To log or not to log that is the question

Barnac1e said:
I really cannot understand anyone contemplating boring a hole in the hull to fit a log paddle-wheel sensor. When I had one it always fouled in a short time and needed constant removal and cleaning - water got in even with sealing flaps. The whole procedure was a pain and is so primitive that I would sooner go back to my Walker Excelsior Mk IV trailing log than re-install it, except that is somewhere in the cellar and not even on the boat, just like my sextant and tables.

The original skin fitting is now blanked off and my Garmin plotter provides a very precise SOG in one of the data windows - far more accurate (to two decimal places) than the original VDO log ever was and it gives trip and total distances too. True that there is no through-the-water speed but I do not need that, SOG is the important thing and just as useful for sail trimming.

I have two other GPS back-ups, including a netbook running OpenCPN with the dashboard readout. I have never known anything ever interfering with full GPS reception and think it infinitely unlikely - too much of the world's navigation/surveying services rely on it.
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Although I understand your logic, I think you express your opinion a little too strongly. Speed through the water is important - it helps you know what is happening to the tidal stream if nothing else. Just as you can't imagine drilling a hole in the bottom of the boat, so I can't imagine sailing without a log. I would want to know whether my SOG was slow due to debris round the keel/prop/rudder or whether the tide had turned earlier than predicted or what? Perhaps its a mental state of mind from growing up navigating without GPS or Decca.

The fouling thing really isn't too much bother. I don't even anti-foul our impeller but remove it and clean it when I go sailing. You just clean the thing (flap or no flap) and sponge out the bilge.

To put my relationship with the log in context, I also carry a Walker Log on board in case the electronic one fails...
 
john_morris_uk said:
Although I understand your logic, I think you express your opinion a little too strongly. Speed through the water is important - it helps you know what is happening to the tidal stream if nothing else. Just as you can't imagine drilling a hole in the bottom of the boat, so I can't imagine sailing without a log. I would want to know whether my SOG was slow due to debris round the keel/prop/rudder or whether the tide had turned earlier than predicted or what? Perhaps its a mental state of mind from growing up navigating without GPS or Decca.

The fouling thing really isn't too much bother. I don't even anti-foul our impeller but remove it and clean it when I go sailing. You just clean the thing (flap or no flap) and sponge out the bilge.

To put my relationship with the log in context, I also carry a Walker Log on board in case the electronic one fails...
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You are right, John, I was too opinionated with that post - so long back I was surprised to see it resurrected to bite me. I suppose I should not have extrapolated my personal circumstances of warm waters of the Adriatic with it's fouling and lack of tidal streams to other areas where speed through the water could be informative.

However, I still wouldn't fit one wherever I was after the luxury of not having to keep attending to them. Each to their own.
 
Those who say that nothing can possibly go wrong with GPS, obviously don't sail in the North and Northwest of Scotland. Here the MOD regularly carry out jamming of GPS, over huge sea areas, as part of their war games.
 
I'm a little surprised that no-one has yet mentioned doing a calibration run on one's log, whether trailed or thorough-hull.... I s'pose that's gone the way of doing a check and calibration swing of one's compass. Just another reflection of the "If it's still on the boat somewhere, it's OK" approach to navigation.....

Me? I have 2 Walkers Knotmasters, a Wasp and a Stowe, a fitted B&G thingy, 2 sextants ( used to be 3 but I gave the bubble-thingy to the RIN's archive ) and 3 GPS units.... no, five. I have a spare hockey-puck antenna somewhere.

'Man is not lost'

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,,,But he sure gets confused sometimes! :rolleyes:
 
NormanS said:
Those who say that nothing can possibly go wrong with GPS, obviously don't sail in the North and Northwest of Scotland. Here the MOD regularly carry out jamming of GPS, over huge sea areas, as part of their war games.
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I'm curious how you think this can happen? Not that I don't believe you, but GPS works on (a kind of) line of sight, so in order to jam, they'd need to blanket you...

Alternatively they could turn off GPS transmissions from a satellite or two, but firstly, I thought the GPS system was operated by the US, and secondly, the implications would be huge for everybody using the system - it's not just used for car and boat satnav systems...?

How often is regularly?
 
nigelmercier said:
Water speed can be measured with spit and a stopwatch, or any other floating marker.
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It's fun doing that.

I believe the towed "Walker, Wasp" type log was still used by many ships well into the 1970's. Something nice to own, both as a back up, and something to play with.

I wonder if they have paddle wheel logs on container ships.
 
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chrisbitz said:
I'm curious how you think this can happen?
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There's no "think" about it. These exercises are announced via notices to mariners, with planned duration and area to be affected. It's not some tinfoil-hat conspiracy-theory explanation for a brief GPS blip.

Pete
 
oldbilbo said:
I'm a little surprised that no-one has yet mentioned doing a calibration run on one's log, whether trailed or thorough-hull
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On the new boat I've just fitted a new log instrument (same old transducer) and will certainly be conducting a calibration as soon as feasible. There's a big section on it in the manual.

I also need to do a calibration of the electronic compass - it's currently about thirty degrees out from the steering compass!

