[CA] [CA11] Boat Speed Calibration Inconsistency

[infotechma04]

Hello, I have an ST60 boat speed head. I have tried: 1) new transducer and 2) the Airmar ultrasonic transducer. Each has been carefully calibrated. However, after a random period (days, months) the calibration is clearly off and the display reading becomes either high or low by a significant amount (10 to 15 percent). I have re calibrated the system several times and each time the value seems to drift. Is there some way to check if the display head is bad. Or?? Thanks, Charles

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum infotechma04,

Instruments sensing speed through water (STW) simply count pulses to measure distance and then perform speed calculations based upon the distance measured over an interval. Such "drift" would not typically be attributed to the instrument display unless it was additionally not retaining other user settings. Most differences such as you have reported result from operating the vessel at a speed which is faster or slower than that which the at which the transducer was calibrated and/or 2) result from marine growth which has accumulated on the transducer or the hull forward of the transducer. Regarding the first item, STW transducers feature non-linear error. ST40, ST60, and i50 instruments are recreational instruments supporting single point calibration. This means that the reported STW will be most accurate when the vessel's STW matches STW at that which the transducer had been calibrated. In accordance and in recognizing the accuracy needs of performance sailing, over the last 20 years Raymarine has offered lines of higher performance STW instruments supporting multi-point calibration (ST290, ST70 MFIDs, and now the i70/i70S MFIDs) to support greater STW accuracy across a wider range of STW.

Regarding the second item, performance sailors commonly utilize retractable speed/temp transducers and a transducer housing featuring a valve. The latter permits the transducer to be withdrawn from its housing and the housing to be capped between uses. When withdrawn, the transducer may be cleaned and prepped for its next use, when it will be reinserted into its housing.