Speed Through GPS Versus Old Fashioned Paddle Log

[valhalla360]

Unless you are into performance it's nice but in no way needed.

I have a good feel for speed thru the water from the feel of the boat and the wake, so for adjustments to current, it's not needed.

I have found it to be very accurate. The speed might blip up or down for a second but I'm smart enough to ignroe those if there isn't a gust or other reason for them. Other than the occasional blip, it is accurate to the 1/10th of a knot.

As a cruiser, the goal of knowing speed is how fast am I closing on my destination not how fast am I traveling thru the water.

[Robin3]

Originally Posted by leftbrainstuff

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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Nothing to do with positional accuracy per se. I had differential GPS on several marine sets years ago in Europe, they stopped bothering with dGPS when Uncle Sam removed the selective availabilty degradation of his GPS signals for non military users some years ago thereby making the basic GPS much more accurate.

[Robin3]

Originally Posted by valhalla360

Unless you are into performance it's nice but in no way needed.

I have a good feel for speed thru the water from the feel of the boat and the wake, so for adjustments to current, it's not needed.

I have found it to be very accurate. The speed might blip up or down for a second but I'm smart enough to ignroe those if there isn't a gust or other reason for them. Other than the occasional blip, it is accurate to the 1/10th of a knot.

As a cruiser, the goal of knowing speed is how fast am I closing on my destination not how fast am I traveling thru the water.

But as a (competent) navigator it should be of concern to know how far you have travelled through the water and in what direction because that data is required to manually do a deduced reckoning of position. The tuning come tweaking aspects are not the only issue. Generally IMO it is sail people that want the boatspeed data and mobo folk that don't bother as they have plenty of (engine) grunt in reserve.

[deblen]

Why not use the VMG GPS display as well as SOG.?

[Robin3]

VMG to a waypoint or to windward (or downwind) under sail is I agree very useful data, but again varies with current effect and/or wind direction. Waterspeed on it's own it seems is seen as irrelevant by many peeps these days, especially if brung up on modern smart technology right from the cradle. Us old codgers like to know the whole picture, from first principles ie the 'workings' not just the answers.

[valhalla360]

Originally Posted by Robin3

But as a (competent) navigator it should be of concern to know how far you have travelled through the water and in what direction because that data is required to manually do a deduced reckoning of position. The tuning come tweaking aspects are not the only issue. Generally IMO it is sail people that want the boatspeed data and mobo folk that don't bother as they have plenty of (engine) grunt in reserve.

Go back and re-read. I have a very good idea of speed thru the water from knowing my boat over several thousand miles.

Also, I can estimate cross currents by comparing GPS heading to the direction the bow is pointed.

I'm doing the same thing but because it's not critical for cruising, as opposed to racing, I don't need the accuracy of a seperate instrument.

I still hold that nice to have but not really needed for cruising.

[Robin3]

Originally Posted by deblen

Why not use the VMG GPS display as well as SOG.?

My plotter come multi function display is set up to display boatspeed, windspeed/direction, SOG and VMG ( to next waypoint usually, but occasionally to windward if on a long upwind leg) XTE, TTG ( to next wpt or maybe last wpt) plus water depth. ALL data is useful if you understand what each figure shown is actually derived from and how it is computed. Otherwise bus and train timetables can tell you when you will get there best.

I expect airline pilots still like to know both airspeed and groundspeed?

[Stu Jackson]

Originally Posted by valhalla360

Go back and re-read. I have a very good idea of speed thru the water from knowing my boat over several thousand miles.

Also, I can estimate cross currents by comparing GPS heading to the direction the bow is pointed.

I'm doing the same thing but because it's not critical for cruising, as opposed to racing, I don't need the accuracy of a separate instrument.

I still hold that nice to have but not really needed for cruising.

And before GPS, how did you learn what your boatspeed STW actually was, so you could even learn that?

One technique was to drop something off the bow and count seconds until it passed the stern.

Some of us learned that by watching our instruments.

Before GPS.

[Robin3]

Originally Posted by valhalla360

Go back and re-read. I have a very good idea of speed thru the water from knowing my boat over several thousand miles.

Also, I can estimate cross currents by comparing GPS heading to the direction the bow is pointed.

I'm doing the same thing but because it's not critical for cruising, as opposed to racing, I don't need the accuracy of a seperate instrument.

I still hold that nice to have but not really needed for cruising.

We used to sail ( race and cruise) in areas where 4 or 5kt cross currents were routine and which affected not just SOG figure but the apparent windspeed quite markedly. On the tide reversal ( OK not instant) the current could add 5kts to a true beam wind or subtract 5 kts from it, big difference when felt on board. Do you have windspeed /direction out of interest?

[valhalla360]

Originally Posted by Stu Jackson

And before GPS, how did you learn what your boatspeed STW actually was, so you could even learn that?

One technique was to drop something off the bow and count seconds until it passed the stern.

Some of us learned that by watching our instruments.

Before GPS.

The boating I did before GPS was an open aluminimum boat with a 15hp outboard. Zero instruments, no charts, all line of sight. If the outboard quit, you broke out the oars.

Not really relevant to this discussion though.

[valhalla360]

Originally Posted by Robin3

We used to sail ( race and cruise) in areas where 4 or 5kt cross currents were routine and which affected not just SOG figure but the apparent windspeed quite markedly. On the tide reversal ( OK not instant) the current could add 5kts to a true beam wind or subtract 5 kts from it, big difference when felt on board. Do you have windspeed /direction out of interest?

Racing in those conditions, I can agree, it's probably critical to have the paddle wheel feeding in as you don't want to miss out on a 0.1kt of SMG.

Cruising, I'll see that the bow is pointed 30-40degrees off from the GPS heading and then I can take appropriate action to adjust for the current (been there, done that).

Again, nice to have but really only needed for fine tuning.

[Stu Jackson]

Originally Posted by valhalla360

The boating I did before GPS was an open aluminimum boat with a 15hp outboard. Zero instruments, no charts, all line of sight. If the outboard quit, you broke out the oars.

Not really relevant to this discussion though.

Yes it is, since someone made the point that faster boats are less affected by current than slower ones. That rig could easily do twice or three times the speed of a sailboat.

I "navigated" in The Gulf Islands with my f-i-l's outboard powered boat with no instruments, too. But if I was in a sailboat, I'd sure like to have had STW and SOG.

But back to the question: How did you learn boatspeed if not with instruments?

[Jonathancpwalsh]

Thanks to all who have contributed to my original question... I guess it's " horses for courses". All replies were very interesting. Personally, having owned my boat for c 14 years, I can normally tell - within 0.2 knot - what the Speed through the water is. I don't need an impeller to tell me. Thus, for me, if I only had to have one, it would certainly be the SOG from the GPS. Good sailing to one and all - and long live diversity!

[Gerrycooper56]

Cruising, Florida to Australia, no paddle wheel, no problem


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[MarkSF]

Originally Posted by Jonathancpwalsh

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!

Around here we have these things called tides and currents.