Calibrating Boat Speed

A very simple way to calibrate your boat speed (readings from your paddle wheel) is to use your trip distance (Home Page / Trip Caculator) measurements on your chart plotter (Distance over Ground & Distance through Water) by sailing for say 10 min in one direction – then turn through 180 degrees and sail back to where you started. You can then look at the 2 distance measurements to caculate the speed calibration factor.

The nice thing about this technique is that it eliminates the impact of current or tide because you travel back to where you started. That said – obviously its best to calibrate the boat speed where you have minimal current or tide.

1) Goto the Boat Speed calibration page (System / Network / Calibration ) and set the boat speed calibration factor to 1.0

2) Go to the Trip Caculator (Home page – Trip Calculator on left menu) page on your chart plotter:

3) Position your boat at the start location – best to be adjacent to a landmark – ideally avoid any strong currents.

4) Reset the trip settings so TRIP and DSTWTR is zero.

5) Motor at around 5-6 kts for aprox 10 min in a straight line then turn through 180 degrees and head back to your start location.

6) Have a pen & paper handy – when you get back to the start location note down the TRIP and DSTWR readings.

7) Lets asume the TRIP distance is 0.8 and the DSTWR is 0.75. Divide the TRIP Distance by DSTWR so 0.8 / 0.75 = 0.9375 is the Calibration factor. Enter that number into the Calibration setting.

8) If you have multiple Chartplotters – I believe that you will need to enter that Calibration number into each device.

9) With that number recorded in the Calbration setting – repeat the test if you have time. Ideally the TRIP & DSTWR should be very similiar readings. You can adjust the Calibration number up or down to fine tune.

Personally – I found my paddle wheel transducer to be quite unreliable. In 2022 I swiched over to an Airmar Ultrasonic Speeed/Depth/Temp transducer – which I have found to be super accurate. https://www.airmar.com/productdescription.html?id=208

Finially – I’d strongly recommend that you also Calibrate your B&G Digital compass regularly. I have found that they can “drift” out of calibration – particularly if you travel a reasonable distance.

To calibrate the Precision 9 Compass:

1. Go to (System / Network / Device List) and find the Compass. Select the compass and you should see a “Calibrate” menu item. Choose Calibrate – and follow the on-screen instructions. It requires you to turn the boat slowly through a 390 degree turn. At the end of that turn the compass will have been Calibrated.