Log reading v Actual miles

[ditchcrawler]

On a trip from Woodbridge to Burnham in my new(to me) boat I noticed that the distance logged by the log was significantly less than the actual known & measured off the chart distance.(I did roughly check the log against the GPS at slack water).
I realised of course that this was due to the fact that like most of us on short trips we tend to plan to go against the tide as little as possible & have the tide with us as much as possible.This means that we sail more miles than we actually log according to the instrument log.
To get to the essence is it OK to fill in the ships log with the measured distance over the ground & personal log ditto & how many of us do this or do we just log miles from the instument reading.

 

[Phoenix of Hamble]

I only ever log distance through the water.... be interesting to see what others do....

As an aside... definitely take the time to check and calibrate your log against a measured distance, allowing for tides.... mine was significantly out when I checked.......

 

[michael_w]

Personally, I record miles run, both through the water(log) and over the ground(GPS); if only to see how well I worked the tide.

You will never sail the exact measured distance, so I don't bother with that one.

After lots of calibration runs, I think the log is as accurate as I can get it.

 

[andy_wilson]

The log is of most use if it gives you miles through the water, in case you have an electrical failure and you have to work up a DR plot.

No harm in putting the miles over the ground in the narative I guess. I would imagine over a season there would not be much difference

 

[tome]

Same here, we use distance through the water. Local sailing (less than a full tide) we'll always do significantly less than the ground distance, but that's what we log

 

[Talbot]

distance through the water is what should be recorded.

 

[Boathook]

I started this year to record the distance from the GPS as well. Makes interesting figures when on 2 weeks holiday and you try to use the tide to your advantage

 

[Goldie]

Distance to log is distance through the water. Hypothetically, imagine you were beating from 5 miles from A to B against a foul spring tide and sailed (say) 15 miles by the log (i.e. through the water). Would you really log just the 5 miles that you'd made good? Extreme example I know, but demonstrates the principle I think. Or by "over the ground" would you record the track distance actually sailed and taken from the GPS? Through the water is the answer; after all, that's what you have actually sailed isn't it?

 

[fireball]

I record both log and GPS at the end of each day - as said above it can give you a good feeling when you work out how much tide you caught.
I also record the engine hours at the end of each day so I can do a rough calc of miles per engine hours ... useless information really - but it is interesting.

 

[Guest]

Commercial shipping records both.

Each watch records log at each end of watch in the log-book ... jot it in position log etc.

Noon-calculations for Ships data to owners and eta's etc. real distance calculated over ground noon to noon is recorded.

But of course we are yotties ... so does it really matter ? As ong as you understand the difference of the two forms and when to use each ...

 

[Birdseye]

I record the gps position (marked on the chart) together with the instantaneous gps speed and track every half hour or thereabouts. This lot tells you where you are and where you are / were heading. So you dont need to save log distance run to work out a DR. Who wants a DR when you have a GPS position?

To be honest, I also record the log speed and distance run. Now you have forced me to think about it, I cant see why I do this. Must be the bureaucrat in me.

I've often thought that if I ever do buy a new boat I wont bother with a log at all. A second GPS costs less, doesnt make a hole in the hull, is more reliable and more accurate.

 

[Guest]

Funny thing is ...

I bought a Dual Display ... Log and E/S with transducers ... I fitted the E/S as that didn't require lift-out and was in-hull. The log has sat in its box never fitted after near 7 years. I keep promising myself that I will cut the hole next time lifted out and fit it. I still promise myself that.

If I don't actually do it by time I am ready to sail to Baltic - I will jury rig a tube and fitting overboard for it - as I do want it.

It's a difficult one to explain exactly why ... but I want it. Electrical power failure will take it out so that can't be reason. I do like to know STW .... as that is actually what engine / sails are producing .... not the final SOG which is vector of STW + current ....
If for example the GPS does fail ... then log + current chart will give EP - which is better than DR (which to be honest is about as much use as a fa*t in the wind) .... so at least you'll have a better idea. Having plotted GPS positions up until loss of GPS is not really an answer to push on - as current / tides change all the time and GPS plots based on actual position - albeit from orbital hardware and not calculator !

Yer takes yer choice ..... I'll have both please.

 

[Cruiser2B]

I've generally taken the logged distance to be "distance made good" - the distance along the charted path between points A and B. On those occasions where we've gone around in circles and B is the same place as A, then I note "distance through water." It really doesn't matter what you use, so long as it's clear to you and anyone using your log.