Log Speed vs GPS [SOG] Speed?

[senormechanico]

Originally Posted by theonecalledtom

Question about plugging in these paddle wheels - I have one too, lying in the bottom of my boat because, well, what sane man unscrews a hole below the water line?

Seriously, do you just unscrew the plug and quickly screw in the paddlewheel?

Yep. I'll never forget the first time I did that. With great trepidation and rehearsing the procedure over and over, I had the plan all worked out.
I was going to pull the pin, unscrew the collar, pull out the paddlewheel, then immediately slam the plug into the geyser of water.

It didn't really work out that way.

As soon as I pulled out the paddlewheel, I was momentarily transfixed at how cool looking the bright greenish light was that was coming up from the now 2 foot high geyser of water. That awe lasted about 1 second before I thought, HEY DUFUS! YOU'RE SINKING!

It still was pretty cool, though.

Steve B.

[Sandero]

I'm with HUD on this. A smart set of instruments and transducer inputs can give you all sorts of valuable data which can save you time and that is a safety factor. Modern instruments with a speedo and a AWI can tell it all as if the water is static. Then refer to the GPS to see what reality says your doing.

[noelex 77]

Originally Posted by Hydra

noelex,
Be careful about current in the Med. Yes, there is almost no tide on the French Riviera (it's different near Tunisia or Sicily) but there are some permanent current patterns (up to 0.5kt) that the wind can increase up to 3kts. See your Sailing Directions.


As far as I know, ARPA radars also use STW to compute the relative motion of ships to provide CPA and TCPA because it is assumed that all ships in an area are submitted to the same current.

The trouble is, a speed log (Pitot, paddlewheel, electromagnetic) is expensive and needs calibrating (except for Doppler, the most expensive). GPS is cheap and fairly accurate, without having to put anything in the water.

Alain

Thanks Alain. I guess I shouldn't make generalized statements, such as there is no current in the Med. As you correctly point out there are places with very significant current. I used to sail in the North of Australia with some large tides and currents. Some of this was before GPS so we still had to guess the currents even though we had a paddlewheel.
The fouling in the Med is signifficant and it makes keeping a padlewheel completly free, so its calibration is accurate, difficult. Not many crusing boats here have a log that reads accuratly, especiall if you want accuracy on both tacks and a range of heel angles.
Its certainly not impossible and probably worthwile for the racing boats,but I am not convinced its worth the trouble for crusing yachts. If it gives inacurate information its worthless and calculated information such as true wind is wrong as well.

[Chief Engineer]

0 SOG 3knot current....you are either anchored or not progressing upstream with the engine

Originally Posted by Ex-Calif

Yup.

Imagine 0 knots SOG against a 3 knot current. The boat isn't going anywhere over the ground but the water is rushing by the boat at 3 knots. Actually the boat is moving "upstream" through the water at 3 knots.

4 kts of boat speed and SOG goes to 1 kt.
8 kts boat speed and SOG is 5 kts.

[captain58sailin]

The longer the interval with the averaging feature on the GPS the more accurate overall speed indication, if you use a 10 minute average, your SOG will be much truer than if you use a 1 minute interval, of course the 1 minute update will tell you about right now. I would not cut another hole in the bottom of my boat to add a paddle wheel for STW, your set and drift can be easily calculated using the cross track error on most GPS units. If you are close enough to shore, your radar can also aid in your calculations on the current strength and direction. I would never depend on any one navigation aid for safe navigation, I would use all the tools available, to insure, I am, where I think I am.

[Hydra]

Originally Posted by Cheechako

"....there are some permanent current patterns (up to 0.5kt)...." You're kidding right?

Cheechako,
I wasn't kidding, just referring to noelex present sailing area. It's *very* different elsewhere.

captain58,
I agree with you. GPS (plus radar if you have one) will tell you precisely where you are and where you are going with respect to shore. Then, if there is some current, you will notice funny things when you change course. For the same engine RPM, you sail faster or slower, depending on the situation, with some impact on your ETA. If you don't mind this changes, it's fine.

Alain

[Cheechako]

Sorry.... I didnt mean to sound snotty.... .5 just sounded like a "non-event" to me!

Even the little cheap Hummingbird GPS, I had in a 22 alum runabout averaged speed every few seconds if you set it to..... ?

