Eddy Surfing a 1-Knot Seastead — Caribbean Loop Feasibility

Executive Summary

~5–7 daysSkillful eddy-position forecast horizon (useful guidance to ~10 days, then persistence + climatology)

≈ 3,900 nmCoast-hugging clockwise loop of the Caribbean Sea (~7,200 km)

105–155 daysEstimated underway time at 1 kt through the water with eddy routing

6–9 monthsRealistic door-to-door calendar time incl. weather holds & hurricane-season timing

Bottom line: Go clockwise. The Caribbean's mean flow runs westward from the Lesser Antilles to Yucatán Channel, so a clockwise loop rides the current for roughly two-thirds of the distance. Expect a voyage-average of 1.3–1.5 kt made-good (peaks of 2–2.5 kt on eddy limbs and in Yucatán Channel; slogs of 0.6–0.9 kt on the eastern return leg). A counterclockwise loop fights the mean flow almost everywhere and could take 2–3× longer. Your intuition is correct: current-selection matters more than hull speed at 1 kt.

1 · What Predicts Eddies, and How Far Ahead?

The chart you linked (myfoxhurricane) visualizes RTOFS, NOAA's Real-Time Ocean Forecast System — a daily-run global eddy-resolving model. That's the right family of tools. Here's the landscape:

Source	Type	Resolution	Horizon	Access	Notes
RTOFS (NOAA)	Daily forecast	1/12° (~8 km)	~7 days	Free, GRIB2/NetCDF	The model behind your fox chart; Atlantic focus
GOFS 3.1 / HYCOM	Daily forecast	1/12°	~7 days	Free (THREDDS/OPeNDAP)	Global; RTOFS's parent model
Copernicus Marine (CMEMS)	Analysis + 10-day forecast	1/12°	~10 days	Free w/ registration	Best all-round global product; assimilates altimetry
OSCAR (NASA/JPL)	Satellite-derived nowcast	1/3°	None (5-day composite)	Free	Great sanity-check on model drift
SWOT + DUACS altimetry	Sea-surface height obs	~15–25 km swaths	Observation, not forecast	Free	SWOT (2022+) dramatically sharpens eddy mapping; feeds the models above
AVISO Mesoscale Eddy Atlas	Detected & tracked eddies	—	Historical + near-real-time	Free	30-year eddy track database; goldmine for climatology
Saildocs	GRIB-by-email delivery	—	—	Free	Delivers RTOFS current GRIBs over low-bandwidth email — ideal aboard
Regional (CARICOOS etc.)	Regional models/obs	~2–5 km	~2–3 days	Free	Better near-coast detail around PR/USVI

How far ahead is reliable?

Days 1–3: Eddy centers typically good to ±20–40 km; swirl speeds to ±0.1–0.2 kt. Trust it for tactical decisions.
Days 4–7: Position error grows to ±50–100 km. Still very useful — eddies are 50–150 km wide, so you usually know which side of an eddy you'll be on.
Days 8–14: Marginal. Use it as guidance only.
Weeks 2+: Switch to persistence + climatology. Eddies translate predictably westward at ~5–15 km/day, and strong isolated rings (Loop Current rings, North Brazil Current rings) follow repeatable tracks. The AVISO Eddy Atlas tells you where eddies of each type live, season by season.

Confidence trick: pull two independent models (e.g., HYCOM and CMEMS). Where they agree, trust the plan; where they disagree by >0.3 kt, treat the feature as uncertain and keep margin.

2 · Routing Algorithms & Software

Your problem is the classical time-dependent Zermelo problem: minimize travel time when your velocity through water is fixed but the fluid moves. It's well studied, and the practical solution space looks like this:

Time-expanded graph search (recommended). Discretize space into cells and time into 6-hour steps matching the forecast. State = (cell, time). Edge cost = distance ÷ ground speed, where ground speed = your 1 kt plus the current's component along the leg. Solve with A* or Dijkstra. Robust, easy, handles "no-go" cells (adverse current, shoals) naturally.
Level-set / ordered-upwind methods. Continuous-space version (Lolla & Lermusiaux, MIT, 2014). Elegant, used in research; overkill for a first implementation.
Receding-horizon replanning (how you'd actually operate). Because forecast skill dies at ~7 days but your trip lasts months, nobody solves the whole voyage at once. You re-plan daily: optimize the next 5–7 days, sail it, repeat. This "surfs" the evolving eddy field and is nearly as good as perfect foresight.

