Well kept secret

Had the same with these instruments, used to think "it's only 10 NM to Cherbourg" (in the pouring rain) and looked at the auto-pilot repeater reading and it's something like 12 or 13 miles to go. V Depressing. Finally found that the log was set to statute miles (not nautical) although the auto readout was set to Knots.

Took ages to work out that to change the repeater (it appeared to be set correctly) I needed to set the log. Damn thing was showing speed in knots too. Could this be the same for you? Works a treat now it's all consistent units.
 
All,

So much talk about log cals, I just had to put my pennys worth in. For my part I sail (not motor), and I race more than I cruise. This might help explain my perspective on logs.

'Surely SOG is important and water speed less so...'
Depends on your type sailing, but if you race definately not.

'GPS for measuring the mile....'
Not as accurate as a transit measured mile which will get it down to the last metre. So it depends how accuarte you need it.

'You need zero tide....'
You don't need it, but it helps. What is more important is the rate of change of tide. You MUST do it both ways, even if the almanac says its slack, it won't be (gravity is not the only thing to make water move). What is more important is that the flow has not changed in the time taken to do the runs. Remeber that even under power wind will make the boat go faster one way than the other. So less wind is good too.

'Anyone noticed how its not linear....'
Absolutely. When I cal'd logs on destroyers, we would do many runs at different speeds to draw a graph of log performance. Unfortunately like everything else sailing, it is anything but linear. More expensive log units do allow for applying calibration at differnt speeds, if this is important to you. Of course as soon as you put your sails up and the boat starts healing, the curve goes to pot. It usually reads different on port Tack to Starboard. GPS being good at picking up those kind of descrepancies. Maybe your boat really is going faster on one tack than another (theres whole thread just waiting to be written)

'Only the relative speed change matters...'
In some respects this is very true. Speed goes up, your doing something right. However most racing boats ' tune up' over a period of months / years, and it is important that they hit target boat speeds. So the reading from one year to the next needs a datum. You possibly have new instruments, maybe even a new boat, and old reading are meaningless without a datum. This only being important to us speed freaks I guess.

Happy sailing /forums/images/graemlins/smile.gif

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