The latter is more or less correct (based on the fact we arrived at SW Shingles from Poole the other week as planned) but I still want to swing it. I'm happy enough with how to check, but adjusting still seems a bit of a black art.

Pete
 
prv said:
There's no "think" about it. These exercises are announced via notices to mariners, with planned duration and area to be affected. It's not some tinfoil-hat conspiracy-theory explanation for a brief GPS blip.

Pete
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I'm sorry to have offended you, I didn't mean to. I'm still curious how these happen. I'm guessing they don't have a 40 mile wide tarpaulin, so do they turn off uk satellites, or do they have an agreement with the us? Or is it some sort of radio signal jammer?

How often is regularly? Monthly? Yearly? Every 10 years?
 
chrisbitz said:
I'm curious how you think this can happen? Not that I don't believe you, but GPS works on (a kind of) line of sight, so in order to jam, they'd need to blanket you...

Alternatively they could turn off GPS transmissions from a satellite or two, but firstly, I thought the GPS system was operated by the US, and secondly, the implications would be huge for everybody using the system - it's not just used for car and boat satnav systems...?

How often is regularly?
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It can happen very easily. GPS depends on you receiving radio signals from a set of satellites in a relatively high orbit. (20,200 km) The signal you receive is incredibly weak, and to get a good fix you need to receive signals from at least 3 satellites (for a 2D fix), and 4 or more for a half-way decent fix. All that is necessary to disrupt the system is for something to broadcast a competing signal on the same wavelengths as the ones the satellites use. Blocking GPS over an area is quite simple, and as others have pointed out, is routinely done during war games off the NW of Scotland. Outages have also been announced in Wales from time to time, though I think those have mostly covered land areas.

Of course, the US Navy can switch the whole lot off anytime it wants, or degrade the accuracy if that turned out to be to their advantage (there's an encrypted signal that can be used for precise navigation even if the public signal is degraded)! But they have promised not to do that, and so far (since Bill Clinton's days) haven't.

Bad solar conditions can also EASILY disrupt GPS, either blocking it completely or introducing random, unknown delays into the "time of flight" of the radio signals. In the early days of GPS it was commonplace to find that recorded position data were useless because the 3 or 4 satellites you'd received had been affected by ionospheric disturbances, which are caused by solar particles. Not so bad these days, as there are enough satellites to provide redundancy, but GPS won't work during a solar storm.

GPS is incredibly clever and extremely useful. BUT it is basically a very fragile system, and relying on it as you only means of navigtaion is not a good idea. That's why the navy practise war games with GPS blocked - they assume it won't be available in the case of a conflict.
 
chrisbitz said:
I'm sorry to have offended you, I didn't mean to.
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Sorry, misapprehension somewhere - I'm not in the slightest bit offended and I hope you're not either :)

My understanding is that it's a jamming transmitter - the signals are fairly weak by the time they hit the earth so it doesn't take much - but I don't really know.

You can buy short-range jammers online, though they're probably not legal. The intended use is for fleet drivers who don't want the boss tracking where they're having their tea-breaks, but they're also used by car and motorbike thieves to counteract anti-theft trackers.

I don't know how often as I've never sailed in that area, but I guess I've seen a mention on here most years.

Pete
 
>My head says get one, my heart says don't cut another hole in the hull.

I removed the log because I got fed up having to clean the paddle wheel. GPS will give you speed over ground experience will give speed through the water. If you go ocean sailing towed logs have been known to bitten off by sharks.
 
dunedin said:
I find the speed readout interesting and helpful for optimising the sail set

I have never looked at the log distance reading in the six years since getting the boat. Each to their own way.
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...but you could also use the speed over ground to optimise the sail set???

I've always thought knowing your speed through the water is nice - but it's pointless if at the same time your speed over the ground is negative... :D

As you say though - each to their own - and I have to admit to being driven by a pathological hatred of any more through hull fittings than the minimum I need.....
 
Fantasie 19 said:
...but you could also use the speed over ground to optimise the sail set???

I've always thought knowing your speed through the water is nice - but it's pointless if at the same time your speed over the ground is negative... :D

As you say though - each to their own - and I have to admit to being driven by a pathological hatred of any more through hull fittings than the minimum I need.....
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The point is that velocity is a vector - it has both magnitude and direction. SOG from the GPS could be at 90 degrees to your heading, and so give very little indication of your performance; to see small changes in your performance you'd have to look at the direction of the SOG, not it's magnitude. Speed through the water relates directly to performance; SOG from GPS doesn't necessarily (or at least, not in a simple, intuitive manner). You could (theoretically) compute the speed through the water from SOG and heading (assuming no leeway), but I don't think it is routinely available from most instruments.

Sorry if this isn't clear; you really need to draw the vector triangles to see what I'm getting at.
 
chrisbitz said:
Speed over the ground is never negative - it just might not be in exactly the direction that you wish it to be... and I don't just mean backwards... :-)
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I stand corrected - you're right of course..... the better analogy might be "your speed over the ground is zero".. which it often has been with me..... :D

Interesting arguments for speed through water - but it still wouldn't make me want to drill another hole in the hull... the risk may be small, but it's still a risk.........
 
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