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Old 06 January 2010, 12:41   #11
Country: UK - England
Town: Fareham
Make: Avon Adventure
Length: 5m +
Engine: Merc 90
Join Date: Aug 2009
Posts: 91
Originally Posted by jyasaki View Post
Exactly so; but typically, a road doesn't creep along in some random direction at a couple of knots.

But groundspeed is groundspeed. It is the speed you are traveling along the surface of the earth, relative to a fixed point on the surface. Doesn't matter if you're walking, driving a car, piloting a boat, or a plane.

In a boat (or in the air), you get other things that screw with measurements depending on what you are sampling: Currents. Water for a boat, air for a plane.

Picture this to get the difference:

A river moving at 3 knots. A run upstream such that groundspeed (i.e. GPS speed over ground) shows 30 knots will have the paddlewheel showing 33 knots (i.e Groundspeed plus the 3kt current.

Turning around and going downstream, again at 30kts on the GPS, and the paddlewheel will show 27 (SOG minus current.)

If you turn to face upstream and adjust power so that your groundspeed becomes zero (or, I suppose, simply anchor), your speed through the water will still read 3 knots.

Thanks for that, I understand what you mean now, I think for what I am going to be doing I will just set up the Ulra Nav to use GPS for ground speed and not bother with fitting a paddlewheel, thanks again for the replys.

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