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GPS and current
Hi speedfreaks!
-Not many GPS-reports give us a hint of the speed and direction of the current.
If there´s a surface drift of 3 knots working with you , it can be easier to make a GPS v-max of 36 knots, than a GPS v-max of 30 knots with current working against your effort to go fast.
When surface drift is going against the wind and your deep speed angle, there´s actually a lot more chop than if the current and wind are going in the same direction.
So a 38 knot GPS v-max on smooth water, with current working for you, can be easy compared to a 30 knot run in heavy chop caused by current working against you.
Open water records over 43 knots for men ( 40 knots for ladies) and Fininan´s 46,82 knots are very impressive, knowing these records are 500 m averages over a defined course ( no short GPS v-max) and official ratification verify that no strong current helped the rider do his / her job.
If you don´t have a GPS, just go faster than your buddies.
-That´s fun too.
-Ulffrom Ulf Astrom ( )
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2 Svar
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post number 7
date 2/18/2005 7:09:02 AM
message Current:I have tried to measure the current at various times at Sandy Point and found that it is usually next to insignificant at the times when it is good for top speeds. We usually speed sail close to low tide as this offers the smoothest water, the deepest water closest to the sand and the least following chop at really broad angles. The Macquarrie Innovation guys were allowed to sail in a two hour window around low tide without any penalty (as I understand it). That usually covers the vast majority of the fastest runs done here.
Even at full ebb tide at 1/3 to 2/3 out, I doubt there is as much as 2 knots favourable flow, and my observations so far suggest it is more like 1-1.5, if that. I will be making more observations when the oppurtunity arises.
Cheersfrom Andrew (sailquik@hotmail.com )
post number 8
date 2/18/2005 10:11:44 PM
message:
Current is a tricky thing to figure–it doesn’t just go back and forth (in/out), but side to side, swirls and eddies, and so on as well. Often in tidal estuaries, rivers, etc, there can be a near shore reverse current.Look at the tide/current charts for a known harbor (NY harbor comes to mind) — it’s pretty complicated.
from tg ( )post number 9
date 2/19/2005 12:22:22 AM
message I would think finding out the current would be hard without a speedo on the board. When we race large sailboats they have knot meters on board, they tell you boat speed through the water, then we would use the GPS to tell us the speed over the bottom, is the gps was slower than the knot meter then we knew we were in bad tide, but if our speed over the bottom was faster the we knew we were in good current, we would check every time we went through what looked like a tide line.
from AB ( )post number 10
date 2/25/2005 5:24:16 AM
message:Yes AB,
when we are beating upwind, against a strong current, on a boat logging speed through the water, it´s easy to see that the VMG upwind is almost zero on the GPS, when the boat seems to be going fast through the water.It´s easy to check surface drift or current near shore by just filling a bottle with water adding little sand until the bottle is slowly sinking.
Put a 80cm rope between that bottle and another plastic bottle, filled with just that much water, that the hole thing floats.Now you can observe any movement of the current meter you´ve just made.
Official ratification has been done just like this, to check surface drift and current.
And as I said, the easy way to see current, is a very flat water surface when current and wind is going the same way.
When you have wind against the current, you have something else…-a very steep chop is developing.If you know this and a lot more.. -you know what boards to put in your car, in different conditions, to be faster than your friends, or your best GPS record.
Keep sailing and have fun!
Ulf Astrom
from Ulf Astrom ( )
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nice info. good to know.
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