COG vs Magnetic heading

Sneaky Duck

Anarchist
621
32
Rochester, NY
Ok, so perhaps this isn't a topic for a gear forum, but it's directly related to GPS units like the Prostart or Sailmon Max, in comparison to a magnetic compass like the prism or Tacktick.  I'm in the market for new electronics but need to understand what I'm buying prior to picking a unit.  I've been told by multiple Pro's that I trust that on my Star I really want a magnetic compass because it is way better for showing wind shifts.  Several of the top Star boats, Paul Cayard and Robert Scheidt for instance use the Prostart on their boats which is GPS.  The people that told me to go magnetic said that GPS isn't nearly as good because it shows your course over ground NOT your heading, so it will be impacted by the boat side slipping, current, and so on.  My question is, have those guys who use the prostart (I don't personally know any of them to ask) just decided that the starting features are more important then the compass and therefore they don't really care that the COG isn't' as accurate in picking out shifts?  Or do they have a way of interpreting the data that they get from the GPS that makes it more accurate for them?  I know these guys are way better then I'll ever be.  And likely very in tune to their boats... but I have a hard time believing that they are just so in tune with things that they give up compass headings all together.

 
On my J/29 I’ve been having instrument problems. Been reduced to using and IPad with a starting app similar to what you would get with the Sailmon for COG/SOG and start timing. Use my old fashion mag compasses to give me header/lifts. Haven’t raced a star in 20 years. Can’t you put on a small Ritchie compass like I used to have on my laser to track shifts and the Max or Prostart and have the best of both worlds? 

 

Sneaky Duck

Anarchist
621
32
Rochester, NY
For the life of me, I can't seem to find where I saw it in the rules, (I emailed a measurer to ask) but I seem to remember that you're only allowed a single electronic unit/compass.  Otherwise.. I'd totally do that.  Get something like a Tacktick and a Max.  That would be an awesome combo.

I ask the question though more from the standpoint of, is there a way to use GPS COG in a way that's adequate to show headings for wind shifts?  They advertise it as if you can.  But I'm being told otherwise by some very good sailors.  Once I know what I'm talking about... THEN I can narrow it down to the specific equipment.

 

PaulK

Super Anarchist
I ask the question though more from the standpoint of, is there a way to use GPS COG in a way that's adequate to show headings for wind shifts?  They advertise it as if you can.  But I'm being told otherwise by some very good sailors.  Once I know what I'm talking about... THEN I can narrow it down to the specific equipment.
GIGO.  GPS headings are based on COG.  The GPS system doesn’t know or care which way your bow is pointing.  Solings trailer backwards, and if you had a GPS on a trailered Soling going down the highway it would show 65mph in the direction the car was pulling, not the direction the boat was pointed.  Digital compasses are not based on GPS.  They have little magnetic compasses inside them that are variously damped to give fairly reliable readouts, and have electronics that provide a display that is easy to read.  You can use the info from a GPS to tell you what is happening to the boat - which MAY tell you if the boat is getting lifted or headed compared to earlier readings- but it won’t necessarily tell you which way the boat is pointing.  It WILL, as the Soling example  shows,  tell you how the boat moving.  This can be much more useful in determining what the fastest route to the finish is than just knowing which way your bow is pointing.  This is why GPS is often not permitted in OD classes - it confers too great an advantage.  Digital compasses, like standard magnetic compasses, simply tell what your heading is. They are easier to read, however, so many OD classes are starting to allow them. Different strokes for different folks. 

 

RImike

Super Anarchist
1,031
125
Newport RI
There are 3 "levels" of heading depending on how much money you want to spend. You covered the basic, which is COG vs Magnetic which relies on a a GPS vs compass heading. The angle of the boat relative to heading isn't know to the GPS.

The next level is a huge price jump as it would require a processor to compute but then a channel called "COG Magnetic" is created. It uses all of the GPS data to calculate with consideration for magnetic declination. The result matches your actual magnetic compass heading (angle of bow related to direction of travel aside). 

I don't know if either units have this capability though. 

The ultimate level which is what the Vende guys use involves 2 GPS antennas at either end of the transom and compare the GPS data of each sensor and determine both the COG, COG magnetic as well as the actual direction of bow related to travel. This is helpful in the southern hemisphere as magnetic field is weaker there and a magnetic compass doesn't deliver the same accuracy as it would in the northern hemisphere.  

 
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weightless

Super Anarchist
5,608
587
^^^ That confused me a little. Invoking Cunningham's law let me say that IIRC common sensor building blocks are:

GPS:  3d position and velocity with respect to a geodetic datum directly.
Magnetometer / magnetic compass:  orientation in a magnetic field
Gyroscope: changes in rotation
Accelerometer:  changes in translation

The gyroscope and accelerometer and sometimes the magnetometer may be combined with some logic into an Inertial Motion Sensor (IMU) which outputs motion and orientation. Lots of consumer electronics have an IMU. MEMS versions of these sensors are small, cheap and use very little power.