[noelex 77]

STW is often in bold numbers for all to see,. For those of you who sail in waters with little current, on a typical cruising boat , I would encourage you to trial using SOG as your primary speed reference and get your instrument package, if possible, to calculate true wind , upwind and downwind VMG numbers from SOG rather than STW, I think you may be surprised how useful it is to achieve exactly the same speed numbers on each tack and know if you do not there is a reason, like poor sail trim, rather than : “ The log does tend to read a bit higher on starboard tack especially with a bit more heel”
Upwind and downwind VMG are suddenly useful instead of trying to make allowances for different heel or leeway angles.
You can tack and gibe without any change in true wind speed.
Anyway it is worth a try, if you have the sort of log with the accuracy I have typically seen in cruising Yachts I think you will be surprised how much of an improvement it is, especially now an SD fix is available in many areas.

[goboatingnow]

Without measuring the speed through water, the navigation system cannot compute "over water" wind, it only gives "over ground" wind. But, if the current or tidal stream is significant, there is a strong difference between these winds,

What .... There is no such thing as "over the water wind". All the boat feels is an apparant wind while in fact true wind is in fact properly SOG computed against measured wind speed. ( think about if the SOG is zero , becuase the current is steming the boat, then true wind calulations say or a STW of 8 knots, will be wildly wrong on a STW/Wind system like raymarine. Its only because we rarely use true wind speed that it doesnt matter).
The fact is until reliable GPS, STW was all that instruments had to use, hence true wind calulations used what was available , ie STW. but STW can give very incorrect true speeds and direction particulary in high current low wind speeds computations. Thats why B&G have switched to using SOG, ( and why , foolishlyRaymarine still do not),

STW is useful as boat performance measure, but it has little use in navigation. SOG will always indicate the presense of adverse currents, whereas in fact STW on its own, does not, contary to some confusing posts here. ( its only with the arrival of instant SOG's can we actually determine currents, otherwise the only way is via a positon fix) STW to SOG computations can be used to create and display tidal vectors and some chartplotters do that , but in reality this is only giving you information you already know, because SOG will have fallen or risen as a result of tidal efforts. I am not dismissing having STW, but the OP is right as to its usefulness, its only really tradition that has log speed as important , primarily because until GPS no other speed calculation was possible.

As far as I know, ARPA radars also use STW to compute the relative motion of ships to provide CPA and TCPA because it is assumed that all ships in an area are submitted to the same current.

Nope.. ARPA is suposed to use STW because, ARPA is computing relative speeds and assumes nor knows nothing about what current the other vessel is experiencing ( nor does it need to know). Radar provides speed and position information of the target vessel and hence that by definition is related to over the water speed. Hence then taking that speed information and comparing it to the ARPA vessels ground speed is like mixing apples and oranges. The resulting collision vectors are based on two seperate reference schemes and are wrong. Radar in a ship not tied to earth cannot by definition provide the ground speed of the target vessel.

As an aside STW referenced ARPA provides incorrect vectors if the ARPA equipped boat is anchored and there is a current running under it!.

AIS ARPA ( to coin a phrase) uses the opposite calulations ( or more correctly it can use) , it uses SOG of you and the target vessel to compute collision vectors, since the AIS receiver does know the SOG of the target vessel, the computation is valid.

[mudbug]

Hi guys . . . this issue came up on another board and I offered the following:

The full transition . . . .

The day Melissa and I left the dock in New Orleans, Jan 1, 2005, the speed log was not working. It mysteriously just broke the day we pulled out. The wheel was free and it spun fine, but no signal. Maybe it didn't like the snow that had just fallen in New Orleans!

Anyway, the speed data that the paddlewheel puts into the system does a few things.

One, it feeds the log, an electronic "odometer" in the B&G system that records data like top speed and miles logged (through the water).

Two, it lets your see your speed through the water (while in currents this speed can be a lot different than the speed your GPS shows across the ground). If I motor into a seven knot current, the speed wheel will spin like a fool and read seven knots, but the GPS will be reading close to zero, because I won't be gaining any ground on the current.

Three, the speed through the water data tells the autopilot how fast you are going and the computer decides how much rudder is appropriate to make corrections and keep you on course. In general slower speeds get more rudder angle, fast speeds get less rudder angle.

Four, to calculate true wind speed, the system's computer takes the speed through the water and either adds (if you are going downwind) or subtracts (if you are heading into the wind) and does all sorts of computer wizardry to figure true wind if you are at an angle that is somewhat between perpendicular to and inline with the wind. If there is no speed through the water data going in, the system still shows wind speed, but it is all apparent wind, even when the true wind button is depressed . . . with no speed through the water data it's all a raw display of the winds hitting mast head. Of course, currents can cause the system to think there is more addition or subtraction of the apparent wind to get to true wind and can result in inaccurate readings.

