What is more accurate for speed, navionics or nasa log?

[Supertramp]

Both are useful. On passage, speed through the water let's me know I'm on track and maintaining my speed even if I have a knot or two of bad tide. And whether the effort of getting the pole up or reefing has worked. Navionics gives me SOG.

My paddle wheel and the bottom fur up and I have to recalibrate a couple of times a year - just match the navionics speed in a tide free spot. My calibration is in increments anyway so exact accuracy is impossible.

 

[johnalison]

Navionics gives very accurate speed over the ground. Your log gives speed through the water and less accurate, Depends what you want.

I would rephrase that as Navionics gives a very accurate speed over the ground but a very inaccurate speed through the water, while a log, properly calibrated, gives an accurate speed through the water but a less accurate speed over the ground.

 

[johnalison]

Both are useful. On passage, speed through the water let's me know I'm on track and maintaining my speed even if I have a knot or two of bad tide. And whether the effort of getting the pole up or reefing has worked. Navionics gives me SOG.

My paddle wheel and the bottom fur up and I have to recalibrate a couple of times a year - just match the navionics speed in a tide free spot. My calibration is in increments anyway so exact accuracy is impossible.

I think there is potential for electronic or Doppler logs to be pretty accurate, but absolute accuracy is not necessarily critical if used for sail trim or checking on power. I normally set my log to read about 0.1-0.2 over, a hark back to the old adage that a distance log should not be set to under-read.

 

[Babylon]

Neither is "less accurate" than the other, they're just different systems.

I'm lowish-tech, like trad nav and am a fan of the log, which I re-calibrate only rarely because it never seems to go out (it helps that I replace it with the blank plug when not aboard and sailing so it rarely gets fouled and easy to clean if it does). Plan passages on paper according to tides and wind, then the log tells me everything I really need to know. Hawk wind-indicator, telltales and the balance and feel of the boat tell me I'm in the grove. Cross-channel or long coastal passages the GPS is helpful - things like XTE, SOG, TTG etc add to the information - but for me they're secondary.

 

[wonkywinch]

Speed through water is now of limited value except to measure change in boat speed, for example after changing sail trim or engine RPM.

The harbour masters around the Solent wouldn't agree with you ;-)

 

[ylop]

The harbour masters around the Solent wouldn't agree with you ;-)

Presumably though they have no easy means to measure the STW of a vessel?

 

[Crinan12]

Apart from trying to compare the equivalent of chips with mash, what state is your paddle wheel and surrounding hull in? Any growth, paddle wheel misalignment, spindle wear will change it's output.

Paddle wheel brand new/just installed and hull is clean
Cheers

 

[Chiara’s slave]

Presumably though they have no easy means to measure the STW of a vessel?

I daresay it’s more of a judgement call about excess wash.

 

[davidmh]

I would disregard the Nasa log in a canal especially the narrow ones. The narrow boat displaces a lot of water and the water accelerates around the hull and the boat moves along. It does the same thing in open water but the difference is minimal.
Dvaid MH

 

[V1701]

I fall into the camp of having removed the paddle wheel, replaced it with the blank & never put it back again. I think there are probably quite a ot of people who will have done the same. If you leave them in they just get fouled & I can't be bothered taking it out & putting it back each time for what you gain...

 

[Boathook]

I like both speed through water and SOG. Both show different things and sailing in the Solent and surrounding areas you can see whether you have caught the back eddies, etc.
Speed through water allows checks on sail trim and engine economy, etc.

 

[johnalison]

The harbour masters around the Solent wouldn't agree with you ;-)

Brightlingsea speed limit is based on SOG.

 

[johnalison]

I would disregard the Nasa log in a canal especially the narrow ones. The narrow boat displaces a lot of water and the water accelerates around the hull and the boat moves along. It does the same thing in open water but the difference is minimal.
Dvaid MH

There was a time when I thought that the Dutch canals would be a good place to calibrate my log, but it didn’t work out that way. My impression, after nearly 20 years often in those waters is that the shallow water both slows the boat and causes the log to over-read. I had not noticed this with my previous, smaller, boat, and maybe it differs between boats. In some canals a log that read well in open water would over-read by 3/4 knots. My feeling is that any depth less than about 5m has some effect. Some Dutch canals also have a stream of up to 1/2 knot, which doesn’t help either. I have always regarded calibration as a dynamic process, and not easily done.

 

[Daydream believer]

There was a time when I thought that the Dutch canals would be a good place to calibrate my log, but it didn’t work out that way. My impression, after nearly 20 years often in those waters is that the shallow water both slows the boat and causes the log to over-read. I had not noticed this with my previous, smaller, boat, and maybe it differs between boats. In some canals a log that read well in open water would over-read by 3/4 knots. My feeling is that any depth less than about 5m has some effect. Some Dutch canals also have a stream of up to 1/2 knot, which doesn’t help either. I have always regarded calibration as a dynamic process, and not easily done.

