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Old 27-11-2008, 00:03   #1
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GPS vs Knot Meter

I have pondered for some time now the actual need for a knot meter with a GPS (or more than one) on board. I recently tried to calibrate my knot meter (with a GPS) and found that when I calibrated it at 3 knots, it would read too high at 6 knots and if I calibrated it at 6 knots, it read too low at 3 knots. We're talking 25%. So a trip log would be quite useless. I'm thinking with a GPS, the log is dead on as well as the speed (on average). The advantages are many...
1) no mechanical log to pull out of a thru hull all the time, which means no excess water flowing in.
2) one less thru hull. Always a good thing.
3) less daily battery draw.
So just a depth sounder and thats it.
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Old 27-11-2008, 00:38   #2
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Unless your GPS has a an impellor it's measuring distance over the bottom. A knotmeter measures speed through the water. The knotmeter is very useful developing improvements in boat trim and sail combinations that can be masked by current with the GPS.

Basically it boils down to, do you want a backup for doing a DR if the GPS quits and do you have a need for measuring speed through the water vice over the bottom.

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Old 27-11-2008, 00:52   #3
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Interesting points. I feel the knot meter, being in the water probably never really gives an accurate reading anyway with factors of current, heal and wave action. Seems GPS has a steadier reading for sail trim, although I think sail trim is better achieved with tell-tails.
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Old 27-11-2008, 01:39   #4
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My though-hull log needs volts just as much as my GPS. I only have one log, but now four GPS on board, so I am more likely to lose the log function. The log wheel often gets jammed by something. Plus the log is inaccurate. I tried calibrating it with GPS in a canal (negligible current) only to find that it reads differently in fresh water than in salt. So much for the log.

However, I do find it useful when sailing. Whenever, I'm storming along but the GPS shows a miserable 4 knots over ground, I know from the log that it's more like 6-7 knots through the water, so adverse current and not dragging another lobster pot.
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Old 27-11-2008, 02:42   #5
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Both are useful.

Knowing how fast your boat should go through the water for given conditions helps with boat and sail trim and can point out adverse currents.

GPS tells everyone when you will get there...
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Old 27-11-2008, 03:19   #6
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If you have a rapidly updating and accurate GPS it can also be used for trim and so forth since if you trim better you speed over the bottom will increase regardless of current.

Calibration of a knot meter requires a series of measured runs, tiomeing and calculation. You can even do this with current and almost have to since most of the time there is some current running.

Shiva has B&G instruments (old Hornet cruise pack) with a speedo that can report to 2 decimal places and it has a litte indicator as well to show speed trend - faster, slower or no change. Precision is user setable, I believre to hundreths, tenths of even integer readings. This is something I don't have on any of my GPS units (3) which report only to a tenth of a knot. The depth sounder likewise reports to one decimal place for depths under 20 feet IIRC and this can matter in shallow areas (anchorages) where inches mean the difference of going aground or being afloat.

Having said all the above. The speedo impellor is prone to misreading becase of fouling on or around the impellor and this is evidenced by the fact that when there is some fouling the speed seems to read slow and often clears after a while of moving (relatively ) fast through the water. I suppose a non mechanical transducer for speed would solve all those problems, but I still like to 2 decimal places readout and trend indicators.

Hundreds of a knot hardly matter, but see them is more useful for trim purposes. No?
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Old 27-11-2008, 05:35   #7
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This is an interesting question that I hadn’t thought much about.

On the larger yachts I used a Doppler speed system which was very accurate and depending on depth could be made to read bottom or water. From that I could easily calculate currents in order to ground stabilize the older True Motion radars. Today, the Radars do this automatically

That was also in the days when GPS was still a US monopoly and they did warn that they might shut it down for security reason.

Now, I think loosing GPS is about the only reason I would still have an alternate log, so as to use for DR calculations. But I wonder if we would ever loose the GPS system?
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Old 27-11-2008, 05:40   #8
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Well, there's nothing wrong with having a sextant and a propeller log on board. 'Waratah' will have them on board for sure. She is almost finished now. Can't wait to do the sea trials.
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Old 27-11-2008, 06:02   #9
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Quote:
Originally Posted by Celestialsailor View Post
I have pondered for some time now the actual need for a knot meter with a GPS (or more than one) on board.......... The advantages are many...
1) no mechanical log to pull out of a thru hull all the time, which means no excess water flowing in.
2) one less thru hull. Always a good thing.
3) less daily battery draw.
So just a depth sounder and thats it.
I have also been thinking along these lines but I have fitted a knot meter (log) just because I like to look at it from the cockpit (my GPS units are below decks). I think the log will go the way of the sextant. Both work OK, neither are essential any more. Yet I also carry a back up Walker trailing log, sextant and leadline .

Quote:
Originally Posted by Pelagic View Post
....... But I wonder if we would ever loose the GPS system?
If we loose the GPS system, I think we will have bigger problems to deal with other than navigating our sailboats even though some of us will be able to fall back on non GPS methods
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Old 27-11-2008, 07:35   #10
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Our new to us boat does not have a speed log and relies on the GPS for speed. At first I did not like this but after a while I have come to the conclusion that this works fine and there is no need to drill another hole in the bottom. I can't imagine a situation where knowing the speed to a hundredth of a knot would ever make a difference. With our previous boat the speedo was always fouled so I was constantly pulling it and cleaning the impeller. So I think we will leave things just as they are. We have two GPS units plus a unit attached to the computer so I doubt all 3 will fail at the same time. If all 3 did fail it would not stop or hinder us from getting home or to safe harbor just because we did not know how fast we were going.
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Old 27-11-2008, 07:52   #11
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Our old Mirage 26 didn’t have a depth sounder, nor a knot log. It had a mag’ compass.
For coastal piloting, I did T/D calc’s when we had visibility; and floated twigs bow to stern to calculate speed in fog. Worked like a charm.
GPS, Knot-Log, and etc are all great improvements, but not absolutely necessary.
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Old 27-11-2008, 08:38   #12
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Don't forget about true wind functions, which require boat speed for the calculations. Most modern boat performance instrument systems give true wind readings based on boat speed through the water and while some of these systems allow you to use speed over ground from a GPS for the calculations, they recommend against it because SOG will give incorrect data as it is mixing a ground referenced speed with boat referenced heading. Wind data will be inaccurate in strong tidal conditions. Also SOG is updated less frequently on the system's network even though the GPS output may update more frequently.

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Old 27-11-2008, 09:36   #13
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Quote:
Originally Posted by Pelagic View Post
This is an interesting question that I hadn’t thought much about.

On the larger yachts I used a Doppler speed system which was very accurate and depending on depth could be made to read bottom or water. From that I could easily calculate currents in order to ground stabilize the older True Motion radars. Today, the Radars do this automatically

That was also in the days when GPS was still a US monopoly and they did warn that they might shut it down for security reason.

Now, I think loosing GPS is about the only reason I would still have an alternate log, so as to use for DR calculations. But I wonder if we would ever loose the GPS system?
They put bottom tracking ADCP's (Acoustic Doppler Current Profilers like RDI's or Sontek's) on mega yachts? Hah...that's pretty impressive! I had the impression they were just on research vessels.
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Old 27-11-2008, 09:45   #14
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Airmar's $800 CS4500 ultrasonic speed sensor is supposed to be accurate to a tenth of a knot, but I haven't heard from anyone using it. It has a surface exposed to the water that I presume can't be coated, so I still needs cleaning, right?
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Old 27-11-2008, 09:47   #15
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You can coat doppler transducer heads. I have seen it done. It can attenuate them some. Check what the manufacturer recommends.
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