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Re: Speedo GPS and VMG [Re: Tony_F18] #67921
02/25/06 09:45 AM
02/25/06 09:45 AM
Joined: Nov 2005
Posts: 64
Switzerland
H2O_Sensations Offline OP
journeyman
H2O_Sensations  Offline OP
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Switzerland
With the Velocitek S3 you have a 20 to 25 hours battery life with AAA batteries...

Tested and approved.
Jr

-- Have You Seen This? --
Re: Speedo GPS and VMG [Re: MauganN20] #67922
02/25/06 11:28 AM
02/25/06 11:28 AM
Joined: Jul 2001
Posts: 851
US Western Continental Shelf
hobiegary Offline
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hobiegary  Offline
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I have VMG and it is amusing when on a long tack, like when sailing 20+ miles from the coast to an island.

One thing that you might not think about until you actually use one:
Let's say you are heading toward a target that is 1 mile directly upwind of where you start from. Your VMG will change, each time there is a wind shift. Also, as you get closer to the target, you will only get good VMG readings if you are nearly on the lay line for the target. The closer you draw near your target, the worse this becomes. The target is rarely near your 12 o'clock position and becomes farther and farther away from the 12 o'clock position as you draw closer, except when you are on a direct (final) approach (sailing right on the lay line).

When starting a mile away, the target is not more than 50º off your bows but, when you get closer, the target may be as much as 100º off of your bows and this turns VMG into a negative number.

You can try to compensate for this by extending a line beyond the target, and projecting a waypoint well beyond the target. But each time you begin a new tack, you would need a new projected waypoint. And each time the wind shifts, so does most everything else.

For me to have an accurate gauge of my upwind performance from an electronic instrument, I would need to know my VMG to a projected waypoint that is projected a constant number of miles beyond the target, in the exact current bearing of the target. A computer can provide this, but the computer will need to know the apparant wind speed and apparant wind angle, combined with boat speed over the ground, location of the boat, and location of the target in order to extrapolate
such a number.

Personally, I think that the most usefull reading, upwind on a catmaran would be apparant wind speed; keep it as high as possible! I want a very faint audio pitch (in my ear from my sunglasses frame or helmet) in my ear that varies in pitch as the wind varies in speed. ("Electronic Shroud Whistle")

GARY



Santa Monica Bay
Mystere 6.0 "Whisk" <--- R.I.P.
Re: Speedo GPS and VMG [Re: hobiegary] #67923
02/25/06 01:23 PM
02/25/06 01:23 PM
Joined: Mar 2005
Posts: 160
claus Offline
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Hobiegary, seems that your gps calculates VMG by measuring the distance to one waypoint, calculating the velocity from the changes in this distance to the waypoint. This is not VMG as it is used in sailing. In sailing it would be the proyection of the real velocity to some diection (i.e. wind direction), the velocitek s5 seems to do such a calculation.

Re: Speedo GPS and VMG [Re: claus] #67924
02/25/06 03:07 PM
02/25/06 03:07 PM
Joined: Dec 2005
Posts: 55
Los Angeles, CA
chrisun Offline
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Quote
Hobiegary, seems that your gps calculates VMG by measuring the distance to one waypoint, calculating the velocity from the changes in this distance to the waypoint. This is not VMG as it is used in sailing. In sailing it would be the proyection of the real velocity to some diection (i.e. wind direction), the velocitek s5 seems to do such a calculation.


What?! I don't agree.

The description of VMG that Velocitek provides is no different than what Hobiegary described. Velocitek simply chose to only illustrate the optimal situation that Hobiegary described:

"when you are on a direct (final) approach (sailing right on the lay line)."

Other than that Hobiegary is right - if you want to know your best speed to a target you need to have data that accounts for wind direction as well.

The Velocitek seems to offer nothing more innovative in the realm of VMG calculation than any other GPS that calculates VMG - at least according to the description that they offer themselves:
http://www.velocitekspeed.com/VMG.html


C-Class USA75
Re: Speedo GPS and VMG [Re: chrisun] #67925
02/25/06 03:18 PM
02/25/06 03:18 PM
Joined: Aug 2005
Posts: 2,921
Michigan
PTP Offline
Carpal Tunnel
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Carpal Tunnel

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Michigan
lets say you are sailing to a windward mark and following your VMG on your GPS. If you get to the point wherein VMG becomes zero then negative, aren't you at the layline for that mark and should therefore tack? (not counting leeward drift). It would mean that if you tacked 90 deg then you would be headed straight for the mark. Am I wrong? We do this occasionally when racing on a monohull.

