LITTLE CLOUD OCEAN DATA Sea temperature, depth soundings and wind from a sailing vessel Brittany (France) to Madeira and the Selvagens, 5 August to 29 September 2026 Version 1.0, published 29 September 2026 Web page: https://sailinglittlecloud.com/ocean-data/ Contact: sam@sailinglittlecloud.com (Little Cloud Sailing, Sam & Oriane) Licence: Creative Commons Attribution 4.0 International (CC BY 4.0) https://creativecommons.org/licenses/by/4.0/ How to cite: Little Cloud Sailing (2026). Little Cloud ocean data: sea temperature, depth soundings and wind from the sailing catamaran Little Cloud, Brittany to the Selvagens, August-September 2026. Version 1.0. https://sailinglittlecloud.com/ocean-data/ THE VESSEL AND THE LOGGER ------------------------- Little Cloud is a Catana 431 sailing catamaran (13 m) sailing from Brittany to the Caribbean, taking part in the International SeaKeepers Society DISCOVERY programme. A SeaKeepers data logger (Yacht Devices YDVR-04 voyage recorder) records every message on the boat's NMEA 2000 instrument network. The raw logs (1,480 files, 3.8 GB) were decoded message by message to produce these files. Instruments: Position, speed and course over ground Garmin GPSMAP 943 and GPSMAP 743xsv (GNSS) Heading Raymarine SPX-30 autopilot compass Sea temperature, depth, speed through Raymarine SeaTalk instruments and hull water transducers, via a SeaTalk-STNG converter Apparent wind Raymarine i60 masthead wind transducer ROUTE AND TIME COVERAGE ----------------------- Stops: The Abers, Molene archipelago, Brest, Audierne, Lorient (Brittany); Viveiro (Galicia); Porto Santo; Baia d'Abra, Quinta do Lorde, Machico, Ribeira Brava and Calheta (Madeira); Selvagem Grande (Selvagens); then at sea towards the Canaries. Distance sailed: 1,794 nautical miles. Recording time: 601 hours on 36 days. The logger records only while the instruments are switched on, so there are gaps, mostly during the stay in Lorient (10 August to 5 September). Bounding box: 29.58 N to 48.60 N, 17.26 W to 3.27 W. All times are UTC. FILE 1: little-cloud_ocean-data_1min_2026-08-05_2026-09-29.csv --------------------------------------------------------------- One row per minute with data (36,075 rows), under way, at anchor and in port. Empty cells mean no valid value. time_utc start of the minute, ISO 8601, UTC latitude decimal degrees, WGS84 (median of GNSS fixes in the minute) longitude decimal degrees, WGS84 underway 1 when moving (2-minute median speed over ground above 1 kn), else 0 sog_kn speed over ground, knots cog_deg course over ground, degrees true (empty when not under way) heading_true_deg compass heading corrected for magnetic variation, degrees true stw_kn speed through water, knots (paddle-wheel log, uncalibrated) sea_temp_c sea temperature, degrees C (hull sensor, just below the surface) depth_m depth below the water surface, metres (median of good soundings, see file 2) app_wind_speed_kn apparent wind speed, knots app_wind_angle_deg apparent wind angle, degrees from the bow, -180 to 180, positive to starboard true_wind_speed_kn true wind speed, knots (calculated, see below) true_wind_dir_deg direction the true wind blows FROM, degrees true (calculated) FILE 2: little-cloud_depth-soundings_10s_2026-08-05_2026-09-29.csv ------------------------------------------------------------------- Echo-sounder depths as 10-second medians (138,931 rows). time_utc centre of the 10-second window, ISO 8601, UTC latitude decimal degrees, WGS84 longitude decimal degrees, WGS84 depth_m depth below the water surface, metres qc_flag 0 = good (131,093 rows) 1 = spike: differs from its neighbours by more than 2 m or 25 % 2 = false echo: a bottom reading where the sea is far deeper (shallow readings far from any coast, and any reading far off the narrow Iberian and Macaronesian shelves) 3 = at the echo sounder's range limit (about 183 m): no bottom 4 = at the echo sounder's shallow limit: it reads nothing below about 1.6 m, so the depth is 1.6 m or less. At anchor in water that shallow (Molene, 6-7 August 2026) it also gave false 4-8 m readings in between, so every reading at anchor from the first to the last floor reading is flagged 4. Depths are below the water surface, as the boat's depth instrument reports them (the instrument adds the depth of the transducer). They are NOT corrected for tide or sound speed. The sounder reads in steps of 0.1 ft (about 3 cm), between about 1.6 m and 180 m, so good soundings come from coastal and shelf waters (about 386 nm of track). PROCESSING ---------- - Timestamps: the recorder's clock was aligned to GNSS time file by file (linear fit, residuals under 0.05 s). - Values are averaged over 10-second windows, then over minutes. Sea temperature and depth use medians, which removes isolated sensor glitches; the rest use means. Angles are averaged as vectors. - True wind is calculated from apparent wind, speed through water (speed over ground when missing) and true heading. It is water-referenced: currents are not removed. - Sea temperature: two isolated one-reading glitches in the raw data (12.0 C, 27.6 C) are removed by the averaging. The 1-minute values range from 14.2 to 26.2 C. - Porto Santo example on the web page (not applied to these files): at anchor in Porto Santo bay (15-16 September 2026, 33 h) the echo sounder recorded the tide. A tide curve (M2, M4 and K1 constituents plus a plane for the boat swinging over the sloping bottom) fitted to those readings (rms 7.5 cm) was used to reduce the soundings of our passages in and out of the bay to the mean water level of the stay. LIMITATIONS ----------- These are the boat's navigation instruments, not calibrated scientific sensors. - Wind: the anemometer is at the masthead, well above the 10 m height used by weather models, and moves with the boat. For comparison with models, average over 10 minutes or more and correct for height. Speed through water reads about 6 % below speed over ground on average, which affects the calculated true wind. - Sea temperature: hull-mounted sensor near the surface, not calibrated against a reference thermometer. Readings in marinas and anchorages may reflect local water. - Depth: single-beam recreational echo sounder; see the notes above. On steep slopes it tends to report the shallowest bottom within its beam. - Not measured aboard: air pressure, air temperature, humidity. MORE DATA --------- We keep the raw logs: every instrument message at full rate (up to 10 per second). Write to sam@sailinglittlecloud.com for full-resolution data, other variables, a custom extract, or measurements on our next legs (Canaries, Cape Verde, Atlantic crossing, Caribbean).