeps those cases separate.

Course and tracker coverage also vary. The 2020 Chicago race was not sailed, and Bayview used only the Shore course in 2020 and 2021 because the Canadian border was closed. The 2011 Chicago fleet predates the track archive. Section 5 uses its radar only to describe the weather line; it does not reconstruct or compare the fleet.

The archive does not show a separate night advantage in finishing results.

The correlation between a boat's night performance relative to its own daytime performance and corrected place is −0.027 across 414 racing divisions in 37 editions. The edition-clustered 95 % interval is [−0.120, +0.062]. Negative would mean that holding more speed at night went with a better place. The interval includes zero, and the design could reliably detect correlations of about 0.128 or larger. A smaller association remains possible.

Compare each boat with itself

For every track leg, we divide speed over the ground by the speed predicted from a published ORC polar at the local analysed wind angle and strength. This ratio is the achieved fraction of polar speed. Night retention is the boat's median achieved fraction at night divided by its own daytime value.

The comparison is within a boat, so a constant error in the reference polar cancels. That cancellation is an assumption: day and night can occupy different wind regimes, where the polar's error may differ. Day and night are assigned at each boat's position using civil twilight rather than a fixed clock.

The comparison covers whole nights. It cannot isolate a particular watch or the hours around 02:00. Maintaining speed at night may still matter, even though this analysis detects no additional association after sunset.

Inside a class, sailing farther usually goes with finishing worse. The pooled Spearman correlation is +0.41 across 434 divisions in 40 tracked fleets, and 39 of 40 fleets are positive. The best third of a class typically sails 4 to 13 fewer miles than the worst third on courses of 204 to 289 miles.

A separate sensitivity analysis over the 37 fleets with wind data and its own eligibility filters gives ρ = +0.434 across 487 divisions, 95 % CI [+0.334, +0.525]. Thinning every track to a common 30-minute cadence gives +0.442 [+0.349, +0.525]. This interval applies to the sensitivity analysis. The positive association persists with its different population and sampling cadence. Tracker cadence and missing fixes still make the mileage gaps approximate.

Comparing boats within a class

Across a mixed fleet, faster boats can sail farther because they cover more water per hour. Comparing corrected place inside one division reduces that confounding by boat speed, though rating and configuration differences remain. The result is observational: a boat that is in phase with the shifts may both sail less and move faster. The archive cannot separate the shorter track from the sailing that produced it.

Distance above the published course

The largest gaps occur when fleets sail much more than the published course:

The relationship does not disappear on a beat. The within-division correlations are +0.400 in upwind years (163 divisions), +0.434 in mixed years (167) and +0.392 off the wind (120). Wind regime changes the available mileage more clearly than it changes the correlation.

The exception: 2019 Chicago

The only negative fleet is 2019 Chicago, at ρ = −0.240; the top third sailed 1.2 miles more than the bottom third. We do not know why.

The race began with heavy upwind sailing and waves. The wind later died for many boats, and not every boat finished before the shutdown. Its 5.5-knot fleet-hour median is therefore not a description of the opening conditions or of every boat's passage. I sailed aboard the J/120 Proof; we won our class and sailed a lot of distance.

The edition's distance correlation is not explained by its wind angle, median wind, darkness, squall signature or retirement rate. Those checks leave other possible explanations unresolved. A boat-by-boat reconstruction of the shifts would be a separate case study.

Measurement limits

Recorded tracks underestimate distance because they draw straight lines between fixes. The archive does not establish the direction of the resulting bias in the best-versus-worst group difference. The sample contains finishers with valid corrected results; including incomplete tracks would create the relationship mechanically. Comparisons are made within editions because tracker cadence differs between years.

Fewer recorded miles accompany better corrected places within a class. The association does not establish what would happen if a boat were instructed to sail a shorter route. Across 72 Transpac and Pacific Cup divisions, the corresponding pooled correlation is +0.221 (+0.239 in the Transpac and +0.194 in the Pacific Cup), about half as strong. Route choices can account for a larger share of total distance on a lake course.

Mesoscale weather shapes a Mackinac race. Lake breezes, nocturnal jets and convective lines are smaller than a quarter-degree global grid. At this latitude one such cell is roughly 20 by 28 kilometres, while Lake Michigan is about 90 kilometres wide. Four or five cells leave these local structures unresolved.