Re: Speedo GPS and VMG [Re: PTP] #67926
02/25/06 03:56 PM
02/25/06 03:56 PM
Joined: Dec 2005
Posts: 55
Los Angeles, CA
chrisun Offline
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Los Angeles, CA
Quote
lets say you are sailing to a windward mark and following your VMG on your GPS. If you get to the point wherein VMG becomes zero then negative, aren't you at the layline for that mark and should therefore tack? (not counting leeward drift). It would mean that if you tacked 90 deg then you would be headed straight for the mark. Am I wrong? We do this occasionally when racing on a monohull.

I think the point is that when using GPS calculated VMG one has to remain aware that as the angle of the bow to the mark becomes large (closer to 90 degrees or greater), VMG will drop - as you said, to zero or negative -

Essentially GPS VMG doesn't tell you how fast you are going in the direction that you need to go in order to make a mark. It only tells you how fast you are going towards a mark (or waypoint).

In response to the situation that you described - a negative VMG reading, or a VMG reading of zero does not always mean you are at your layline. Currents, for instance, can have significant effects on how far you have to sail "past" a mark in order to lay a mark. I can recall several instances where tacking/gybing at a GPS VMG reading of 0 would never get you to the mark.

But, yes, discounting all of the common exceptions (current, obstructions, etc.), sailing past a VMG reading of 0 means sailing beyond the layline. But in general, except on the last tack, one would want to tack before VMG dropped to 0 or below in order to stay inside the course.


C-Class USA75
Re: Speedo GPS and VMG [Re: PTP] #67927
02/25/06 03:57 PM
02/25/06 03:57 PM
Joined: Jul 2001
Posts: 851
US Western Continental Shelf
hobiegary Offline
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US Western Continental Shelf
Quote
lets say you are sailing to a windward mark and following your VMG on your GPS. If you get to the point wherein VMG becomes zero then negative, aren't you at the layline for that mark and should therefore tack? (not counting leeward drift). It would mean that if you tacked 90 deg then you would be headed straight for the mark. Am I wrong?


If you are trying to use VMG for its best use, then yes, you are wrong. If you thought that simply because you are moving in a direction that is 90º to your target, that you are at the perfect place to tack, then yes, you are almost certainly wrong.

But using the VMG function, or using a bearing for determining a tack angle is not a good idea UNLESS: a) there is no current, and b) There is an exact 90º tacking angle on the boat upon which you sit, and c) there are no deviations in the direction from which the wind blows during your journey (wind shifts), and d) there are no seas (waves) which will affect your progress, and e) your boat does not do better on one tack than the other tack, and f) there are no wind velocity changes that will affect your boat's pointing ability.

If you are using this reading to determine when to tack, then you could just as well use the (proportionate) "bearing to mark" reading. When it hits the desired tacking angle, ... go for it.

As for the usefullness of VMG To Mark, it is very limited without the added datum "true wind angle." Additional data to include 'true wind direction,' 'true wind speed,' 'delta of distination:true wind,' and average boat speed (over the ground) on each tack,' would IMO be certainly be helpful.

GARY


Santa Monica Bay
Mystere 6.0 "Whisk" <--- R.I.P.
Re: Speedo GPS and VMG [Re: claus] #67928
02/25/06 04:01 PM
02/25/06 04:01 PM
Joined: Jul 2001
Posts: 851
US Western Continental Shelf
hobiegary Offline
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Quote
Hobiegary, seems that your gps calculates VMG by measuring the distance to one waypoint, calculating the velocity from the changes in this distance to the waypoint. This is not VMG as it is used in sailing. In sailing it would be the proyection of the real velocity to some diection (i.e. wind direction), the velocitek s5 seems to do such a calculation.


Respectfully, I disagree.

