I sometimes answer questions on facebook and realized some of them might be useful to share here.
For example, in the photo above, we are making a whopping 1.8 knots. That is GPS, or actual speed. Water speed was about 5 knots.
There are different kinds of speed indicators for your boat.
Knowing the difference is useful when you are dealing with currents, be they caused by rivers, tides or wind.
Boat speed in water is the actual speed that the water is passing by the sensor and the actual speed your boat is making though the water. This is a combination of your boat progress and any currents. Those currents can add speed or take it away. One knot current against you will show the same water speed because your boat is moving though the water, and water is flowing past the hull.
GPS speed is measured against the surface of the earth. It is the actual speed, actual progress, you are making over the earth. It's your true speed.
If you are directly facing a one knot current and are showing five knots of water speed, your true speed would be about four knots.
If you are facing a five knot current, in a five knot boat, your true speed can be zero.
This is why it's useful to know the difference between the two numbers. They can tell you a lot about your surroundings.
Remember, your water speed is relative. The sum of your water speed and currents is your actual speed.
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