I've always wondered how fast we are realy going. In an advert i saw a great device, Velocitek S10 (velocitekspeed.com).

This tool promises to show real time speed and even has a velicity made good function.

Can anybody, who has bought it tell me how it performs in the real life?

if you do a search for speed, there is plenty of advice on how to measure your speed. In short, GPS speed readings are at best, suspect. I once registered a speed of 73 knots on the GPS during the tybee.

And you REALLY believe you where 73 knots? Maybe in your dreams!!!! LOL. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

Doug

The GPS on my fishing boat has read as high as 45kts, the boat is 20' with a four cylinder inboard. No way it goes over 20kts with a tailwind. Can't be trusted. Measured and timed is the only way to get solid numbers. 74kts on a cat, what a ride.

I've heard good things and some bad things about the units. I haven't used one on a boat but I have had an opportunity to play with one briefly. The developers of the thing really seem to be intune with what people want and I know they've been making changes to the device based on feedback...so some of what I say might be out of date.

The unit I saw displays only two digits of speed data. If you are at 5.4 knots - great. If you are at 15.4 knots - you only got to see "15". In order to establish VMG, you first much tell the unit the wind direction by clicking a button at a downwind point, then clicking a button after sailing to a point directly upwind. IMHO, this would be very difficult to do accurately and/or in a timely fashion before a start - especially on a lake where the wind shifts regularly and often dramatically.

As far as the build quality, the unit is bulletproof. I was very impressed with the construction. I wish they would come out with a unit that functions like the basic tactic unit with a magnetic compass instead of a GPS compass. A unit with angled displays and some better mounting options to provide some competition with tactic would be a market killer.

I'm very certain that was tongue in cheek.

True, if you're course is "square" to the wind .... If you had time to mark "C" and "A" buoys (or anywhere along this line that is a sufficient distance apart for the GPS to calibrate) wouldn't the VMG be displayed as the fastest point of sail to this (imaginary) line?


Ex: If the wind is backed then port-tack is favored and your VMG will be very high. When tacking to stbd-tack, you're VMG will be much lower. Compared to a perfectly square course you'd expect your VMG on either tack to be similar.

Gadgets are cool and all. BUT, my take on this is to get my head OUT of the boat. If I am passing boats I am going fast. If I am falling behind I am going slow.

These items are all good for number values, but around the race course I have no time to be looking inside the boat. It is just another thing to distract me.

OF COURSE! this is my take on this matter.

True, but what if you're practicing with no other boats to pass (or pass you).

Time and place for everything <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Gosh, I don't know, wouldn't it be really hard to keep your tongue in your cheek at 73 knots? I'd think your lips would be flapping in the breeze a little too much... Must be true.

Are you going to do this before the race starts or chalk the first leg up to setting up the GPS? There is no time to program in the marks around the course before the race since racing usually starts shortly after the RC has a course set. Additionally, I don't have a hand free at any mark rounding to be pushing buttons on a device. If it automatically could recognize when I rounded a mark - that would be something (until the race committee moves A due to a wind shift...then it becomes useless again).

I think it is a decent training device, but I don't think I would find it practical on a double handed spin cat around a race course.

No... you set your down wind position ... where ever you are on the course.

You sail one minute on port... you sail one minute on starboard. Stop... set the upwind....

This device extrapolates the wind bearing to infinity.. and caclulates your VMG to the wind. your GARMIN is always trying to send you to a fixed point.... not infinity,. so your VMG is to the bearing... not to the wind.


If the wind shifts during the race... You will see it as different VMG's on port and starbord... Hmm... you may not have noticed this shift.

I use the Velocitek for training. I use the VMG mode as that is the fastest way to get around a course. Who cares if you are going 25 in the wrong direction?

I think it helps a bit in determining and getting the feel for the fastest angle of attack in different wind and sea conditions.

I have sent in my list of things I would like it to do but have not heard back on whether they were programmed.

Later,
Dan

The newer GPS units that are WAAs compatible are far more accurate then the older units. You may still receive some erroneous readout, but none the less your speed will be more accurate.

From the Garmin website

The origins of WAAS

The Federal Aviation Administration (FAA) and the Department of Transportation (DOT) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

How it Works

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

Who benefits from WAAS?

Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does.

Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.

GPS speeds are a pretty simple distance/time calculation... and erroneous results are easily explained. Most units track position at fixed increments of time, default is usually 10 to 30 second intervals. When you export your track and look at the table of data, you can identify where the errors are introduced. Here is an example from a recent bicycle training ride... Notice that for one interval, the unit lost reception and placed my position with no movement and then calculated the speed as double when it found the position in the next interval. This often happens under bridges or very large trees. On a boat it can happen when your handheld is blocked by your lifejacket. You will notice that the odd max speeds are an even increment of your expected speed on a boat for the conditions... 2, 3 or 4 times expected.

391 9/21/2006 19:02 909 ft 299 ft 0:00:10 20 mph 111° true N40.12869 W83.15433
392 9/21/2006 19:03 902 ft 312 ft 0:00:10 21 mph 109° true N40.12839 W83.15333
393 9/21/2006 19:03 901 ft 309 ft 0:00:10 0 mph 109° true N40.12839 W83.15333
394 9/21/2006 19:03 898 ft 303 ft 0:00:10 42 mph 114° true N40.12781 W83.15124
395 9/21/2006 19:03 891 ft 288 ft 0:00:10 20 mph 119° true N40.12748 W83.15025

A friend of mine also logged a rediculous speed whilst participating in the Fast Cat I.O.W race. Whilst having a few beers after the race we discovered his GPS was placed on a string around his neck, whilst tacking our gybing we reckoned it might have swung to a radical speed but are they really that accurate? I doubt it.

On one race in the Keys, I think it registered "999" for a second.

I was tempted to put aluminum foil on my head to prevent the brain scan by the aliens who had obviously placed a tractor beam on the boat .... <img src="http://www.catsailor.com/forums/images/graemlins/ooo.gif" alt="" />

Turns out the batteries were crapping out...

Does this mean you have found your unit to be accurate most of the time Mike? and what brand was it
regards

Yes, I have found it to be very accurate when it has a clear view of the sky. It is a Garmin Etrex Vista C. I use it for a range of land, water and air activities and am quite satisfied. Unfortunately they have discontinued it but did replace it with the somewhat pricey Vista Cx It is my 4th garmin as I have upgraded several times.

I ride with a wheel speed sensor and they are almost always identical to the tenth of a mph. Errors are usually explainable by having it in a bad spot or under bridges and dense forest.

The lanyard attaches to the bottom of the unit so it will hang upside down with the antenae pointed down... not so good for best reception even in open water on a boat. It is best to keep in a pocket pointed up.

Agree with Mike - I have the original Vista and have used it on the boat for four or five years now. Only major wierd number I ever got was when I forgot to reset all the numbers before the Horn Island Hop one year. We got back to Ocean Springs YC after a particularly hairy, wet, screaming jib reach in flat water on a Nacra 6.0 - my skipper turned to me at the bar where we were in the drip-dry cycle and asked me loudly what our top speed had been. A hush fell over the crowd as I punched a couple of buttons and proudly announced our top speed was 79 mph - in the truck on the way over from Pensacola. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

User error aside, Garmin makes a good, reliable rugged product, and the Vista with the compass is quite useable for cat racing.