So, the issue: if the speedwheel transducer is not sending data to the computer, what happens?

Well, on my B&G system, you can choose the speed data inputs. One setting selects the paddlewheel transducer, one setting selects a GPS feed (if the GPS is NMEA'ed into the system), and another is simply a "manual" speed setting.

The manual mode has three easily chosen settings of "slow" "medium" and "fast" and you can manually set each of those as well. I set mine at 5, 7, and 9 knots.

So, if the speedwheel gets clogged, or if I have it out, or if it breaks, or whatever, I simply switch to manual and choose the manual speed setting closest to what speed I am doing and the autopilot works just fine (if there is no speed data at all the autopilot goes into a "no speed" alarm mode, so the manual settings serve to overcome that).

As for an odometer/log, the Furuno GPS/radar system I have has a real mileage over ground log that is more useful anyway . . . real miles, not miles through currents while perhaps actually going nowhere, so to speak.

As for the true wind, having no speed transducer gives me a chance to show off amazing math skills, such as: Gee, I am going down wind and the GPS says I am doing seven knots. The apparent wind speed (which will be the same as true on your system) says eleven knots. Hmmm . . . seven plus eleven is . . . wait a minute . . . I know, I know . . . don't tell me . . . . . . IT'S 18 KNOTS true wind!

And if heading into the wind and the GPS says you are doing six knots and the apparent and true wind speed is fourteen knots . . . . hmmm let's see, fourteen minus the six knot speed's fake breeze created by heading into it is . . . 8 knots true wind.

After the first year or so out here, we didn't look much at the wind speed intruments at all, we just know about how hard its blowing from seeing routine sea conditons and feeling it, day after day. Of course, when it starts ripping, though, we enjoy gawking at the wind speed indicator and reefing accordingly.

As for boat speed in currents, by using the speed log the first year, I just learned what kind of SOG the boat makes in all conditions when currents are not present. I know that I should make 7.3 knots while motoring in still water and winds under ten. I will do 6.9 to 7 dead into winds in the high teens to twenty knots. I know how fast the boat is at various wind speeds and angles and seas in non-current conditions.

So, for my purposes, the speed transducer can be lived without pretty easily. And that is what I do for the most part when the boat will sit more than a week. When on the go and working our way for long distances, only stopping a day or two here and there, I'll leave it in. When the inverse is the case . . . only sailing a day or two now and then and staying put for long periods in between hops, like hurricane seasons in marinas in Venezuela and Guatemala, I take it out and leave it out.

I bought a new speed transducer in 2005 to replace the one that quit on our maiden voyage and it lives mostly in the clean bilge and the plug stays in its place in the hull.

For kicks, sometimes I switch the system to transducer and go spin the speed wheel to see how fast I can get it to go. Melissa does not think this is as fun as I do.

Regardless, I like all my equipment to be in perfect operating condition, but have found that the hassle of cleaning the speed transducer's paddlewheel and is not worth it sometimes.

I remember being VERY reticent about pulling the transducer out and putting the plug in (and visa versa) when I first got the boat. Seeing water shooting in through an open hole is disconcerting.

But, it's no big deal. I take a rag beach towel and wrap it around the through-hull area to catch all the water and I dry everything off immediately after the switch is made

I keep a little tub of silicone "O-ring" grease from the SCUBA shop in my tool bag and use it all on sorts of stuff. Whichever component is taken out (plug or transducer) I immediately clean it perfectly and apply some o-ring silicone grease to the o-rings and leave it ready to go back in.

Also, a guy I met in the BVI swears by coating the paddlewheel with vaseline to keep it clean longer . . . he says that crud won't grow on the vaseline. I tried it once and didn't monitor it enough to really document any advantage.

Anyway, I was upset when the speedwheel broke in 2005, now I have a perfect one that lives a very quiet and generally unmolested life in the bilge.

All the best,

Buddy

[senormechanico]

goboatingnow said,

"Radar provides speed and position information of the target vessel and hence that by definition is related to over the water speed. Hence then taking that speed information and comparing it to the ARPA vessels ground speed is like mixing apples and oranges. The resulting collision vectors are based on two seperate reference schemes and are wrong. Radar in a ship not tied to earth cannot by definition provide the ground speed of the target vessel."

