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I had an Airmar ultrasonic log the whole of our circumnavigation. The advantage of ultrasound is that it doesn't need to be raised and cleaned all the time - is maintenance free. You often learn that the paddle wheel needs cleaning when you most need to know your speed - particularly if you have been stationary for a week or so. The disadvantage of ultrasound is that it is very sensitive - it measures the speed of the boat as it hunts on its anchor - even a light cross current. 0.1-0.2 knots. If you spend a week at anchor you can easily find the boat has measured a 50-100 mile passage. Switching the depth sounder instrument display off may stop the system logging the distance - or not - depending on how it is set up.
Eventually, I gave up and fitted an Airmar paddle wheel. However, 4 years on and I am thinking of returning to the ultrasound because i am fed up with cleaning the crud off!
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