The map compares the same wind field and hour at two resolutions. Coarse averaging obscures local structures.

Wind archive

The analysis uses hourly 10-metre wind from HRRR at 3 kilometres and RTMA at 2.5 kilometres. HRRR covers races from 2014 onward. RTMA reaches back to 2013 and provides an independent comparison during their overlap. Missing hours remain missing; editions before either archive stay in analyses that do not require wind.

These analyses estimate the weather that occurred. Section 6 uses forecasts published by each decision time.

Wind angle to the next mark

A boat beating at 45° to the wind can look like it is reaching if we classify its own course over the ground. Instead, each hour is classified by the angle between the wind and the bearing from the boat to its next mark. Under 60° is upwind, 60° to 120° is reaching, and over 120° is running. Cove Island boats point toward the Cove mark until they round it. Shares are weighted by time so tracker cadence does not give one edition more influence.

The 2018 Chicago fleet beat for nearly the whole race. The 2024 fleet ran for most of it. The classifier is deliberately coarse and does not turn those categories into a description of every boat or every hour. In particular, 2019 began with heavy upwind sailing and waves before a later shutdown pulled its fleet-hour median wind down.

Squalls in hourly wind data

A convective line can produce a large direction change and speed jump in the same hour. A front may turn the wind without the same speed spike.

The 2022 Chicago race is the clearest convective signature in the archive. The 2017 Chicago race retired 32.4 % of its fleet but sits near the middle of this ranking, while the 97 %-upwind 2018 race also had heavy retirements without a sharp wind discontinuity. A high retirement rate alone does not identify a severe convective event.

A squall crosses a fleet quickly. A long beat can wear down boats and crews for a day without producing a dramatic radar image. The archive contains both, and retirement rate does not rank them the same way as convective intensity.

Radar coverage

NEXRAD Level II provides raw reflectivity and radial velocity. Its beam rises with distance from the station, while the wind that matters to a boat is near the water.

We use a three-kilometre beam-height limit. Beyond it, reflectivity can still show the middle of a storm but cannot represent what reached the deck. The maps distinguish a valid clear observation from water the radar did not sample.

Five nights with radar coverage

For each boat, the analysis finds the worst reflectivity over its track and the last clear frame before that peak. This estimates when rain arrived. A gust front can precede the radar echo.

The 2026 Bayview race had the archive's highest retirement rates, 47.2 % on Cove Island and 42.7 % on Shore, after a severe squall. The wind analysis captures the jump, but the national archive has no Detroit radar files for the relevant days. Gaylord did not cover the fleet's southern position. The radar view remains blank for that race.

The 2011 Chicago–Mackinac

On 17–18 July 2011 a severe line crossed the Chicago–Mackinac fleet. WingNuts capsized and two sailors died. This paper does not reconstruct the fleet or turn the event into a performance comparison. The US Sailing independent safety report is the record of the accident and its lessons.

The surviving radar records the line’s development and movement. The strongest return intensified from 22.5 to 63.0 dBZ in thirty-six minutes, and the 50 dBZ core advanced at about 25 knots toward 127°. Reflectivity is not wind, so we do not convert it into a gust estimate. A line moving at 25 knots covers fifteen miles in thirty-six minutes. This retrospective calculation does not establish how much warning crews had.

Holding speed through a line

For each boat, speed retention is its average speed over the ground in the hour after the line arrived divided by its average in the hour before. We use raw speed because the reference polars end below the winds in these squalls.

Negative means that holding more speed went with a better corrected place. Greater retention went with better place in 2022 and 2021. The detected 2016 relationship points in the opposite direction; 2017 and 2024 do not separate. Pooled across 71 racing divisions, the interval includes zero. Five nights cannot establish which conditions produce either pattern.

Later retirees held 0.071 less of their pre-line speed than finishers in this five-night sample. A within-night permutation gives p = 0.006, but resampling the five nights gives an interval of [−0.121, +0.050]. Speed retention is not a reefing decision, and slowing may already reflect damage. This association does not establish that maintaining speed was safer.