I cannot see that. I compare the GPS speed to the log in the Walcheran canal & I do it both ways. Experience shows typically .1 kts of current. If the boat goes slower then the GPS records a slower speed.
One adjusts the log to match, because that is the speed through the water. Then one does a check the other way to cancel out the effect of the current. The Walcheran canal is about 3-3.5M deep in the middle, I seem to recall. I do the check whilst motoring in the centre. Years ago one might have been trying over a measured distance( typically between 2 bridges on that canal), but now one would almost always use the GPS to reduce any errors.

Of course one can normally only be accurate at one typical speed (I choose 5kts) & the error varies depending if one is going slower or faster than this. That is just the nature of the beast & I usually write down the log reading at, say 3kts & 6kts, so I know the variation.

I cannot believe that a relatively small dynamically efficiently shaped yacht will be pushing water back under itself, in the way large square barges do when travelling along canals.

How do you know in open water the log is reading out by 3/4kt? Are you using the same speed that you calibrated the log in the canal? Currents,from wind & tide plus wave motion, being present, thus not an accurate prediction even if one does reverse direction etc.

 

[oldmanofthehills]

All paddle wheel logs need calbrating. Fairly simple setting of percentage (of logs apparent reading at the paddle). The problem is that if one sets it at launch it under-reads as grot grows on the paddle, and if one sets it mid season it is wildly out by 10 % or more once hull and paddle cleaned when next dried out.

I did buy a trailing log which should have been more consisted as not kept in the water, but it was ancient electronics so quite unstable

 

[Daydream believer]

All paddle wheel logs need calbrating. Fairly simple setting of percentage (of logs apparent reading at the paddle). The problem is that if one sets it at launch it under-reads as grot grows on the paddle, and if one sets it mid season it is wildly out by 10 % or more once hull and paddle cleaned when next dried out.

I did buy a trailing log which should have been more consisted as not kept in the water, but it was ancient electronics so quite unstable

Obviously , if one clogs it up with carp it will not work properly. However, it is a 10 minute job to draw a log impeller out,( modern version) Check it, clean it & replace it. Then of course one can always apply a coat of the correct antifoul paint, as well as lift the boat & have it jetwashed mid season & start of the season. Just depends how clean one wants the hull etc. There is no point trying to calibrate with a dirty paddle wheel.

 

[oldmanofthehills]

Obviously , if one clogs it up with carp it will not work properly. However, it is a 10 minute job to draw a log impeller out,( modern version) Check it, clean it & replace it. Then of course one can always apply a coat of the correct antifoul paint, as well as lift the boat & have it jetwashed mid season & start of the season. Just depends how clean one wants the hull etc. There is no point trying to calibrate with a dirty paddle wheel.

 

[oldmanofthehills]

Those of us on moorings can only get lifted out once a year, and drying out is not possible for many so all we can do is put antifoul on paddle.

As taking out our log paddle spews water over boat electrics and some electronics, I avoid that. I only really care in absolute terms about speed over ground , so log is just a backup in case plotter fails

 

[johnalison]

I cannot see that. I compare the GPS speed to the log in the Walcheran canal & I do it both ways. Experience shows typically .1 kts of current. If the boat goes slower then the GPS records a slower speed.
One adjusts the log to match, because that is the speed through the water. Then one does a check the other way to cancel out the effect of the current. The Walcheran canal is about 3-3.5M deep in the middle, I seem to recall. I do the check whilst motoring in the centre. Years ago one might have been trying over a measured distance( typically between 2 bridges on that canal), but now one would almost always use the GPS to reduce any errors.

Of course one can normally only be accurate at one typical speed (I choose 5kts) & the error varies depending if one is going slower or faster than this. That is just the nature of the beast & I usually write down the log reading at, say 3kts & 6kts, so I know the variation.

I cannot believe that a relatively small dynamically efficiently shaped yacht will be pushing water back under itself, in the way large square barges do when travelling along canals.

How do you know in open water the log is reading out by 3/4kt? Are you using the same speed that you calibrated the log in the canal? Currents,from wind & tide plus wave motion, being present, thus not an accurate prediction even if one does reverse direction etc.

My belief in what should be the speed is based on averaging the results over time and from different places. The canal disparities also seem to vary in relation to depth, many canals being 2m or less. There also seem to be significant currents in the deeper Kiel canal, but, as I say, this may be a feature of some boat designs.

 

[Snowgoose-1]

With 'clever' navigation systems the log speed is used to calculate and display optimum sailing tack, effect of current, etc. Our B&G Triton displays all sorts of information that uses log speed, in conjunction with true and apparent wind speed and direction.

Just wondering if modern plotters can say use the nearest tidal diamonds , or interpolate and display estimated speed and direction of tide on the plotter ? I there will of course be lost of localised variations. I found paddle wheel logs were ok when there was nothing else but too frustrating otherwise. Woul be interested if anyone still uses a trailing and what they feel about it. For fun, I still use time and distance when anchored using bow/stern . Good for localised knowledge.