GPS or speed/depth Instrument truth?

[george2064]

I was out on the water yesterday (it was a gorgeous day) ... and my GPS game me a speed of 20.5 knots at 3400rpm and my depth and speed instrument was showing about 14.5 knots ... that gives ?? I am thinking the GPS must be true ... but how to I fix the other instrument?

 

[Henry Boyd]

It is entirely possible that the depth/ speed device is off, BUT it is more likely you were just traveling in a current.

The GPS gives you SOG Speed Over Ground. It is the speed the boat is moving across the face of the earth.

The depth/speed gauge gives the speed through the water. It does not tell you if the water is moving.

Yesterday you were traveling in a 6 knot current.

Henry

 

[george2064]

Good point ... thank you for the info ...

 

[tobnpr]

Are they usually in agreement?
Speed sensors are paddle-wheel type thru-hulls that often get fouled with growth.

They're prehistoric, anyway.