Log vs GPS Speed - Why the Difference?

[mario f]

The log speed measure speed of water passing boat.
GPS speed over ground.

[DEAN2140]

Most transducers are located off center. When the boat is running or reaching, it is relatively flat but when close hauled the heel may affect the flow of water and thus the reading. My speed through the water is consistently faster close hauled on starboard.

[moseriw]

GPS = calculated via Position Difference = SOG Speed over Ground
Log = mechanical weel in the water = STW Speed trough water


if speed more than 3 kn SOG is accurate.
STW is not really accurate, but good enough, as the water flow is not constant.
If you have current and you know your log is thoroughly adjusted then

the difference between SOG and STW is the current.

[DEAN2140]

The difference may be due to the transducer not being on the center line of the boat. When you are running or on a reach, the hull is relatively flat but when close hauled the heel may affect the flow of water on an off center transducer. On my boat, speed through the water is about 20% faster on close hauled on starboard.

[Lihuedooley77]

yep,

Agree with the GPS speed over the ground and knot meter, or taff rail log,
speed thru the water ( mechanical readings ) .

Plus if you are sailing close hauled, close reach, and a beam reach, you are likely to have leeway, pushed down wind, no straight line course .

Are you plotting positions and ETA's, using coastal navigation on paper charts, D.R's, E.P's, fixes or running fixes, as well as using the GPS ?

The O.P. is getting some great ideas as to the difference in GPS vs Log speed. Seems it could be a number of different reasons for the difference.

Also, when navigating we use all aspects of navigation, Coastal Navigation and Piloting, GPS, and even fathom lines ( depth sounder ) .

[ozdigennaro]

It's very simple. Log is movement relative to the water. GPS is movement relative to the planet.

Log does not include movement to the lee and current. GPS is accurate. Log is an estimate.

[MJH]

It bugs me when owners compare speeds but don't state the instrument used (GPS/paddle wheel) and weather the paddle wheel has been calibrated.

Every knotmeter I have owned has included instructions on calibrating it to the boat, always with a test run over a defined mile...Coast Guard has some of these posted on NOAA charts, I have one in my area. Have you done a calibration run against a measured mile?

FWIW, on my previous boat I would calibrate it coming home after the haul with a clean bottom. I did get small varying corrections from calibration to calibration. On my current larger boat, I just don't worry about it, looking more for variations within the instrument than against the GPS. Also, going slow to docking, etc., when the GPS doesn't give a faster response, is when I use it more.

~ ~ _/) ~ ~ MJH

[Dooglas]

You might also think about how the paddle wheel on the log works when you are close hauled. The boat is likely making significant leeway and is not pointing in the direction of the CMG. Thus water is flowing under the boat in a direction not perpendicular to the axis of the paddle wheel. Surely that introduces some error. The GPS, on the other hand, is measuring your actual SOG regardless of leeway, which way the bow of your boat is pointing, etc.

[jmschmidt]

The main purpose of a log or paddle wheel type knot meter is for sail trim. Position error, and therefore accuracy, will vary on differing points of sail for the reasons stated. On a given point of sale with a constant wind condition even a slight trimming of sails could result in a slight increase or decrease in indicated speed. This tells you if you made the right trim adjustment or not. This is highly useful while racing.

Other that I suppose a paddle wheel knot meter could be used to approximate current and/or as a rough indication of SOG for updating the DR when all your electronic fail. A Taffrail or trailing log with a long tether could be much more consistent than a paddle wheel.

[Jan Huisman]

Paddle wheel logs are susceptable to the waterflow under the hull. Mine can be turned in its holder, is is not locked in position by a key, so and I can turn it bit by bit until the maximum reading is obtained.

Try to do this: loosen the holding nut, turn the log and let your partner read the output. Good luck.

[Stu Jackson]

Just because the displays have a decimal point doesn't mean much. I believe people read (pi!) too much into them and the differences between them. I use the differences between ours to indicate the current's relative strength. I haven't sailed on a lake in 35 years! Neither readout is absolute. Close enough for planning purposes.

[Paul L]

Paddle wheel log calibration on most systems is a fixed offset. In reality they can be pretty nonlinear in their measurements. So if you calibrate at 5 kts it doesn't mean that it is accurate at 2 or 7kts.

[Rapanui]

Happens on our cat, about 1 knot difference port tack to starboard. Transducer is not on centreline of one hull so I put it down to leeway causing different flow angles to transducer. When I had a cat with the Tx on a hull centerline it was fine. Low priority do address it though

[Lepke]

As others said: over ground speed compared to thru the water speed. Also in big swells the paddle is measuring the up and down distance and would show the greater distance.

[Dave22q]

I suggest you investigate as follows. Select 2 points at least 20 minutes apart with the current flowing between them. Motor from the first to second and back recording log readings every 10 minutes or so as well as the time for each run. A lot of variation in those readings signal an instrument problem right there. Then use simple math to compute average speed over ground in each direction. Compare that to your average log reading each way. Each of the differences is a good estimate of the tidal flow (one +, one -). If your computed tidal flow stimates differ by a lot or are not reasonable, you almost certainly have a log instrument problem. Repeat several times including cross tide/wind courses and you might be able to come up with a log correction factor. I suggest motoring not sailing for maintaining straight courses and minimal vulnerability to wind shifts confusing the data.
You might try cleaning your log sensor before doing this. A bit of shell or sand could easily cause bad readings.