That assumes the speed and direction of the current is the same for the vessel on the radar screen and your vessel. In Puget Sound and it's environs, not necessarily the same thing! The current (and wind) can be 180 degrees out within a couple of miles.

Steve B.

[s/v Jedi]

ARPA radars only detect the distance and bearing of a target relative to their own scanner, that's it, nothing more. All the rest of the information is calculated. By doing timed measurements (using their internal time-base which is very accurate) they can calculate the target's speed and course relative to their own scanner. From this information they can further calculate CPA and TCPA but still no course and speed of the target vessel relative to ground nor water. For this, it needs more sensor input, like from compass, GPS, and log sensors. I do not know how they use this sensor data in their calculations but all of that is not needed for CPA and TCPA and these calculations are correct because all it needs is relative bearing and distance to the target.

Windspeed and direction over water: a very important sensor input for a sailboat because the sailboat has nothing to do with the seabed; it moves with the water it is in. This is the only important information for decisions about reefing, sailtrim etc. No info from GPS is helpful. For navigation however, our target is a bay, port, island etc. and these are all attached to ground and not moving with the water. So here, speed and course over ground are the preferred sensor data to work with. 80% of learning navigation is about correcting the flawed data from log and compass and much of that has a high level of educated guesses. When using GPS, all these errors are avoided from the start so this is much better.

Like Mudbug wrote: your autopilot (assuming it is a decent & modern one) really needs speed through water to decide how much rudder is needed for a correction. Everyone not having or using their log sensor should realize they are blocking their AP capabilities leading to less comfort, less correct course-keeping and more wear and tear on the AP drive and rudder.

cheers,
Nick.

[goboatingnow]

ARPA radars only detect the distance and bearing of a target relative to their own scanner, that's it, nothing more. All the rest of the information is calculated. By doing timed measurements (using their internal time-base which is very accurate) they can calculate the target's speed and course relative to their own scanner. From this information they can further calculate CPA and TCPA but still no course and speed of the target vessel relative to ground nor water. For this, it needs more sensor input, like from compass, GPS, and log sensors. I do not know how they use this sensor data in their calculations but all of that is not needed for CPA and TCPA and these calculations are correct because all it needs is relative bearing and distance to the target.

You are mistaken and confused , Radar gives range and bearing and hence from a series of such measurements , relative speed and direction are computed. I was answering the fact as to why SOG is not used in Arpa its becuase relative vectors cannot be computed with absolute vectors, it has to be the target vessels relative vectors against the arpa vessles relative vectors and the result is a collison vector that is also in the same dimensions.

Windspeed and direction over water: a very important sensor input for a sailboat because the sailboat has nothing to do with the seabed; it moves with the water it is in.

This is teh old chestnut,

Firstly aparent wind is generated by the speed of teh boat OVER THE GROUND as wind speed is not relative to water speed. you to not need STW or SOG to measure apparent wind, you merely measure the windspeed directly.( teh little cups) A sailboat has everything to do with the ground speed as SOG is what develops aparent wind. ( Not STW). think about it I am doing 8 knots ie STW =8 and a counter current is 8 knots, ie SOG =00 then the aparrant wind is if I am stopped, if I an not sailing but am carried along by a current of 8 knots my STW=0, but the apareant wind is a function of my ground speed.

Summary

A sailboat feels the apparent wind as a function of its SOG not its STW

The only reason true wind is calculated by the silly instruments using STW is that was what was all that was traditionally available. Its proper calulation requires SOG.

I absolutly agree that STW applies to sail trim etc ( other then decisions related to true wind). however in most cases you can use SOG ti sail trim as well as say in the middel of the atlantic SOG is just as good as STW for sailtrim.

absolutly autopilots require STW as they are dealing with relative motion.

[captain58sailin]

Nathaniel Bowditch 1773-1838 published some very nice tables for all kinds of nautical information. Including how to calculate true wind from the apparent wind. His works have been the bible for professional mariners for years. I believe you can obtain a software version of his works through Baker /Lyman, where all you have to do is enter in the relevant data and the answer is automatically calculated. This book or set of books depending on which year it was published is very helpful. It is worth a look. I do not work for Baker/Lyman.

[s/v Jedi]

Originally Posted by goboatingnow

You are mistaken and confused , Radar gives range and bearing and hence from a series of such measurements , relative speed and direction are computed. I was answering the fact as to why SOG is not used in Arpa its becuase relative vectors cannot be computed with absolute vectors, it has to be the target vessels relative vectors against the arpa vessles relative vectors and the result is a collison vector that is also in the same dimensions.

