Log calibration

[brian_neale]

I have just taken delivery of a new boat, and during the delivery trip back from Falmouth to Solent I had the feeling that the log was under-reading. During a supposedly "slack" tide I "calibrated" the log by setting it to the SOG as per GPS. This gave a calibration factor of 1.25 (i.e. log was under-reading by about 25%). Log is Raymarine ST60, boat traditional long keel with paddle wheel fitted about halfway along hull, under turn of bilge.

This kind of error seems more than I would have expected, and later this season I shall recheck against Walker log (once fitted!) and maybe measured distance. However, the speed readings now do seem consistent with GPS/tidal stream figures.

Has anyone else gone through this exercise? What kind of "errors" have you seen?

 

[ccscott49]

Done it with other logs, not ST60, but I've seen up to 30% errors, but after calibration, it seems to stay accurate, I did mine over measured mile, but gps on a straight course should be adequate.

 

[jimi]

I've got the same feeling with mine, I think its underreading by 20%. I'm going to do 2 waypoints 1 mile apart then go both ways between them. That should eliminate the tidal effect.

Jim

 

[WayneS]

Had to do exactly the same last week-end to my Danaplus instrument. 125% seemed to put me spot on with the GPS at slack water.

Suddenly the boat seems to be going soooooo much faster and I can now stop worrying why my engine is not working properly.

Cheers

Wayne

 

[burgundyben]

Measured mile.

THere is a measured mile in Southampton Water and one I think between Anvil Point and St Albans Head, these are very useful for calbrating logs and are I think marked on Admiralty Charts.
RUn both ways and take an average, noting your log radings as you do it


Ooo, good evening officer.........

 

[johnsomerhausen]

A simple way to calibrate a speed indicator is to start motoring after hitting the "Man overboard button" going in a sraight line for one mile as indicated by the GPS and then returning to the "MOB" position. That way you've pretty much eliminated all tidal influence on the reading.
john

 

[brian_neale]

I am reassured to hear that other people have under-optimistic logs! The amount of the correction seemed a lot to me, but seems like it is not unusual. Will try the measured mile in Soton Water some time - the ST60 has some funny mode that automatically calculates calibration factor after doing a two-way run to cancel tidal effect.

 

[yachtbits]

we recently fitted & calibrated a Silva Star Log. we ended up with a correction of 1.44 to get a correct reading. All logs tend to need calibrating to some extent, this is normal.

The location of the transducer in the hull and the shape of the hull affects the calibration, so every installation is different.

regards
kev

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