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.
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
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]
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.
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"
"Dweidle Dweidel Dweidle Fall Off Fall Off you going to slow - Dweidle Dweidel Dweidle"
I could use one of those! 
How do I get one in South Australia?? (I won't every use a credit card over the internet)
Hi Alec,
Interesting product, how do I get to look at and possibly buy an S5 in the 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.
Darryl, Jalani,
You can see all the Velocitek Dealers from Velocitek website http:/
for australia the exclusive importer is http://www.kasail.com/
Regards,
Jr
"Dweidle Dweidel Dweidle Fall Off Fall Off you going to slow - Dweidle Dweidel Dweidle"
That would be too cool. I could definitely use one of those. How do they work on airplanes? I think we have a new invention in the making.
Mike Hill
www.stlouiscats.com
I could use one of those!
It's usually a little metal tab mounted on the leading edge of the wing (on a Cessna type, not a jet), just low of middle (top to bottom), normaly it is held down by airflow and gravity, which keeps the warning horn silent, but as you approach the "stall" angle of attack (usually around 17 degrees to relative wind) the flow over the wing begins to separate, the little metal tab moves up, which closes a switch and makes the alarm sound. We have the same thing on sails; Tell Tales, just no aural warning.
All you would need for an aural warning is to make the tell tales out of metal tape, with a metal conductor patch in the sail where they should normally flow, and then when they lift, the alarm would sound...of course you also need to run wires in the sails, bateries, (or solar panels) and a warning horn of some type. OR...just pay your crew to tell you not to pinch!
Maybe I'll try fastening a whistle to my forestay's bridle wire. Like one of those deer/elk warning gizmos.
GARY
Mine would be squawking at me all the time. I actually had the good fortune at the last race of the year (last year) to have an experienced crew who would just quietly say "pinching" as soon as he felt the speed drop off. It was a great help and in a short while kept me more attentive to that situation.
DavidN
H20 781