GARY


Santa Monica Bay
Mystere 6.0 "Whisk" <--- R.I.P.
Re: Speedo GPS and VMG [Re: hobiegary] #67929
02/25/06 04:27 PM
02/25/06 04:27 PM
Joined: Aug 2005
Posts: 2,921
Michigan
PTP Offline
Carpal Tunnel
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Posts: 2,921
Michigan
I understand what you are saying, but I wish I knew how to include diagrams for this but I don't. If you want to limit the number of tacks, because tacking loses ground in cats, then why not sail to the outside where your VMG to the waypoint nears 0 then tack. Wouldn't you be optimizing your distance on a tack as opposed to tacking more times and staying in the center of the course? This is, of course, all things being equal (same wind throughout the course, no current -although sailing to the side of the course until VMG=0 with a current would still limit tacks if the current is straight from the windward mark).

Re: Speedo GPS and VMG [Re: PTP] #67930
02/25/06 06:42 PM
02/25/06 06:42 PM
Joined: Feb 2004
Posts: 3,528
Looking for a Job, I got credi...
scooby_simon Offline
Hull Flying, Snow Sliding....
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Looking for a Job, I got credi...
Quote
BTW I still use an old style Garmin GPS V, but plan on getting a 201 as soon as the impulse takes me....


Me too...

As for VMG calcs the only way you can do it is to set the upwind (or downwind) point to be miles (and yes I mean miles) downwind and then use it that way; if you have the point too close to you them the "VMG Calcs" all go wrong and it comes up with meaningless answers as the target is too close.




F16 - GBR 553 - SOLD

I also talk sport here
Re: Speedo GPS and VMG [Re: chrisun] #67931
02/26/06 04:48 AM
02/26/06 04:48 AM
Joined: Mar 2005
Posts: 160
claus Offline
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claus  Offline
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Posts: 160
Quote

What?! I don't agree.

The description of VMG that Velocitek provides is no different than what Hobiegary described. Velocitek simply chose to only illustrate the optimal situation that Hobiegary described:


velocitek writes:

The device then uses
trigonometry to extract the component of your actual speed that is aligned with the upwind-downwind
line defined by TWO reference locations you stored in the device's memory.

This means VMG is calculated in a correct manner, and for example, will not be close to 0 or negative when you are close to the layline but on the other tack than the one that is laying the buoy directly (say short before tacking)

When a gps calculates VMG as velocity of approach to ONE certain point, VMG will be close to zero in the situation described above. This is a very important difference to sailors. Simple trigonometry.

Re: Speedo GPS and VMG [Re: claus] #67932
02/26/06 05:17 PM
02/26/06 05:17 PM

A
Anonymous
Unregistered
Anonymous
Unregistered
A



Hello,

I'm the designer of the S5. I hope I can contribute to the conversation by explaining a little bit more about how our device works.

The S5 gives the magnitude of your velocity projected onto the rhumb line between the windward and leeward marks. You define the direction of this line by entering two reference points. These points are only used to define a DIRECTION. For example, you could use two points on the beach or in the parking lot, as long as one was directly upwind of the other.

Your velocity projected onto the rhumb line depends only on two things: the magnitude of your velocity vector (your speed) and the direction of your velocity vector (your heading) relative to the direction of the rhumb line. It does not depend at all on your position relative to the marks. No matter where you are on the racecourse the S5 will read the same thing for a given speed and heading.

Other GPS devices don't give VMG, they give closing speed. You can get them to approximate VMG by giving them a waypoint far upwind of the windward mark (say 50 miles). Picking the exact location of this point 50 miles upwind can be a big headache when the wind direction changes. If you just used the windward mark as your waypoint, the closing speed given by the device would not look anything like VMG once you got close to the windward mark.

To sum it up, our device tells you how fast you are moving in the upwind (or downwind) DIRECTION. Other devices tell you how fast you are moving towards (or away from) a pre-programmed LOCATION.

When we first started testing this device, we used closing speed like everyone else. Our pro team complained that it did not "feel" like real VMG and got all screwed up close to the marks. This made us figure out the difference between closing speed and real VMG. The pro team is now confident that the output of the device is the number they really care about.

I hope this helps. Please let me know if any of this doesn't make sense to you.

Thanks for reading,

Alec

Re: Speedo GPS and VMG [Re: ] #67933
02/26/06 06:56 PM
02/26/06 06:56 PM
Joined: Dec 2005
Posts: 55
Los Angeles, CA
chrisun Offline
journeyman
chrisun  Offline
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Posts: 55
Los Angeles, CA
Quote
Hello,

I'm the designer of the S5. I hope I can contribute to the conversation by explaining a little bit more about how our device works.


