gps speed question

**Little Five** · 23-02-12, 10:25

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?

**flaming** · 23-02-12, 10:29

Originally Posted by Little Five

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.

**BlowingOldBoots** · 23-02-12, 10:33

5.19 kts

**Little Five** · 23-02-12, 10:35

Originally Posted by BlowingOldBoots

5.19 kts

Thanks B.O.B. How do you do the maths?

**JayBee** · 23-02-12, 10:36

5.196 knots, approximately.

**NickRobinson** · 23-02-12, 10:37

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)

**john_morris_uk** · 23-02-12, 10:37

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...

**chockswahay** · 23-02-12, 10:42

Originally Posted by Little Five

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