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  1. #1
    Little Five is offline Registered User
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    Default gps speed question

    If, according to the GPS I am making 6 knots SOG but have a 3 knot cross tide on my beam, how much of the 6 knots is forward speed as against sideways slip due to the tide?

  2. #2
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    Quote Originally Posted by Little Five View Post
    If, according to the GPS I am making 6 knots SOG but have a 3 knot cross tide on my beam, how much of the 6 knots is forward speed as against sideways slip due to the tide?
    Whatever is shown on your log.
    You never know, I might be right!

  3. #3
    BlowingOldBoots's Avatar
    BlowingOldBoots is online now Registered User
    Location : Scotland, The Firth of Clyde
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    5.19 kts
    Having time is unavoidable.

  4. #4
    Little Five is offline Registered User
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    Quote Originally Posted by BlowingOldBoots View Post
    5.19 kts
    Thanks B.O.B. How do you do the maths?

  5. #5
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    5.196 knots, approximately.

    Better roughly right than completely wrong

  6. #6
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    NickRobinson is offline Registered User
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    The vectors are added together arithmetically

    If you drifted your SOG (no wind) would be 3kts but not in the desired direction.

    Obviously if you tuned into the current and stood still (SOG=0) you would need to see 3kts on your log (if working!)

    So you sail or motor, with the rudder pointing you higher than your next wpt so you describe one of those lovely triangle plots from DS night classes.

    I bet this doesn't help.. and it's an a.m. not midnight posting too....

    Nick (Rivendell)

  7. #7
    john_morris_uk is online now Registered User
    Location : Near Exeter Boat is near Rhu.
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    If its EXACTLY on the beam, then see above. The trouble is tidal current is very rarely exactly on the beam and a little different either f'wd or aft makes a big difference to the answer as to what you are actually doing 'through the water'. It's one of the reasons we like to have a log as well as a GPS...
    Wishing things away is not effective.

  8. #8
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    Quote Originally Posted by Little Five View Post
    If, according to the GPS I am making 6 knots SOG but have a 3 knot cross tide on my beam, how much of the 6 knots is forward speed as against sideways slip due to the tide?
    If everything remained constant you would indeed travel six miles in one hour but of course that won't happen

    If the cross tide is genuinely at 90 degrees to your track then you will actually have a speed thru the water in excess of 6 knots. If the cross tide is say 30 degrees forward of your track then you will actually being doing in the region of 7.5 knots thru the water, if the cross tide is 30 degrees aft of your track then you will only be doing about 4.5 knots thru the water!

    Now where the fun really starts is if you perceive the cross tide to be on the beam but in practice it is forward or aft by 60 degrees...........in this case your boat speed thru the water may be as high as 9 knots or as low as 3 knots..........you see where I'm coming from!!

    Chox
    Last edited by chockswahay; 23-02-12 at 11:34.

  9. #9
    Little Five is offline Registered User
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    I am interested in how to do the calculation if, say your log isn't working or you suspect it is giving false readings.

  10. #10
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    The OP was enquiring about speed over the ground, was he not?
    Better roughly right than completely wrong

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