Log v GPS

Blue5

Blue5

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Earlier this year I fitted a new Garmin 152 GPS and I have been comparing the total trip odometer against the boat log. I had no reason to suspect any discrepancy but for 330 nautical miles recorded by GPS the log reads 260.
I assume the GPS is giving the correct reading but is there any other possible reason for such a difference, the log appears ok and running free.
 
The GPS records distance over the ground but the log records distance through the water. You have obviously been doing a lot of downstream sailing (as we all do if poss).

If that isn't the answer then it's because logs are unreliable and don't work at low speeds if at all and why would you want one?

That'll bring them out of the woodwork - we haven't a log or not thread for days /forums/images/graemlins/grin.gif
 
Why?

[ QUOTE ]
GPS needs to be treated with respectful scepticism

[/ QUOTE ]Why would you be sceptical of GPS data? It's likely to be more accurate than any log.
 
...and pre-summer calibration won't stay valid with some summer fouling on the log paddle wheel: the boat is not slower, but the log grows lazy...
 
Has the GPS been left switched on whilst at anchor etc? We find that quite a few miles are clocked up on our GPS that way.

Try checking the GPS distance travelled at the beginning and end of each leg, and comparing that with your log. After making any necessary allowance for tide, that should show up any significant discrepancy.
 
I think I may sort of know the answers to these but..... what is a log and how do they work and are they expensive and is it worth fitting one to a mobo?

My Honwave 3.2m airfloor may be available for sale with or without 8hp Honda o/b. PM for details. Am I gonna get shot for adding this?
 
There is also the possibility of the paddle wheel becoming fouled. Coming back from Yarmouth mine stopped for period of time, don't know how long for just noticed the speedo not working, 20 mins later it was off again.
 
My last boat with a log sat on a half tide mooring in Newport. The log didn't usually work until I hit a wave to knock the plug of mud out of the paddle wheel, usually somewhere off Cowes /forums/images/graemlins/smile.gif
 
On my last trip,I covered 500 nm and my old Wasp towing log only showed a mile less than the GPS.It may have been chance but assuming there are no strong tides to complicate things it's usualy very acurate.
 
And I'm really wondering why I bought a NASA Duet rather than just the Echo. Certainly if you live on a mud drying mooring you're wasting your time with a log unless you always remove it and replace with the plug.

As for speed-thru-the-water, it's just as easy to work back from SOG as it used to be the other way around! ..... and probably more accurate.

Vic
 
Gps = distance over ground
Log = distance through the water

You need to ‘calibrate’ the log (if you trust GPS) in at least 5m depth of water and with NO tide, against the SOG, from the GPS to get a reasonably accurate log reading, however it will NEVER = the GPS, due to tide, depth of water and general ‘muck’ lurking around the transducer or even the orientation, since they seldom are ‘locked’ fore & aft, or port/starboard.

It is also better to calibrate using distance run, rather than absolute speed over several Kms if you can.

Trust me; I’ve had five years trying to equate the two of ‘em and they have NEVER agreed for all of the above reasons.

BTW ‘NDH’ have you taken any RYA courses? This is pretty basic navigational stuff?
 
When a boat enters shallower water, the speed of flow of water under the hull increases (particularly in narrow channels) this tends to show a log reading well in excess of the ACTUAL speed through the water. I have found in practice that anything under 3-4m depth significantly alters the LOG-vs.-GPS comparison (this may be only applicable to OUR boat with a keel depth of 1.5m) but it does make a significant difference when to comparing Log vs. GPS over a days run.
 
[ QUOTE ]
BTW ‘NDH’ have you taken any RYA courses? This is pretty basic navigational stuff?

[/ QUOTE ]Indeed. I've been wondering the same thing especially since the tides flow pretty fast around Haslar and that part of the world!
 
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