# Sketch: time-expanded A* over (cell, time-step)
V_BOAT = 0.514                      # m/s  (1 knot)

for each 6-h forecast step k, cell i:
    c = current[i][k]               # from HYCOM / CMEMS / RTOFS
    for each neighbor j:
        u = unit_vector(i -> j)
        sog = dot(c, u) + V_BOAT    # ground speed on this leg
        if sog < 0.15: continue     # refuse to lose ground
        cost = dist(i, j) / sog
        relax((j, k+1), cost,
              heuristic = great_circle(j, goal) / V_BOAT)

# Post-filter the winning path:
#   reject any leg where forecast OPPOSING current > 0.7 kt
#   (keeps a 0.3 kt margin for model error + wind)

Existing / open-source tools

Tool	What it does	License	Fit for your seastead
OpenCPN + weather_routing_pi	Full time-dependent weather routing; accepts GRIB current fields (e.g., RTOFS via Saildocs)	GPL, open source	★★★★★ — Works today. Set your "polar" to a flat 1 kt and feed it current GRIBs.
VISIR (CMCC)	Graph-based open ship router (developed for the Mediterranean)	Open source	★★★☆☆ — Methodology reference; wave-focused but adaptable
py-eddy-tracker	Detects & tracks mesoscale eddies in SSH fields (Okubu-Weiss / winding-angle methods)	Open source (Python)	★★★★★ — Feed it CMEMS/HYCOM sea height; get eddy centers, radii, swirl speeds
xarray + cartopy + networkx/scipy	DIY stack: read NetCDF/GRIB, build the A* above in ~200 lines	Open source	★★★★☆ — Full control; the snippet above is most of the work
Saildocs	Email-delivered RTOFS current GRIBs for a region you define	Free service	★★★★★ — Low-bandwidth data aboard
MIT MSEAS planners	Research-grade level-set path planning in dynamic flows	Research	★★☆☆☆ — Read the papers; code not turnkey

Honest answer: there is no polished turnkey open-source "eddy router" yet — but OpenCPN's routing plugin plus RTOFS current GRIBs gets you 90% of the value with zero coding, and the DIY A* closes the gap if you want eddy-aware logic built in.

3 · Does This Actually Work in Practice?

The governing arithmetic is simple. On any heading, your speed over ground is:

SOG = 1.0 kt (you) + (current · your-heading)

Ride a 0.5 kt fair current → 1.5 kt (+50%).
Fight a 0.5 kt adverse current → 0.5 kt (−50%).
Avoid adverse water entirely and you never pay the penalty — that asymmetry is the whole game.

In the Caribbean, mesoscale eddies commonly carry 0.3–0.8 kt swirl, and the mean Caribbean Current adds another 0.5–1.0 kt westward. So a well-routed day is routinely 1.5–2.0 kt made-good, and ring limbs or Yucatán Channel can deliver 2–2.5 kt. Realistic voyage averages:

Strategy	Typical voyage-average SOG	Comment
Point at destination, ignore currents	~0.9–1.1 kt	Adverse eddies cancel favorable ones, minus lost ground
Daily replanned eddy routing (clockwise Caribbean)	~1.3–1.5 kt	The realistic target
Excellent execution, patient waiting for rings	~1.6–1.8 kt	You'll stop and loiter to let a ring get into position — worth it

Two operational realities: (1) eddies themselves drift west at 0.2–0.4 kt, so "meeting" one means leading it like a ferry crossing; (2) sometimes the optimal move is to stop (hold position, minimal power) for a day or two until the field rotates in your favor — cheap at 1 kt economics, and the routing algorithm will discover this on its own.

Watch wind, not just current. A high-freeboard solar platform has real windage. 20–25 kt of trade wind can add 0.3–0.6 kt of leeway/drift and steep seas that hurt a 1 kt hull far more than any eddy. Current routing optimizes one term of a two-term problem.

4 · "Eddies Can't Push Me Ashore" — Mostly Right, With Caveats

What's true: Mesoscale eddies are 50–150 km wide, hundreds of meters deep, rotating lenses of water. They cannot exist over a shallow shelf — they decay at the 100–200 m isobath. A 2 kt eddy swirl will never be found at the beach. For eddy-scale currents, your reasoning is sound.

