View Full Version : GPS Speed-o

River Rat
05-26-2006, 10:16 AM
I'm wondering if anyone makes a GPS based speed-o? I don't have the cash for perfect pass and I'm getting tired of my old system plugging and not working correctly :mad: I figure if it exist then someone on here knows about it :rolleyes:

05-26-2006, 10:30 AM
Yes they do exist, though I am not so sure that you are going to be much cheaper than perfect pass by the time that you are done.

Try this link:

Livorsi Marine (http://www.livorsi.com/catalog/gps.htm)

Let us know how it works out if you go ahead with it!

05-26-2006, 10:45 AM

05-26-2006, 11:44 AM
Save longer for the PP, it's worth it.

05-26-2006, 12:19 PM
Be carefull with checking your speed with a GPS. Check out this article.

88 PS190
05-26-2006, 12:35 PM
ok so this seems inaccurate to me.

GPS speedos judge your overall speed based on triangulation, this is not affected in the least by water current.

GPS should provide the 100% what your speed is, a paddle wheel or pitot varies w/ water speed, ie upstream 2 mph current will read 2 mph + your boat speed, downstream it will read slower.

That article is back*sswards.

What is true is that GPS does not provide speed in relation to water, but overall speed, which is the same thing that timing through the course indicates, so that if you are taking a slalom pass into 2mph current the boat speed will be 36 mph over the ground, and your balls will come up at the same rate, they do not slow the boat down 2mph so that the skier has the same percieved feel from the boat.

The viewpoint they are providing is that when using a GPS speedo into a 2mph current your boat speed is 2mph lower because you are slowed by current even though your pitot tube or paddlewheel is indicating the speed you'd expect.

For all intents and purposes the speedometer should be set accurately when using GPS for watersports. If you have a GPS and dual tournament speedos it'd be common practice to calibrate one for up current and one for down current.

05-26-2006, 12:45 PM
I agree it doesn't make sense to me but this is not the only time or place I have either heard or read this.

05-26-2006, 12:50 PM

05-26-2006, 02:03 PM
I believe the article is correct. The thing to remember is this article is for a Data Bank instrument/guage that seems to be intended for mechanics/prop designers. If you where a prop designer, you would want to know your speed of the boat in relation to the water, not land. This is what this device is measuring.

Now, the majority of us want to know our boat speed in relation to land, which GPS could give us, or a pitot tube/pressure setup that is adjusted for upstream and downstream runs.

05-26-2006, 02:27 PM
... but the rotation of the earth is taken into account.

88 PS190
05-26-2006, 02:39 PM
I agree, the article is making a point about performance tuning, in which case if you're going 70 up current vs. the water you are doing the same as 70 down current with it ie. your prop rpm is the same etc.

BUT that doens't apply to skiing

05-26-2006, 03:15 PM
So, what is backwards about the article?

05-26-2006, 03:26 PM
GPS measures SOG (Speed Over Ground). Slalom course timing measures SOG as well, so the two should be identical. I don't know of many slalom courses that are on flowing water, but if they are, you will be off by the speed at which the water is flowing (faster or slower depending on which direction you're traveling)

GPS is an extremely accurate measure of speed and position. It wasn't that way when Selective Availability (SA) was activated, but since the government deactivated SA, it's been extremely accurate.

These GPS speedometers look good, but they don't act like the PP cruise control, so you still have to monitor throttle, right???

River Rat
05-26-2006, 03:26 PM
All I want is a consistent speed-o at a reasonable price! I'm tired of mine plugging up.

88 PS190
05-26-2006, 03:41 PM
The backwards comment is in regards to our sport and the warning against a GPS speedo, modern GPS speedo's have very fast refresh rates, and a good way to set your speedos on open water.

It doesn't make up for timing or PP in the course, but this article's warning about GPS speedos is exactly the reason to use GPS in our sport.

05-26-2006, 04:08 PM
If you're adamant about a correct speed, your speedos are the way to go, i.e. + / - 1/2 MPH for corrections and allowable tolerances in a course.

GPS may not be accurate in real time and operating conditions, including an aggressive skier VS a less aggressive skier, etc.

To calibrate them the old fashion way, use a stop watch and time your speed in a course or some set distance.

16.5 secs in 855 feet should be close to 36 MPH + / - 1/2 mph IIRC.

This will get you dead on.... Thus the seating setup in the Pro Stars for a timer, as once was.

GPS is for city slickers :uglyhamme :uglyhamme

O.k. I like the idea of the GPS speed-o. It does eliminate the tube clog if nothig else. I have a perfect pass, but on my 93 my friend ,who placed 3rd in the mens 2 state in 95 and would probably know, has always said my 93 seemed "hot". Feel of speed vs speed-o reading. I wanted a way to check that my PP was right and the timing deal sounds better,......and cheaper. So since I dont know, how long is a course? Total and from entry gate to say 3 ball? From each ball to the next? I have acess to a course and could time mine that way but I need a reference other than 855feet because I have no way to judge that distance. Does that make sense?? :confused:

05-26-2006, 07:33 PM

05-26-2006, 10:46 PM
Thanks!! So from the entry gate to the exit gate should be 855 feet and if you make it from one :steering: to the other :steering: in 16.5 seconds you should be going 36mph more or less. Is that correct? Sorry to be stupid, I just wanna make sure I got it right cause I will time it tommorrow if I can find my stop watch.

05-26-2006, 11:40 PM
I'm pretty sure that's right and I guess it's a six buoy course.

City slickers! lol

05-27-2006, 09:12 AM
Yeah, and thanks!

05-27-2006, 09:31 AM