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S&S Swan General - True wind and apparent wind
18 October 2014 - 19:55
Join Date: 01 February 2007
Posts: 57

True wind and apparent wind
I am having a discussion with my local electronics expert about this topic so wanted to get some additional views. Of course the boat only feels apparent wind, then the electronic wind instrument calculates the true wind from the boat speed. In my opinion the speed used for this calculation should be SOG as provided by the GPS. This is the wind you would feel when standing on land. The expert says that boat speed is taken fron the log paddle wheel because the SOG from the GPS is not updated in real time. Accordingly the instrument is actually measuring wind speed over the water not the land, so it is not strictly speaking "true" wind (currents and tides can make the water move relative to the land). The other issue is that log paddlewheels are not very accurate (when they are not clogged up with marine life) so I question the accuracy of this "water wind" Has anyone resolved this enigma ?

19 October 2014 - 15:42
Join Date: 30 January 2007
Posts: 461

A unique answer does not exist because it depends on the navigation system of the boat.
Nowadays the GPS COG is indeed given in real time but as the GPS signal propagation is subject to fluctuations, in order to make the value meaningful an average over many measurements is needed. The length of the average time and the algorithm to compute the optimal mean value depends on the navigation system software. Accepting that the GPS COG is not a true istantaneous value but a mean value over some length of time, its absolute error is actually very small and can be used to compute efficently the true wind. Your friend is right if he refers to old systems, like the B&G of 20 years ago; at that time the GPS had a high error inserted on purpose for security reasons and the navigation systems could use only speed over water measured on board.
Now the systems are so smart that can use both GPS COG and paddlewheel output to obtain the optimal results.
Daniel, 411/004

31 October 2014 - 13:46
Join Date: 25 October 2014
Posts: 9

I am having a new system installed, Raymarine screens below and on the binnacle. C series and E series and i60 and i70. Garmin AIS.

I will ask the question as in the Solent we do have strong tides .

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