Speedo Calibration
September 14, 2024 · 12 min · 2K views · Chester River
What this video covers
A paddlewheel speed sensor on an Island Packet 40 named Navigator read 15 percent high, confirmed by a calibration run on the Chester River in October 2020.2:23 Tom Tursi describes a 10-step procedure for measuring and correcting a boat's water speed and distance instruments using two fixed navigation aids rather than floating buoys.0:45 The measured course ran 0.6 nautical miles between Channel Junction Light LC and Green dayboard G1 on bearings of 276° and 096° true, oriented perpendicular to current flow to minimize current error.4:56 The crew motored at 2,400 rpm, timed each leg with a stopwatch, and computed speed over ground by dividing chart distance by elapsed time.6:58 Dividing SOG by indicated water speed yielded a speed correction factor of 0.85 outbound and 0.86 return, averaging 0.855.10:15 Because the same paddlewheel drives both speed and distance readouts, the distance correction factor equals the speed factor.11:17 Five runs across multiple cruises produced correction factors of 0.85–0.88, and applying the factor to dead reckoning plots and running fixes improved consistency over uncorrected readings.12:19 The procedure also covers TVMDC compass conversion, use of the US Coast Guard light list to calculate inter-NAVAID distance, and the critical distinction between water-referenced speed and GPS ground speed.5:29
Boats in this video
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