gps speed question

[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?

[flaming]

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]

5.19 kts

[Little Five]

5.19 kts

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

[john_morris_uk]

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...

[Little Five]

I am interested in how to do the calculation if, say your log isn't working or you suspect it is giving false readings.

[JayBee]

I am interested in how to do the calculation if, say your log isn't working or you suspect it is giving false readings.

In this case you can just use a bit of good old Pythagoras*, as we are looking at a right angled vector diagram (triangle).

The hypoteneuse is the actual SOG, 6 knots
One side is the tidal set OG, 3 knots
The third side is your unknown forward SOG

*In a right angled triangle,The square on the hypoteneuse is equal to the sum of the squares on the other two sides

So: SOG ^2 = 6^2 - 3^2 = 36 -9 = 27

and SOG is the square root of 27, which is 5.1961524....

Hope this helps

[chockswahay]

I am interested in how to do the calculation if, say your log isn't working or you suspect it is giving false readings.

I use a 'rule of thumb' from my day job (flying) however you do need to know your boat speed to apply it.


For practical purposes if the current is up to 30 degrees off the nose then the current is ALL against you

If the current is 60 degrees off the nose then the current is HALF against you and ALL on the beam.

This comes from a 'clock code' that substitutes once around a clock face with degrees and assumes any thing more than 60 degrees is MAX.

SO.......... if current is known to be say 6 knots ......then

On the nose = 6 knots against you (max) and NO cross current

30 degrees off the nose still equals 6 knots and barely any cross current

45 degrees off the nose = 4.5 knots against you and 4.5 knots cross current

60 degrees off the nose = barely any knots against you and 6 knots cross current

These figures are generally accurate until 60 degrees and then maybe about 0.8 kt pessimistic

Does this make any sense

[JayBee]

5.196 knots, approximately.

[NickRobinson]

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)