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Old 11-03-2015, 05:13   #1
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Smile Speed Through GPS Versus Old Fashioned Paddle Log

Can anyone tell me why so many boats still use the "old fashioned" impeller log to record their speed? GPS obviously gives much more accurate speed over the ground which is, ultimately all you really want, isn't it? Or am I missing something? I'm not a racer - just cruising! Would appreciate your thoughts!
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Old 11-03-2015, 05:15   #2
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Re: speed through GPS versus old fashioned Paddle Log

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Originally Posted by Jonathancpwalsh View Post
Can anyone tell me why so many boats still use the "old fashioned" impeller log to record their speed? GPS obviously gives much more accurate speed over the ground which is, ultimately all you really want, isn't it? Or am I missing something? I'm not a racer - just cruising! Would appreciate your thoughts!
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Old 11-03-2015, 05:23   #3
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Re: speed through GPS versus old fashioned Paddle Log

Impeller log gives you speed through water which is probably a better indicator of sailing performance than SOG in regions with wicked currents like where I sail. Using the speed through the water and SOG together also indicates if you're going with the flow or against it (and at what rate), which is quite useful actually.
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Old 11-03-2015, 05:26   #4
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Re: speed through GPS versus old fashioned Paddle Log

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Originally Posted by Reefmagnet View Post
Impeller log gives you speed through water which is probably a better indicator of sailing performance than SOG in regions with wicked currents like where I sail. Using the speed through the water and SOG together also indicates if you're going with the flow or against it (and at what rate), which is quite useful actually.
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Old 11-03-2015, 05:27   #5
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Re: speed through GPS versus old fashioned Paddle Log

I think most have both. The log can give you speed through the water which can determine current. Personally I don't trust the log 100% and it tends to foul if we stop too long. It's also used by our system to calculate true wind speed and direction, although the gps would probably give a better result. Here's our impeller log readout at 8K, compared to the opencpn gps readout at 6K mainly due to adverse current...Click image for larger version

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Old 11-03-2015, 05:38   #6
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Re: speed through GPS versus old fashioned Paddle Log

I pulled the paddle wheel transducer and replaced with the plug a couple of years ago.
I have GPS at the nav station and at the helm. Don't care at all about speed through the water and the GPS is more accurate. IMHO the old style knot meter is obsolete.
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Old 11-03-2015, 05:44   #7
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Re: speed through GPS versus old fashioned Paddle Log

Both have their uses. My boat has only the GPS and that's fine with me.
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Old 11-03-2015, 06:08   #8
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Re: speed through GPS versus old fashioned Paddle Log

I think having both is very useful as other have said. I use it as a quick diagnostic tool. I know my boat generally cruises at 7 knots under power at a particular RPM. If my GPS says I'm only doing 4.5, the first thing I do is look at the log. If the log is reading 7 knots, then I can immediately rule out any mechanical issues, fouled bottom or running gear etc. because I know the loss of speed is due to an adverse current. More info is always better.

To keep my log clean and clear, I remove it, and install the dummy plug if the boat is going to sit unused for a while.
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Old 11-03-2015, 06:17   #9
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Re: speed through GPS versus old fashioned Paddle Log

SOG is useful as it sum total of what the boat is doing relative to the ground... But it is not effective to analyze and tweak sailing performance.

I find having the basic raw data useful and important... I don't know that I would use only GPS generated data.
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Old 11-03-2015, 06:27   #10
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Re: speed through GPS versus old fashioned Paddle Log

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SOG is useful as it sum total of what the boat is doing relative to the ground... But it is not effective to analyze and tweak sailing performance.

I find having the basic raw data useful and important... I don't know that I would use only GPS generated data.
The GPS readout for speed is in 10ths of knots and updates constantly. I can see changes in boat speed due to trim adjustments as easily as with the speed through water knot log.
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Old 11-03-2015, 07:33   #11
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Re: speed through GPS versus old fashioned Paddle Log

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The GPS readout for speed is in 10ths of knots and updates constantly. I can see changes in boat speed due to trim adjustments as easily as with the speed through water knot log.
No you can't.

