Dear All,
Would a VMG function be important to you on a Speedo GPS or not?
thanks,
Jr
Indespensible, IMO - otherwise, you'd know that you were going very fast in a straight line on a map, but you wouldn't know if you were making best possible speed toward your objective. VMG gives you the ability to determine if you are sailing the shortest course with speed.
Stupid question of the day? Are these things class legal? I'd do almost anything to improve my downwind performance from dismal to merely mediocre! 
Depends on the class.
Hobie classes only allow electronic compasses / watches.
They're making electronic weenie-bikinis now?
Pete,
nothing stops you from training with a GPS, if you can't get somebody to coach you. Then you _know_ how to run your boat when you get on the racecourse.
The best and fastest way to get faster, is to have somebody to train with who is pretty fast.
I'm with JW on this one. not only is it useful in telling you how fast you're going towards your destination, it can also let you know just how long you'll be bobbing along in the atlantic ocean with some bloke whose gotten to much sun and who sings "she thinks my tractor's sexy" to sea turtles.
I might be the only one though....
. . . Then you _know_ how to run your boat when you get on the racecourse. . .
I was thinking that as well. Nobody here to train with.
Boudicca:
They're only making them for fat men men like me! 
Very distracting to the competition!
Stupid question 
?! Possibly but not less than the answers...
Open Forum discussion is for open questions or only for Scientific one? 
I thought getting CatSailors view point would be interesting as a several world champion in J Class told me that VMG is crutial on monohull (even more on boat longer than 12m). For him, the only interesting value on cats is the real speed of the boat... Even more that we often need to go a little down wind to improve speed before going up again.
Nevertheless, I'm each time amazed by friendlyness of some people in this forum.
Jr
H2O:
I'm sorry, but I think you misunderstood! 
I was refering to MY question being the stupid question of the day, because I'm supposed to know those rules!
Personally, I don't think any question is stupid.
Yeah, you gotta love the catamaran community. Even though they'll argue boats to death, at heart we'll all rooting for each other, especially turning a kind ear to beginners...P.S. maybe it's a west coast thing but out here it usually goes by the term banana hammock...
Or "Grape smuggler"
That was a joke, I wouldn't be caught dead in one! I just hadn't rattled Boudaccia's cage in day or two!
can you recomend on a gps product for beach catamaran?
if not allowed on the forum then use p.m
i want one 
thank you,
I have strong comments about the Suunto M9. Not favorable. for more specifics, send me a PM, I would be happy to expand.
Try this:
http:/
My crew got one of these on e-bay for something like 60bucks. I would recommend it.
The Foretrex 201 may be a better bet with its rechargeable Li battery. Several of the guys at my club have these Garmins and the general consensus is that the 201 battery life is better than the 101. Both are quoted at 15 hrs 'normal use'. In fact the 101 has shown itself to be nearer 10 or less while the 201 is more like 13 or 14 or maybe it CAN even do 15!
This is all anecdotal changing room and bar talk of course. BTW I still use an old style Garmin GPS V, but plan on getting a 201 as soon as the impulse takes me.... 
I recommand the Velocitek S3 http://www.velocitekspeed.com/.
This is small company (but great product with 2 year guarantee) in the US company based on Oregon River. The GPS as a great display, big, readable, precise and refresed every sec. Each movement, tunings you do that has an impact on your speed in instantly displayed...
Big advantave vs other is that you share data with your partner.
On the top of that, this GPS is waterproof at 25m.
Tikipete,
No problems.
Regards,
Jr
This is all anecdotal changing room and bar talk of course. BTW I still use an old style Garmin GPS V, but plan on getting a 201 as soon as the impulse takes me....
Whats great about the Foretrex 201, apart from its exceptional battery life, is the computer interface.
After sailing I upload the data to the Garmin mapping software to see my track, tacking angles, high speeds, etc.
When sailing long distance I upload the necessary waypoints and create a route, that way I can focus on the sailing and not go blind staring for markers.
With the Velocitek S3 you have a 20 to 25 hours battery life with AAA batteries...
Tested and approved.
Jr
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
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:/
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.
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
Respectfully, I disagree.
GARY
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).
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.