Measuring Speed

[tgalea]

I need some help in understand this situation whereby my speed log reads let say 4.9 knots and a Garmin GPS reads say 6 knots.

Is the GPS to be assumed more accurate and reliable or the other way round ? I did hear or read in the past that GPS ground speed cannot just be read as being your forward speed ?

Am i mixing the whole thing up.. ? anyone feel like explaining this to me since i'm finding this quite intriguing and never really understood it. Just a note - the GPS always seems to read higher speed than the log.

Thanks

Tyrone.

 

[halcyon]

A few thoughts to start off, gps reads speed over ground, log reads speed through the water. The log if through hull is effected by flow over hull, cavitation of impeller, airation of water, angle of heel and logs sensor error. GPS used to quoted as a bit unreliable at slow speed ?, satalite error, gps system error's.


Brian

 

[Chris_Stannard]

Your GPS will measure speed over the ground, whereas your log will measure speed through the water. They both will read the same when there is no tide.otherwise the log will read the water speed and the GPS will read waterspeed plus or minus tide as appropriate. So if you are heading into a two knot tide with a log speed of six knots the GPS should read four knots (6-2). Turn and go down tide and the GPS will read eight knots (6+2).
In your case the log reading seems to be inaccurate, since you say the log always is lower than the GPS. In your instructions/handbook for the log it should tell you how to put a correction in. The best way to do this is to go out at slack water, and check it is slack by looking for flow round a buoy, and take a GPS reading. Correct your log to give you the same reading. Turn 180 degrees and check just to make sure.
If you have an integrated system, such as Raytheon you should be able to tell the log to accept the GPS reading.
Hope this helps


Chris Stannard

 

[Chris_Stannard]

I forgot to say that you should check to ensure you log impeller is lined up with the fore and aft line, if not you will get a low reading. However if the log is off centre, and it is a sail boat. go from tack to tack and see if there is a difference in your log speed, if this is the case it can usually be cured by canting the log about two or three degrees in towards the centre line of the boat. You will still need to correct te overall log reading.

Chris Stannard

 

[Modulation]

I agree with Chris. You should check your calibration if the log reading is nearly always higher than the gps.
Also, tides don't always work in reciprocal so you can get a negative effect most of the time - but not in mosst unperturbed coastal sweeps.



Brendan

 

[Guest]

Have you checked that the GPS is reading in Knots and not MPH?

 

[vyv_cox]

Since Selective Availability was turned off, GPS is very accurate for speed measurement, as others have said, over the ground. But it may be necessary to play about with the sampling rate of the GPS, or someone else may have done that already. Check the setup of the GPS before jumping to conclusions.

My experience is that on non-moving water the log and GPS measure identical values. In tidal waters a very useful comparison of the two is available. Logs go out of calibration very easily, by being fouled, by painting with too much antifouling, by being twisted in the housing, by being located in the wrong place on the hull, by not being fully pushed down, for various electrical reasons, etc. Check everything and then calibrate it against the GPS.

 

[bedouin]

What my GPS tells me is how inaccurate my log is!

As others have said, GPS is totally accurate in measuring speed over the ground. The log, on the other hand, attempts to measure the flow of water across the hull, and translate that into speed through the water. With my set up that is approximate at best.

My log in particular is very 'non linear' at present (possibly due to fouling on the hull). It is calibrated against the GPS to be accurate at 5kts, but consistently underreads below that (and therefore presumably overreads above)

 

[VMALLOWS]

I've totaly given up on the old paddle-wheel log. It was always either fouled or producing absurdley high readings (wheel vibrating about the read switch). I know it's a 'nice-to-have' so you see the tidal influence, but in reality it was so unreliable it could never have been used for DR.

For fun, take the GPS with you (and a passenger to read it) on a longish journey in the car. You may be amazed at how innacurate your speedometer is (it usually reads 5-10% high). The total mileage (odometer) readings will however probably be very close (depending on how worn your tires are!).