While not used on small boats IME, a GPS with two antennas on a fixed baseline can calculate heading.

Many GPSs can output true or magnetic COG. To be clear, that's just a coordinate change much as a line on a chart can be read as true or magnetic.

 

Sneaky Duck

Anarchist
621
32
Rochester, NY
Not going to lie, the more you guys talk the more things I realize I don't know...  the key will be determining how much I need to learn for my application.

I actually purchased 2 units.  I purchased a Prostart, and I was able to find a deeply discounted left over stock Velocitek Shift.  I'm planning to play with them both and see which I have more confidence with.  I'm thinking maybe I put the shift on for club racing on the small lake, and other local stuff, and swap out to the prostart when I'll be on a long line, or when I'll be likely to encounter current.  Who knows.... Maybe I wasted my $$$ on the Shift and I should have just gone with the Prostart.  Maybe I find that the only time I use the prostart is training.  We'll see.

 

Hitchhiker

Hoopy Frood
4,830
1,482
Saquo-Pilia Hensha
I haven't raced a Star in over ten years.  I thought all electronic heading devices were not allowed, or maybe it's speed devices.

Regardless, in order to get accurate electronic heading you need a gimbaled fluxgate type sensor.  As previously pointed out, heading from a GPS is speed dependent.

The device I use for the Viper 640 is a Novasail NS 360.  It has a GPS sensor for speed and a gimbaled fluxgate sensor for heading, which enables it to get wind checks and to  monitor headers and lifts.  Although that seems a bit redundant when racing in one design (or almost one-design where the Star is concerned!).

With the Novasail one can tun off the speed display (if not allowed under the class rules). But, I don't know how that would stand up in the portest rom if some-one was to take you to task on it!

I have this model (discontinued). http://www.nova-sail.com/en/NS360W.php#

Screen Shot 2020-04-14 at 2.36.12 PM.png

But, they have a newer version. http://www.nova-sail.com/en/NS360 Pocket V2.php

Along with other versions and products.

 
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Sneaky Duck

Anarchist
621
32
Rochester, NY
I haven't raced a Star in over ten years.  I thought all electronic heading devices were not allowed, or maybe it's speed devices.

Regardless, in order to get accurate electronic heading you need a gimbaled fluxgate type sensor.  As previously pointed out, heading from a GPS is speed dependent.

The device I use for the Viper 640 is a Novasail NS 360.  It has a GPS sensor for speed and a gimbaled fluxgate sensor for heading, which enables it to get wind checks and to  monitor headers and lifts.  Although that seems a bit redundant when racing in one design (or almost one-design where the Star is concerned!).

With the Novasail one can tun off the speed display (if not allowed under the class rules). But, I don't know how that would stand up in the portest rom if some-one was to take you to task on it!

I have this model (discontinued). http://www.nova-sail.com/en/NS360W.php#

View attachment 357797

But, they have a newer version. http://www.nova-sail.com/en/NS360 Pocket V2.php

Along with other versions and products.
I'll have to look into if there's prohibited functions, but below is the list of approved electronics.  I liked the look of the NovaSail, and the Sailmon, but honestly... my buddies have the Velociteks, so I went with what I knew in that way.  Plus it was way easier to get my hands on them.

Approved Electronic Devices for 2020
> Velociteck  -  Prism Dingy Compass   
> Velociteck   - ProStart 
> Velocitek     - Shift                       
> NovaSail     - NS360.Pro                    
> TackTick     - Micro Compass        
> TackTick     -   TO70  Compass 
> Sailmon    -   Max      

 

weightless

Super Anarchist
5,608
587
Regardless, in order to get accurate electronic heading you need a gimbaled fluxgate type sensor.
[pedanticLevel, "absurd"] You could go that route but I think you may be over-specifying the technology.

They say it has "a 9 axis gyro gimballed magnetic digital compass." I'd read that as a 9 axis IMU. Typically 9 axis (dof) IMUs are 3-axis x gyro, accel, compass plus some fancy digital motion processing. I suspect there are no physical gimbals (it's all in the quaternions) and the magnetometer could be one of several types but I think Hall effect is most common.

[/pedanticLevel]

 

Hitchhiker

Hoopy Frood
4,830
1,482
Saquo-Pilia Hensha
[pedanticLevel, "absurd"] You could go that route but I think you may be over-specifying the technology.

They say it has "a 9 axis gyro gimballed magnetic digital compass." I'd read that as a 9 axis IMU. Typically 9 axis (dof) IMUs are 3-axis x gyro, accel, compass plus some fancy digital motion processing. I suspect there are no physical gimbals (it's all in the quaternions) and the magnetometer could be one of several types but I think Hall effect is most common.

[/pedanticLevel]
Just offering an alternative.

