GPS v Log

[Whiskey2]

Powering in to Portsmouth Tuesday afternoon against a strong spring ebb making all of one knot on the GPS from the no'4 bouy in, I wondered had my log been working how many miles it would have recorded for the trip? Coming from the aviation world I have kept a logbook since starting to sail in February and have nearly 300 miles actual miles logged, what should I be logging if as one day I may like to do a YM? Surely taking 4 hours to flog against a foul tide returning from Cowes counts for more than 8 miles? Mel.

 

[ccscott49]

No it doesn't, but it's all valuable sea time anyway, of course if it was at night it would have counted as night hours! I mean I've spent many an hour at anchor due to foul tides, the log would show me travelling at about three knots and logging distance, but I was still enjoying myself!

 

[rogerroger]

I'd take it from the log - this way things are bound to even out. You'll log less miles with the tide and more against it - swings and roundabouts.

However, cross the Atlantic in 3 weeks from Canaries to Carribean and you'll "lose" hundreds of miles due to the current with you.


Roger Holden
www.first-magnitude.co.uk

 

[Whiskey2]

Good point but logging GPS miles takes no account of tacking, it measures from A to B So it can't even itself out, can it ?

 

[webcraft]

GPS measures distance covered over the ground irrespective of direction, as it's measuring and calculating on a continuous basis.

If you leave the marina and sail 20 miles due West, then 20 miles due East back to the marina, the GPS log will show 40 nm.

- Nick

BlueMoment.Com
http://www.bluemoment.com

 

[rogerroger]

yes - I'd have to agree with that. Otherwise SOG would be inacurate as well wouldn't it?

Oh and Nick - looking at your photo I'm making a mental note to go to Photoshop tonight to see if I can rival it ! (which I doubt, yours deserves a forum prize I think)



Roger Holden
www.first-magnitude.co.uk

 

[Guest]

If you are worried about miles to qualify to take an RYA exam please read the instructions on page 2 of RYA publication G15/98 the Cruising Logbook. Particular attention should be paid to the requirements for the 5 qualifying passages.

 

[webcraft]

Re: Photo

A forum photo comp - excellent idea - it's a real challenge to do much in 65x75 pixels.

Look forward to seeing yours, Rog.

- Nick

BlueMoment.Com
http://www.bluemoment.com

 

[rogerroger]

Re: Photo

Well I powered up my Mac last night and had a play - but the realisation that having Photoshop doesn't make one a graphic designer anymore than owning a saw makes one a carpenter soon hit me !

Don't hold your breath - I think your pole position is safe !!

Roger Holden
www.first-magnitude.co.uk

 

[peterb]

The point about the differences between GPS and log distances is settled on p 32 of G15/98. Under the heading "Distance logged" it says that the distance recorded should be that "sailed by the log, in the open sea, outside natural or artificial harbours..."

 

[Guest]

...which is at odds with the requiremenst for the 5 "qualifying passages" as described on p2 which says that they should be ...over 60 miles measured along the rhumb line from port of departure to the destination.
Care should therefore be taken when entering these in the log book. I have sailed from Plymouth to Fowey, rhumb line distance about 16 miles, and logged as much as 50 miles when beating against a foul tide in a gaffer.

 

[Mirelle]

Yes, indeed. The word "log" in the sense used by the RYA means "log book" not "patent log", surely? I.e. "five passages where the distance is over 60 miles measured on the rhumb line, as recorded in the log book" ?

 

[Whiskey2]

Thanks for all the replies it sounds like I am recording distances the correct way using the GPS, I have done many a long task in my glider where I flew an extra 25% looking for thermals or dodging showers but you only log the task distance. Thanks again.

 

[peterb]

Rhumb line passages

I've never liked the RYA requirement for the passage distance to be "measured along the rhumb line from the port of departure to the destination". Yes, it works nicely when you are measuring the distance from Lymington to Cherbourg; you get a nice 68 nm, well over the specified 60.

But try sailing from Bradwell to Calais. You have to find your way across all those Thames Estuary sandbanks, do careful tidal calculations to get to North Foreland with a favourable tidal stream, then cross what is often claime to be the busiest TSS in the world. I reckon you are doing well to cover the route without sailing at least 75 nm over the ground. But then you measure along the rhumb line and you get 59 miles. Of course, the rhumb line goes across the middle of Kent and you can't sail it, but that's the line the RYA specify.

Twenty years or so ago I complained to the RYA so much that they changed it. It became "by the shortest navigable route". But only for one edition of G15, after which it reverted to the rhumb line. I've never been able to summon up the energy to go through the campaign again, but I still think that "by the shortest navigable route" is better.

 

[HaraldS]

Over time it doesn't seem to matter much. I have kept both figures in my log book for the last two season and the difference over about 2500 miles is 4%. The water log reads a bit more and that could well be just an adjustment problem.