Mechanical log

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No you're not being dense - and neither am I, I don't think. Surely GPS gives me COG but not course to steer which is what I need to have speed through water for in order to calculate - otherwise I am surely just fighting the tide the whole time?

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Ok, so I have a chartplotter, it shows my speed & COG as a line projected ahead of the boat. All I do is aim that "pointer" at my next waypoint/ destination and hold that course. It will change a little as winds/ tides vary so I adjust my course.

With a GPS, you have set a COG and it will show you cross-track error, adjust course to minimise CTE.

Paper charts are fine as back-up or for interest's sake. But why not use your expensive electronics in the way they were intended? Would you use a lead-line if the echo-sounder was working?

The key problem with paper calculations are that tidal estimates are exactly that & applying them hourly is a further aproximation. The electronics are sensing & displaying what is actually happening in real time (or as near as d@mn it).

Well, that's the way I see it anyway, you may think otherwise /forums/images/graemlins/grin.gif

Enjoy
 
If tyou must have a mechanical log then the Walker was accepted as the most accurate. I remember articals in PBO when they compared different logs and they used the Walker as a master to compare the others. But as others have said there are many disadvantages. 1. Weed can stop them or worse slow them down, so you have to check them regularly to be sure they are clear. 2. They can get caught on fishing lines/ lobster pots. 3. Don't make turns close by navigation bouys. 4. If the wind drops and the boat stops they can hang down close by the prop, if you forget to bring it on board the line can finish up around the prop. I must admit I identified all these problems many years ago when on the steep part of the learning curve. So keeping spare rotors is advisable. And finally I am convinced that fish will eat them as I lost one when I was sure none of the previous reasons applied
 
Dave - you're correct. A point that the GPS followers often miss. Of course it doesn't matter so much where the tides are less fierce. It's the difference between the multi-correction method and the single correction method. But to work up a Course to Steer you need to ignore the GPS. ID your track and use the tidal diamonds to predict the tide you will encounter for the intended time, plot those on the chart and find your course to steer. Then set off at the intended time of departure otherwise the tide effect will be different! Then use your GPS to check progress. Now a mechanical log (subject to its faults that have been described above) is useful in that it will tell you if you are more or less maintaining the expected average boat speed on which basis you plotted the tide effect. If you are using the single correction method, the cross track error on the GPS may give you a fright as you will be allowing the tide to take where it will, safe in the knowledge that it gives you the fastest passage for the journey and will bring you back to the track at the last knockings - assuming you stay on schedule. But there is a big 'BUT' here for you with the single correction method. The down side of benefitting from the single correction method is that it takes you from the track and therefore you need to ensure that such an approach does not set you down on navigation hazards, in which, of course, the Channel Islands has a special place. Before GPS, people used the multi-correction method to keep close to the track plotting the tide for each hour or each half hour and working up a course to steer for each time period. Effectively you are breaking the journey down into a number of smaller sectors. Longer in distance but then if you needed to keep to the track that was the only way.

So a mechanical log does have a place. Like everything else in boating, its a personal judgement. I like it as I like to know that I'm maintaining a minimum boat speed. Otherwise if I've got say two knots tidal help, the SOG from the GPS is kidding.

Are you old fashioned? Umm, well I certainly am. I enjoy working the tides and knowing what is going on.
 
Sorry, but I completely disagree. If you follow a straight line COG to your destination using the GPS/ Chartplotter you will automatically be making compas course adjustments to compensate for the real time tidal rate and drift.

Stands to reason dunnit? Shortest route is a straight line & naturally also quickest. All your calculations do is try to pre-plan the compass course changes in advance - bit of a complete waste of time as you will be using averaged data from tables rather than compensating in real time for actual conditions.

Prove me wrong please, & I will apologise.
 
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Stands to reason dunnit? Shortest route is a straight line & naturally also quickest

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Tiller girl is correct. Attempting to maintain a straight COG for a passage across tidal waters by constantly altering the course steered in order to stay on that course will not be the quickest.

Traditionally you would plot hourly estimated positions from the log reading and the published tidal data but with GPS you can avoid this by plotting the GPS position. With a GPS plotter of course this will all be done automatically. You will see on the plotter how the tide carries you away from the straight line course and back again but that is still the quickest route.
It will be the shortest distance measured through the water even though the distance travelled over the ground will be greater.
 
Thank you Vic!