Why 2017 looked empty

The 2017 Chicago race retired 32.4 % of its fleet, yet only 32 of 296 boats met 35 dBZ. The radar was operating and covered 88.1 % of the analysis grid; its strongest echo was simply not over the fleet. The archive cannot determine whether the event was a dry downdraft.

Radial velocity does not recover a hidden low-level signal here. The median peak velocity difference was 6.5 m/s in 2017, against 27.5 m/s in 2022, but the usable beam was about two kilometres above the water and Doppler velocity requires scatterers. In 2017, 93 % of fleet samples were below 5 dBZ. The measurement cannot distinguish weak outflow from an absence of particles to measure.

The 2017 event left little reflectivity and only a weak velocity signature at the radar’s beam height. A sailor watching the radar colours would have seen an almost empty screen despite the difficult conditions.

Start with the newest forecast

The rolling deterministic policy follows the fastest route in the newest forecast and replans every six hours.

The replay starts at 20:00 local from a real boat: the tracked boat nearest the middle of its division's geographic spread. From there, the navigator takes the newest usable HRRR route and sails it for six hours through the hourly HRRR/RTMA analysis. It then routes again from the position it actually reached.

Every new route starts from the boat’s updated position. The black track shows the distance already sailed; the forecast control advances to the next decision. An early choice can leave the boat poorly placed for the next forecast.

Comparing routes across forecasts

The alternative policy compares routes across several recent forecasts.

At each update, as many as four recent extended HRRR cycles provide candidate routes. We keep a decision only when at least three routes are complete. Each forecast's own optimum becomes one possible action. For every action, we calculate how much time it loses in the forecast or forecasts least favorable to it.

The rolling hedge chooses the route with the lowest adverse time loss among those no more than 3 % slower in the newest forecast. If the newest forecast's fastest route already minimizes that loss within the limit, the hedge selects it. Both navigators then sail for six hours through the same analysed weather and make their next decision from wherever they arrived.

These successive operational runs are correlated. Their disagreement measures forecast changes over time and covers only some plausible weather conditions.

Each forecast hour enters the replay only after publication. Older HRRR runs end at f18 or f36; current extended runs reach f48. If an adverse forecast is missing, we do not let that absence make a route look safer.

Did the hedge help?

We attempted the earliest fully covered racing evening in 22 edition-course cases. Sixteen cases from fourteen independent editions had every route, forecast and decision needed for a complete replay. If any comparison was missing, we did not score the case.

At the 3 % limit, the hedge finished about twenty seconds slower per case on average. The median difference was zero, and the 95 % interval ran from roughly three minutes slower to two minutes faster. We detected no average advantage.

Results varied between cases. The best hedge gained seven minutes; the worst lost thirteen. The hedge chose a different route in ten of sixteen cases. Six cases finished unchanged because the deterministic route also minimized adverse time loss within the limit, or because the routes later converged.

We repeated the selection at four limits on added forecast passage time:

The 5 % hedge lost 6.6 minutes per case on average; the 10 % hedge lost 8.4. The extra sailing time is incurred immediately. The adverse weather that motivated the choice may never occur, or the forecasts may converge at the next update. A short remaining passage leaves less time to recover that cost.

The result applies to this policy using successive HRRR forecasts. Every case uses one J/111 reference polar, the earliest evening with complete coverage, and the largest class with no more than forty boats. The cap excludes trophy and whole-fleet tracker groups, but it remains a design choice. The interval resamples editions, keeping paired Bayview courses together.

When forecast differences change a route

Different wind predictions affect route selection only when they favor different positions. A ten-knot difference can leave every route in the same lane. A small direction change beside a shore or passage can split them.

Further analysis could measure how far apart the candidate routes place the boat and whether any of those positions proved useful in the weather that followed.

The recent HRRR cycles can also move the same squall in the same wrong direction. In that case, the routes agree with one another and miss the useful position together. Selection is limited to the routes generated by those forecasts.

Averaging route coordinates may produce a path that no forecast supports. Compare complete, reachable routes and weigh their added time against the adverse outcomes they could avoid. The available recovery time before the finish limits how much a hedge can cost.

Forecasts and routing can inform those decisions. Maintaining boat speed, avoiding mistakes and executing maneuvers remain central to the result.