 

weightless

Super Anarchist
5,608
587
Just offering an alternative.
And a good looking one. My apologies. I just couldn't help noticing that your statement about what technology was required was not consistent with your specific suggestion. It's not important. Let's blame the lock down and move on. :)

 
If your typical fail on the race course is straight line moding and identifying key shifts, I'd recommend magnetic.  As pointed, it's more accurate (refresh rate), especially in down-speed boats.

If the only thing that's missing from your game is key positioning and leverage off the starting line, and you don't rely on compass heading feedback to keep the boat in the correct mode or speed, then COG GPS might be sufficient.  At the level the Star guys are sailing at, a boat length at the line might be the difference of 1st or 20th at the weather mark.

As most have eluded to, "both" is ideal, but you've got to have all the other pieces of the puzzle before having that much data distraction makes sense, especially on small boats.

 

LionessRacing

Super Anarchist
4,383
615
Myrtle Beach,
A digital compass that has a header/lift indicator is nice, if you don't have a pencil and the ability to reliably do math in your head.. 

Alternatively just recording the pre-start heading on each tack (45 degree lubber lines) on a piece of masking tape, and comparing the heading of the moment,  gives you apparent headers and lifts. 

That doesn't correct for wind/boat speed changes that make the AWD change (lulls are headers, puffs are lifts)  and on a Star you may not care, as you sail the wind you are in.

A more sophisticated system (probably not class legal) would, if correctly calibrated eventually indicate TWD, and that would show you changes in actual direction. which would be guidance to tack if you are expecting shifts as compared to riding through the lull. 

 
A digital compass that has a header/lift indicator is nice, if you don't have a pencil and the ability to reliably do math in your head.. 

Alternatively just recording the pre-start heading on each tack (45 degree lubber lines) on a piece of masking tape, and comparing the heading of the moment,  gives you apparent headers and lifts. 

That doesn't correct for wind/boat speed changes that make the AWD change (lulls are headers, puffs are lifts)  and on a Star you may not care, as you sail the wind you are in.

A more sophisticated system (probably not class legal) would, if correctly calibrated eventually indicate TWD, and that would show you changes in actual direction. which would be guidance to tack if you are expecting shifts as compared to riding through the lull. 
Bump.  We just got the new ProStart for the 105 (hyooge upgrade from the old unit btw), and I’m trying to decide which mode to use for us, magnetic or COG.  I share much of the OP’s naivete with regard to interpreting the data, so I’m inclined to begin with the magnetic mode, but we have so much current differential in the bay that I’m concerned the simple magnetic headings without further context are actually worse than useless if our primary goal is to identify lifts & headers.  Flame away.

 

Mr Moab

Anarchist
651
132
If you are looking to sense lifts and headers magnetic is the way. Yes, if you cross a current line, it may change apparent wind strength and even apparent wind direction, but overall you want magnetic for shifts. COG is key for tracking to a mark. I find COG  more valuable if you have a magnetic reference. You can then detect impact of crossing a current line. We sail in SF bay and have one of the new Prostarts. We have it set to magnetic and then use a garmin watch for COG. Has worked well for us. Really pleased with the Prostart instrument 
 

I do wish  we could set the Prostart to either cycle between mag and COG. Or put Mag and COG on the display simultaneously 

 

LionessRacing

Super Anarchist
4,383
615
Myrtle Beach,
Bump.  We just got the new ProStart for the 105 (hyooge upgrade from the old unit btw), and I’m trying to decide which mode to use for us, magnetic or COG.  I share much of the OP’s naivete with regard to interpreting the data, so I’m inclined to begin with the magnetic mode, but we have so much current differential in the bay that I’m concerned the simple magnetic headings without further context are actually worse than useless if our primary goal is to identify lifts & headers.  Flame away.
I doubt its material, as long as any lay lines are in the same units of Mag or TRUE. 

You are looking for headers and lifts, and a favorable change in the heading you are making (COG) will take you to a ground anchored mark sooner, yet its pretty likely that such a change in course will also accompany a change in Heading that followed a change in TWD. If you cross a current line into an eddy, you would want to know that it was favorable vs unfavorable, even if the heading didn't change. 

 

axolotl

Super Anarchist
1,656
184
San Diego
This is helpful in the southern hemisphere as magnetic field is weaker there and a magnetic compass doesn't deliver the same accuracy as it would in the northern hemisphere.
Not true.  There is the South Atlantic Anomaly where the magnetic field is up to 50% weaker than elsewhere but there's still plenty of field strength for magnetic compasses.  The rest of the Southern Hemisphere's magnetic field is as strong as the Northern Hemisphere's.

SAA_2020.png

You may be talking about the fact that a compass set up for the Northern Hemisphere  won't work well in the Southern Hemisphere.  What happens to the compass needle is that it becomes unstable, wanting to dip a great deal but unable to do so.  Compasses are specific and pre-set for whichever hemisphere they are to be used in. This is because the needle 'dips' in an opposite direction in the northern hemisphere (head down) to that in the southern hemisphere (tail down). 

 
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