Just to add, without a mechanical log (and I accept all the points about accuracy that have been made) you have no (easy) way of knowing if your boat speed is maintaining the average on which basis you calculated your passage plan. For example if you worked out your passage plan on the basis that you would average 6 knots boat speed and your SOG is say 6.25kts, are you maintaing your hoped for boat speed? Well you could go to the chart, look up the tidal diamond and estimate the tide effect at that place and time. You might have 2.2 knots of tide helping and you are falling behind your passage plan. Does that matter heading across the channel - well probably not. Does that matter if you are trying to arrive at the Alderney Race close to a certain time or trying to avoid the whole of the ebb out of the Essex Rivers? Well it might! The point about a mechanical log is that its an easy way of monitoring boat speed and, hey, with modern stuff (just to show I'm not a complete luddite), it will give you your average speed as well. Well it would if a certain well known manufacturer's gear didn't break down so often.

If you want an authority, try Michael Reeve-Fowkes. I'm sure there are others.
 
Sorry, neither Vic nor I have explained it. If I could draw it for you it would be easier but I'm sure you have Reeve-Fowkes in stock. The basic reason is that by fighting to stay on the line of your track you are mimising or reducing some of the beneficial effects of the tide. The best way to see it is to passage plan two identical passages, once using the single correction method - plotting all the expected tidal effects and then drawing one course to steer line, the second time using the multi correction method - say half hour plots. The 'lets have no cross track error' method via GPS is taking the multi-correction method to the Nth degree (sorry didn't mean to pun) - look on the bright side you can legitimately tell customers that they need a mechanical log when they buy a chart plotter!
 
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Sorry, neither Vic nor I have explained it

[/ QUOTE ] Really too difficult to explain within the limits of the forum as it needs some good diagrams. Could be done i suppose doing drawings, scanning them, uploading them to Photobucket etc. but it's easier to refer any one who doubts it to a good book on navigation. It is one the things one is taught on a YM or Coastal skipper course so I was a bit stunned that anyone would believe otherwise.
 
I think it is easy to understand when you compare a single correction plot with a multi-correction plot and you see that for each of the (say) one hour plots what you are doing is aiming (and counteracting the tide) for an intermediate point on the line between you and your destination - rather than aiming for the destination. And I think that's where the confusion can arise because if you cut out the 'sub-plots' in your mind which is what you do when you follow the GPS method, then you don't realise the impact of the tide on your vessel. It is a myth that you've counteracted the tide by having the X track error as zero; you've fought it - unless of course its straight up and down.

But I agree, it needs diagrams to explain. But to quote from the RYA Manual (old version I admit)

"Finding a single course to steer thoroughout a passage instead of working out a new couse to steer each hour as the stream changes reduces the amount of chart-work and also cuts down the time taken on passage. The accuracy of the couse found depends upon the correct assessment of the speed at which the passage will be made.......... (They mean boat speed there so a mechanical log is required)

Fixation with planned tracks is a common failure of inexperienced navigators, the fact that the yacht is on the line giving a misplaced feeling of confidence that all is well. This fixation leads to much wasted time, with the navigator unnecessarily fighting adverse streams to regain the line and having a reluctance to alter course for other vessels for fear of failure to follow the plan and consequently getting lost".

The getting lost bit is a bit 'pre-GPS' I think.

Dave, I hope you are following this and not letting the modern technologists persuade you their gadgets are the only way to do it. Helpful, of course, but you are right.
 
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On ebay, Item 180171006629. ........................ This one looks complete.

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It appears to have only one mounting plate. There should be two. They are handed and marked, one for the port side and one for the st'b'd side. It looks as though the entire line has been used in one length (on a large vessel maybe) The spare bits should include a spare sinker and crimps for the line.

It looks as though it has had very little use.
 
Don't know that this will help much but it will help stir up the discussion....
Rememer Newtonmeters from your school science lessons? A complex piece of scientific apparatus that involved a spring in a plastic tube with a scale drawn on the tube. The more force applied to the spring the more it stretched. Genius. Surely you could attatch a bit of string with a bit of ply on the end which you bung overboard. It would need a bit of calibration (with a GPS at slack with no wind?) but you could mark off knots instead of Newtons and then you have the worlds second simplest log (1st being a log).
I haven't tried it but reckon I'll give it a go next year when I get my brand new, all singing, all dancing Hurley 18 on the water.
Sailfish
 
When I went to school nobody talked about Newtonmeters! Foot Pounds dear boy, Foot Pounds!

Most difficult problem would be to keep the drag on the piece of ply constant. Wouldn't you end up with a bit of a complex shape with weights. Something to stop it spinning. Except it wouldn't I reckon so you better build something that spins at a constant rate - but then would that have enough drag to pull the rusty spring - so we better invent something that converts the spin into a reading - whoops that's a Walker log. Sorry to be a pessimistic old B*** but I reckon that the design stages of the Walker log. I think the spinning of the ply would stop what is otherwise a sound idea.
 
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