Is my speed log showing incorrect speed?

[muckypup]

Up the river Crouch on sunday, about 3 hours after high tide going upriver, my GPS was showing SOG of 7 knots, and the river I believe would have been running out at 3 knots. Should my speed log have been showing 10 knots? It was actually only showing 4.5 knots....

S.

 

[flaming]

Surely you can tell the difference between 4.5 and 10 knots just by looking at your wake!!

 

[mortehoe]

Ergo the esturine flow was

2.5 knots as recorded by your speed log /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[Lakesailor]

If you head against the flow and stem it (or anchor) so the GPS shows 0 knots, what would the log show?

 

[muckypup]

Interesting thought! I will try that next time I'm out.

I should also try and get out at slack water time, so the SOG & log should show the same?

S.

 

[flaming]

The normal way of Log calibration is to do a run at a constant speed between 2 objects at a known distance apart and then use the distance travelled rather than the speed to calibrate. Either mile transits or buoys are good for that. By running both ways and doubling the distance you eliminate the tide effect.
Be careful of only calibrating your log at slow speed. The calibration is normally done by a percentage basis, so you may want to do several runs at different speeds to check that the percentage out is the same, and if not choose the best fit.

 

[pyrojames]

Re: Ergo the esturine flow was

In the wrong direction..... /forums/images/graemlins/smile.gif

 

[muckypup]

[ QUOTE ]
The normal way of Log calibration is to do a run at a constant speed between 2 objects at a known distance apart...

[/ QUOTE ]

That's an excelent suggestion. Next time I'm out I will pick a couple of bouys and try this.

Thanks
S.

 

[jonathankent]

Log only shows your speed through the water... regardless of any tide whatsoever.

For your GPS to be showing 7kts and you believe an adverse tide of 3kts, you have got one hell of a quick boat to achieve that! /forums/images/graemlins/smile.gif

If your log was correct of 4.5kts, against a 3kt tide your GPS would have only shown SOG of 1.5kts.

Personally, I suspect the GPS wasn't functioning spot on that day... or the tide was doing the opposite to what you believe /forums/images/graemlins/smile.gif

 

[muckypup]

[ QUOTE ]
For your GPS to be showing 7kts and you believe an adverse tide of 3kts, you have got one hell of a quick boat to achieve that! /forums/images/graemlins/smile.gif

[/ QUOTE ]

That's only 10 knots, Harlequin is good for 13-14 flat out.

[ QUOTE ]
If your log was correct of 4.5kts, against a 3kt tide your GPS would have only shown SOG of 1.5kts.

[/ QUOTE ]

We were well faster than that as we went down river about 6 miles, and at 1.5 knots it would have taken us some 4 hours to get back (the entire trip was only 2.5 hours)

[ QUOTE ]
Personally, I suspect the GPS wasn't functioning spot on that day... or the tide was doing the opposite to what you believe /forums/images/graemlins/smile.gif

[/ QUOTE ]

Definately going out as SOG was showing 11 knots downstream at only half throttle.

Steve

 

[jonathankent]

Ahh.... just looked at your profile.... a mobo - should have known really, heres me thinking you were sailing /forums/images/graemlins/smile.gif

Well, knowing what the boat can do... now I tend to lean more to the log requiring either a clean or a recalibration.

 

[muckypup]

[ QUOTE ]
Ahh.... just looked at your profile.... a mobo - should have known really, heres me thinking you were sailing /forums/images/graemlins/smile.gif

[/ QUOTE ]

ooo no, all those ropes and cloth and tacking.. far too complex for me /forums/images/graemlins/smile.gif

S.