Where it breaks down:

Tidal streams in passages hit 2–4 kt (Yucatán Channel, Mona, Anegada, reef passes) — absent from daily eddy charts and stronger than you.
Wind-driven drift + windage can exceed 1 kt in squalls/trades.
Submesoscale coastal jets (0.5–1 kt) are unresolved by 1/12° models — precisely near shore, where you need accuracy most.

Verdict: You don't need to fear eddies shoving you onto land, but you absolutely need margin against tides and wind near coasts. Keep a 0.3 kt propulsion reserve, transit passages near slack tide, and carry an anchor. Treat "currents can't push me ashore" as false; treat "mesoscale eddies can't" as true.

5 · The Caribbean Loop Estimate

Clockwise loop: west along the southern Caribbean (with the current), north through the western Caribbean and Yucatán Channel (with the current), then east along the Greater Antilles (the hard leg), and south through the eastern passages.

Leg-by-leg estimate (1 kt through the water, 24 h/day, eddy-routed)

#	Leg	Distance	Current situation	Expected SOG	Days
1	Barbados → Curaçao	550 nm	Inflow jets W/NW 0.3–0.8 kt; ride rims of incoming rings	1.5 kt	15.3
2	Curaçao → Cartagena	380 nm	Caribbean Current W 0.5–1.0 kt	1.7 kt	9.3
3	Cartagena → San Blas	230 nm	W 0.5–1.0 kt	1.7 kt	5.6
4	San Blas → San Andrés	230 nm	NW 0.4–0.8 kt	1.6 kt	6.0
5	San Andrés → Roatán	360 nm	W 0.5–1.0 kt	1.7 kt	8.8
6	Roatán → Cozumel	250 nm	N/NW 0.5–1.0 kt	1.7 kt	6.1
7	Cozumel → Grand Cayman	330 nm	Mixed eddy field, 0–0.5 kt	1.2 kt	11.5
8	Cayman → Jamaica	270 nm	Adverse W flow 0.2–0.5 kt	0.9 kt	12.5
9	Jamaica → S. Haiti	180 nm	Adverse 0.2–0.4 kt	0.9 kt	8.3
10	Haiti → Mona Passage	450 nm	Variable; hop eddy limbs	1.0 kt	18.8
11	Mona → St. Croix	250 nm	Passage jets; transit near slack	1.1 kt	9.5
12	St. Croix → Barbados	410 nm	Adverse trade drift 0.2–0.4 kt	0.9 kt	19.0
Total		≈ 3,890 nm (7,200 km)	~65% of distance with favorable/neutral current	avg ≈ 1.3 kt	≈ 131

Bottom-line numbers

Optimistic (surf everything, patient loitering): ~1.6 kt avg → ~100 days underway.
Central estimate: ~1.3 kt avg → ~130 days (≈ 4.3 months) of continuous steaming.
Conservative: ~1.05 kt avg → ~155 days.
Add 15–25% for weather holds, zigzag around reefs, stops, and maintenance → plan on 6–9 months door-to-door.

Hurricane season (June–November) dominates the calendar. A 1 kt vessel cannot outrun anything. Sensible plan: depart the Lesser Antilles in late November–December, do the long western/northern legs in the dry season, and stage the slower eastern return leg February–April, finishing before June. Or split the loop across two seasons with a secure storm-hole stop.

Why not counterclockwise? You'd fight the 0.5–1.0 kt mean westward flow for ~2,000 nm. Legs that take 9 days clockwise become 20–40 days or simply impossible during adverse eddy phases. Total loop: plausibly 2–3× longer. The sea's conveyor belt only runs one way here — go with it.

6 · Eddy Atlas: Other Cruising Grounds

Rule of thumb: eddy energy is highest on the western sides of ocean basins (western boundary currents shed rings) and lowest in gyre interiors and the eastern tropics.

