[CA] [CA11] Speed over ground vs through water

[philipdawsonladds]

I noticed something odd on my display yesterday which someone might have a thought about. I recently removed and cleaned the paddle wheel on my ST60 speed transducer. The speed is reading out fine. On my C120 MFD I have the SOG displayed which I understand will be affected by the current. So I was travelling against the tide but SOG was reading HIGHER than the ST60 display - when travelling with the tide, same thing (but to be expected this time). It seems that my SOG is always higher than speed through the water. Something's not right - any ideas on what I might check? Thanks

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum Philip,

STW sensors must be calibrated via the instrument which they have been interfaced to (see the Calibration section of the ST60/ST60+ Speed/Tridata instrument's owner's guide). Calibration should be performed under slack conditions and will accordingly only equal SOG under such conditions. STW sensors have non-linear error. Accordingly, the reported speed value will be closest to SOG when the vessel is operating under slack conditions and at the speed of operation during calibration. Some more advanced STW instruments (ex. i70S/i70 MFIDs w/iTC-5 Analog Instrument Transducer Converter) support multi-point STW calibration.