I'm afraid you are the one who is confused ;-) You say "range" and I say "distance" but wouldn't you agree we mean the same. I say "bearing" and you say "direction" but that's the same too. When I say "calculated" you say computed but we mean the same again. You say "series of such measurements" and I say "timed measurements" and my wording is actually more precise because the timing is crucial.

But you keep talking about the target's relative vectors against the ARPA vessel's relative vectors and that is just irrelevant. An ARPA radar can work without any input from log or GPS and it also has no clue at all about the targets sensor readings. All it does is calculate/compute from those distance and bearing measurements so speed over ground is not used and speed over water isn't used either. Okay, compass course is needed for ARPA operation to correct for not steering a straight course all the time. The operator will see a relative vector, CPA and TCPA only in this setup.

But hey, I see the target's speed and course too!!! This is because you connect a GPS to the radar. The radar uses SOG and COG to calculate true speed, bearing and COG of the target. It uses the position info from the GPS to calculate the target position.

So, SOG is used, not STW. You are correct that you can't compare ground-related vectors with water-related vectors but ARPA uses OR relative between the two boats/ships (without GPS info) OR relative to ground, never relative to water.

Firstly aparent wind is generated by the speed of teh boat OVER THE GROUND as wind speed is not relative to water speed. you to not need STW or SOG to measure apparent wind, you merely measure the windspeed directly.( teh little cups) A sailboat has everything to do with the ground speed as SOG is what develops aparent wind. ( Not STW). think about it I am doing 8 knots ie STW =8 and a counter current is 8 knots, ie SOG =00 then the aparrant wind is if I am stopped, if I an not sailing but am carried along by a current of 8 knots my STW=0, but the apareant wind is a function of my ground speed.

I am sorry that I say "irrelevant" a lot but that's because I watch a lot of Star Trek, it is not meant to annoy you... but your statement is irrelevant ;-) The sail boat doesn't care about it's SOG at all, it can't see the ground, doesn't feel it and the ground does not influence it in any way, well may be wave action if it's shallow but that counts as water again. Only the apparent wind and the water matter. It is the CREW who care about SOG and COG or at least the navigator. So, you are right that apparent wind is a function that includes SOG but your sail trim doesn't care about the ground, it's irrelevant to sail trim. If apparent wind speed goes up a lot one might decide to reef regardless of SOG. If the apparent wind angle changes one might decide to adjust the sheets regardless of SOG or even COG. You can be frustrated that your SOG is only half of your STW but the sailboat doesn't care. So again: SOG & COG are used for navigation, not for sail trim.

Another often made mistake is about True Wind. Many sailors think that this is the wind relative to the ground but it is not. It is the wind relative to the water. So if your boat doesn't move through the water, apparent wind is the same as true wind, even when there is an 8 knot current.

The only reason true wind is calculated by the silly instruments using STW is that was what was all that was traditionally available. Its proper calulation requires SOG.

No, true wind speed is defined as wind over water. If one would use SOG instead of STW you end up with wind over ground which is not true wind speed. You might not agree with that definition but it is exactly what is needed for sail trim because it is the wind that the boat has to deal with, and this measurement is used by every competition sail trimmer in the world. Only when the majority of the sailors want the definition changed, it might happen.

When you say that SOG as a basis is just as good as STW for sail trim, you know that this is a flawed statement; you mean that you don't care about the small difference. Example: sailing down wind and wind-over-water is 15 knots while wind over ground is 23 knots (8 knots favorable current). You must select a sail plan for 15 knots of wind, not for 23 knots. If you set sail for 23 knots of wind you will sail slow through the water and loose all the advantage of the favorable current. However, we normally don't have that much current so you don't care.

If you have B&G instruments: read the manual, it explains exactly how true wind and VMG is calculated without the use of GPS.

A difference in true wind angle on alternating tacks is mostly caused by upwash. This effect can easily cause a 10 degree error. The wind instrument is not to blame because the apparent wind angle is the same on both tacks (but showing a wrong value if not calibrated). When you see a 10 degree error in true wind between tacks, this means that the real apparent wind on each tack is 5 degrees more than what the instrument shows. This is why boats with cheap instruments can sail closer angles to the wind... hmmmpfff ;-)

cheers,
Nick.