Taking an exaggerated example, if you're on a heading of 90 degrees traveling at 5 knots, in a 90 degree cross current moving at five knots, and your SOG speed changes by .5 knots, how much has your STW changed? How much of the change in your SOG is due to boat trim or variation in the current?

Without STW data, how do you even know what the true current IS, other than looking at tables? You can look at your course and heading and the delta between them, but some of that is leeway of the boat through the water and it requires some calculation. A simple glance at SOG vs. STW will give you an instant appreciation of where you stand, and it's much easier to determine the results of trim and course changes.

In areas with variable currents, which is most coastal sailing, or in the gulf stream for example, having accurate STW data can be an important element of both effective navigation and boat trim.

Yeah you can of course get by without it, but there are circumstances where the differential between the two can give you some very useful information.
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Old 11-03-2015, 07:43   #12
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Re: speed through GPS versus old fashioned Paddle Log

This is true. Recently we were trying to find the canary current. The charts showed we were in it and should have had .5-1K with us, but we had 1K against us. So compare the STW to SOG is really the only way to try to improve the situation. You can soon figure out the direction and maximum speed of the current ( usually on the nose) and select the best course to suit..
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Old 11-03-2015, 07:49   #13
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Re: speed through GPS versus old fashioned Paddle Log

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Originally Posted by Jonathancpwalsh View Post
Can anyone tell me why so many boats still use the "old fashioned" impeller log to record their speed? GPS obviously gives much more accurate speed over the ground which is, ultimately all you really want, isn't it? Or am I missing something? I'm not a racer - just cruising! Would appreciate your thoughts!
Gps is a crude speed readout while underway but without differential gps at slow speeds its not very accurate over short distances. Over a full day its very accurate.

Hull mounted speed sensor gives your speed while moving and current flow while anchored. Combined with gps you can easily calculate the effect of current on your speed.

Many of the modern thru hull sensors also include water temperature which is helpful when trying to predict weather from synoptic charts and grib files. A thru hull speed sensor is also a simple hall effect sensor which are very reliable and cheap to make. They draw no power unlike gps which is very power hungry.

Like most systems onboard redundancy is a good thing.

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Old 11-03-2015, 07:55   #14
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Re: speed through GPS versus old fashioned Paddle Log

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Originally Posted by Suijin View Post
No you can't.

Taking an exaggerated example, if you're on a heading of 90 degrees traveling at 5 knots, in a 90 degree cross current moving at five knots, and your SOG speed changes by .5 knots, how much has your STW changed? How much of the change in your SOG is due to boat trim or variation in the current?

Without STW data, how do you even know what the true current IS, other than looking at tables? You can look at your course and heading and the delta between them, but some of that is leeway of the boat through the water and it requires some calculation. A simple glance at SOG vs. STW will give you an instant appreciation of where you stand, and it's much easier to determine the results of trim and course changes.

In areas with variable currents, which is most coastal sailing, or in the gulf stream for example, having accurate STW data can be an important element of both effective navigation and boat trim.

Yeah you can of course get by without it, but there are circumstances where the differential between the two can give you some very useful information.
Agree. 1/10 of a knot output is just a mathematically derived value. It aint that sensitive. Doing the math, its simple trig, will highlight the resolution you'll see would be in the order of 30+ meters at best.

Differential gps can give very accurate readings but I'm not aware of any nautical gps systems that use differential gps. (with the exception of Pi Systems gear)

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Old 11-03-2015, 08:29   #15
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Re: speed through GPS versus old fashioned Paddle Log

I had the same arguments/dilemna when installing all new electronics on our current boat. Having spent a lifetime sailing in the strong tidal currents of the English channel and learning 'proper' navigation techniques where cross tide/currents must be accounted for, I cannot envisage not having boatspeed as well as GPS speed available.Also how about where speed limits are involved, they surely are for boatspeed not groundspeed so in a strong current you might think you are breaking the postedspeed limit and slow right down but in fact are way under it or you might be well over the set limit because you are battling a foul current and are using a lot more engine rpm to get to the ground speed you want?.
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