Region	Character	Typical speeds	Rideable?	Notes
Caribbean Sea	Westward stream + rings + passage jets	0.5–1.5 kt	★★★★☆	Your home waters; excellent
Gulf of Mexico	Loop Current rings shed ~yearly, drift west for months	1–2 kt swirl	★★★★☆	Big, slow, well-tracked targets
US East Coast / Gulf Stream	Warm/cold-core rings	1–2 kt	★★★★★	Best-observed eddies on Earth; busy & windy
Gyre interiors (Sargasso, S. Pacific middle)	Quiet	<0.3 kt	★☆☆☆☆	Nothing to surf — but nothing to fight; easy motoring
Mediterranean	Algerian anticyclones drifting E along N. Africa; persistent Ierapetra & Rhodes gyres	0.3–0.6 kt	★★★☆☆	Usable! Microtidal (predictable); real hazards are Mistral/Bora winds, traffic, borders
South Pacific (Tahiti→Fiji)	Weak interior gyre	0.1–0.3 kt	★☆☆☆☆	No eddy help needed or available; pleasant 1 kt sailing
Coral / Tasman Seas (EAC)	East Australian Current eddies	0.5–1.5 kt	★★★★☆	Strong, well-modeled (Australia's BLUElink)
Equatorial band	NEC/NECC/SEC "rivers" + Tropical Instability Waves	0.5–1.5 kt; TIWs 0.5–1 kt	★★★★☆	TIWs alternate on ~30-day cycles — poor man's eddies, very phase-predictable; great for E–W crossings
Eastern South America	North Brazil Current rings off the Amazon; Brazil Current warm rings; Brazil–Malvinas Confluence	NBC rings up to 1.5–2 kt	★★★★★	Arguably the world's best rideable rings; they march NW toward — the Caribbean. Natural extension of your loop
Agulhas system (S. Africa)	Huge retroflection rings into S. Atlantic	1–2 kt	★★★★☆	World-class eddies, world-class weather. Expert-only
Somali / Arabian Sea	"Great Whirl" during SW monsoon	1–2 kt	★★★☆☆	Seasonally repeatable; regional security considerations
Kuroshio / Japan	Rings galore	1–2 kt	★★★★☆	Superb regional forecasting (JCOPE)

Direct answers: South Pacific interior — mostly no usable eddies (fine, you don't need them there); Tasman/Coral Sea — yes. Mediterranean — yes, modestly (the Algerian eddies are the prize). Eastern South America — outstanding, top-three eddy venue on the planet, and its rings conveniently deliver you back toward the Caribbean.

7 · Daily Operating Playbook

Morning: pull the latest 7-day HYCOM/CMEMS currents (Saildocs GRIB if bandwidth-limited).
Detect: run eddy detection (py-eddy-tracker or an Okubo-Weiss threshold) → list of centers, radii, swirl speeds.
Plan: run time-expanded A* for the next 5–7 days; hard constraint: no leg with forecast opposing current > 0.7 kt.
Beyond day 7: fall back to persistence — extrapolate each eddy's westward drift (~5–15 km/day) — plus seasonal climatology from the AVISO Eddy Atlas.
Re-plan daily. Treat anything past day 5 as advisory, not gospel.
Cross-check two independent models; disagreement > 0.3 kt ⇒ add margin or wait.
Passages (Yucatán, Mona, Anegada, reef passes): consult tidal-stream tables; transit near slack. These are the only currents that can genuinely overpower you.
Reserve: always keep ~0.3 kt of thrust in hand for wind and waves. If made-good toward goal drops below 0.3 kt for 12+ hours, stop and hold position rather than burn energy sideways.
Learn: log GPS drift vs. forecast every 6 hours; after a month you'll have a personal bias correction for your local waters — worth more than any model upgrade.
Season: June–November, always be within 48 h of a defensible storm hole.

8 · Data & Software Links

RTOFS (your fox chart's engine): polar.ncep.noaa.gov/rtofs · GRIB via nomads.ncep.noaa.gov
HYCOM GOFS 3.1: hycom.org/data/gofs-31
Copernicus Marine (10-day global forecasts): marine.copernicus.eu
OSCAR satellite currents: podaac.jpl.nasa.gov/Oscar
SWOT mission: swot.jpl.nasa.gov · AVISO/DUACS & Mesoscale Eddy Atlas: aviso.altimetry.fr
py-eddy-tracker: github.com/py-eddy-tracker
OpenCPN: opencpn.org · Weather Routing plugin: github.com/rgleason/weather_routing_pi
VISIR open ship router: visir-model.net
Saildocs (low-bandwidth GRIB by email): saildocs.com
NOAA CoastWatch ERDDAP (easy subset downloads): coastwatch.noaa.gov/erddap
CARICOOS (PR/USVI regional observations): caricoos.org
Further reading: Lolla & Lermusiaux, "Time-optimal path planning in dynamic flows" (2014); Chelton et al., global eddy statistics from altimetry.