GPS SOG vs Speed transducer calibration effects

[Derek - Raymarine - Moderator]

The differences between GPS SOG and Speed through the water can be affected by many things, mostly external influences.

These factors can be described in this FAQ read here.

If the Speed transducer is more than 2 years old, it is possible the shaft and paddlewheel bearings are worn and a paddlewheel service kit can be used to change these components.

Check the speed comparison is carried out at slack water when there is no tide or current affecting the boat.

Water flow under the boat can also affect the calibration of boat speed, it is important the transducer is aligned accurately to the direction of water flow, underwater fittings of the boat can affect water flow, so can hull features such as bow thruster tunnels, sacrificial anodes, water inlets and outlets, keel and spray rails, all can create levels of turbulence or redirect water flow which can affect the accuracy of the speed transducer.

The two guides either side of the paddlwheel are intended to guide the water over the paddlewheel, however, if the transducer is misaligned, these guides can cavitate causing poor performance too.

RAYFAQ#