Welcome Alec!

The explanation helped - especially the contrast versus closing speed calculations. Thanks!

Does the S5 still calculate closing speed?
Does the S5 display SOG/VMG and closing speed at the same time?

Closing speed is still of interest in certain situations.

Howcome no photos of the S3/S5 displays (with something displayed) on the website? [please point out links if I missed them]


C-Class USA75
Re: Speedo GPS and VMG [Re: chrisun] #67934
02/26/06 08:08 PM
02/26/06 08:08 PM

A
Anonymous
Unregistered
Anonymous
Unregistered
A



Thanks for your questions.

The production version of the S5 displays either real VMG or SOG depending on the operating mode chosen by the user. It does not display closing speed.

We'll fix the photos of the display on the website in the next few days (there was a mix-up with our photographer and the devices were not switched on for the catalog shots). For now, to get an idea of how the display looks when the device is on please look at this page with photos of an older version of the S3 speedometer: www.velocitekspeed.com/product

The display numbers are huge and can be easily read from well over 30 ft away.

Re: Speedo GPS and VMG [Re: ] #67935
02/26/06 08:57 PM
02/26/06 08:57 PM
Joined: Dec 2005
Posts: 338
Nimrod Offline
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Posts: 338
Being a neophyte to cat racing the only question I can think of with all of this - is do you really have the time to fiddle with a GPS trying to figure what is the best course –tack in the heat of a race? From my personal experience last summer, my hands were full handling the boat, watching for other boats, and feeling the wind watching the water & working off my gut. That seems to work for me with my recent collection of mantel hardware. IMHO I would rather learn how do this with instinct in place of electronics for the mere fact what happens should the batteries go dead.

What's next stall warning horns for apparent like they have in airlines?
"Dweidle Dweidel Dweidle Fall Off Fall Off you going to slow - Dweidle Dweidel Dweidle"

Re: Speedo GPS and VMG [Re: Nimrod] #67936
02/26/06 09:31 PM
02/26/06 09:31 PM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
Carpal Tunnel
Jake  Offline
Carpal Tunnel

Joined: Jun 2001
Posts: 12,310
South Carolina
Quote
What's next stall warning horns for apparent like they have in airlines?
"Dweidle Dweidel Dweidle Fall Off Fall Off you going to slow - Dweidle Dweidel Dweidle"


I could use one of those!


Jake Kohl
Re: Speedo GPS and VMG [Re: ] #67937
02/26/06 11:41 PM
02/26/06 11:41 PM
Joined: Feb 2004
Posts: 1,012
South Australia
Darryl_Barrett Offline
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Darryl_Barrett  Offline
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Posts: 1,012
South Australia
How do I get one in South Australia?? (I won't every use a credit card over the internet)

Re: Speedo GPS and VMG [Re: ] #67938
02/27/06 03:18 AM
02/27/06 03:18 AM
Joined: Feb 2005
Posts: 1,382
Essex, UK
Jalani Offline
veteran
Jalani  Offline
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Posts: 1,382
Essex, UK
Hi Alec,

Interesting product, how do I get to look at and possibly buy an S5 in the UK?


John Alani
___________
Stealth F16s GBR527 and GBR538
Re: Speedo GPS and VMG [Re: Nimrod] #67939
02/27/06 03:21 AM
02/27/06 03:21 AM
Joined: Feb 2005
Posts: 1,382
Essex, UK
Jalani Offline
veteran
Jalani  Offline
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Posts: 1,382
Essex, UK
Spot on Nimrod,

There really isn't anything better than getting your head 'out of the boat'!

But, once all the things you've described become semi-automatic (practice, practice and practice), toys like GPS, and TackTic become useful.


John Alani
___________
Stealth F16s GBR527 and GBR538
Re: Speedo GPS and VMG [Re: Jalani] #67940
02/27/06 05:53 AM
02/27/06 05:53 AM
Joined: Nov 2005
Posts: 64
Switzerland
H2O_Sensations Offline OP
journeyman
H2O_Sensations  Offline OP
journeyman

Joined: Nov 2005
Posts: 64
Switzerland
Darryl, Jalani,

You can see all the Velocitek Dealers from Velocitek website http://www.velocitekspeed.com/Global_dealers.html
for australia the exclusive importer is http://www.kasail.com/

Regards,
Jr

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