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26-11-2008, 23:03
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#1
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Registered User
Join Date: Nov 2006
Location: Back in Northern California working on the Ranch
Boat: Pearson 365 Sloop and 9' Fatty Knees.
Posts: 10,466
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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.
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: Wow - what a ride!"
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26-11-2008, 23:38
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#2
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Moderator Emeritus
Join Date: Aug 2005
Location: Carlsbad, CA
Boat: 1976 Sabre 28-2
Posts: 7,505
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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.
Aloha
Peter O.
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26-11-2008, 23:52
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#3
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Registered User
Join Date: Nov 2006
Location: Back in Northern California working on the Ranch
Boat: Pearson 365 Sloop and 9' Fatty Knees.
Posts: 10,466
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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.
__________________
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well-preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming: Wow - what a ride!"
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27-11-2008, 00:39
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#4
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Registered User
Join Date: Mar 2007
Location: Algarve
Boat: Coronet Elvstrom 38
Posts: 59
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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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27-11-2008, 01:42
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#5
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Moderator Emeritus
Join Date: May 2007
Location: Ohio
Boat: Now boatless :-(
Posts: 11,580
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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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27-11-2008, 02:19
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#6
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Registered User
Join Date: Apr 2006
Posts: 4,413
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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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27-11-2008, 04:35
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#7
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CF Adviser
Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 10,280
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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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27-11-2008, 04:40
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#8
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Registered User
Join Date: Sep 2007
Location: Shanghai, China
Boat: Dix 43 CC, steel, 43 ft
Posts: 149
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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.
__________________
'How inappropriate to call this planet Earth when it is quite clearly Ocean.' - Sir Arthur C. Clarke
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27-11-2008, 05:02
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#9
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 20,301
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Quote:
Originally Posted by Celestialsailor
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.
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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
....... But I wonder if we would ever loose the GPS system?
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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
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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27-11-2008, 06:35
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#10
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Registered User
Join Date: Mar 2003
Location: Wherever the boat is!
Boat: Marine Trader 34DC
Posts: 4,619
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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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27-11-2008, 06:52
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#11
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Senior Cruiser
Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 49,139
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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.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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27-11-2008, 07:38
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#12
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Registered User
Join Date: Apr 2007
Posts: 718
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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.
Eric
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27-11-2008, 08:36
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#13
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Moderator Emeritus
Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,406
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Quote:
Originally Posted by Pelagic
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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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.
__________________
David
Life begins where land ends.
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27-11-2008, 08:45
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#14
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Senior Cruiser
Join Date: May 2008
Location: near Annapolis
Boat: PDQ 36 & Atlantic 42
Posts: 1,178
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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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27-11-2008, 08:47
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#15
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Moderator Emeritus
Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,406
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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.
__________________
David
Life begins where land ends.
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