Originally Posted by Wobble
Good question. Then, how fast was it on that particular day?and then you go to the Detroit River with a 7 mph current, you do not have a 75mph boat because your GPS said so on that particular day?
And, if you always boat on the detroit river, and so do your friends, how fast is your boat if it reads 75mph on your way to the bar?
Originally Posted by Wobble
68Originally Posted by Emaginashun
79.5Originally Posted by Emaginashun
This might expain how the donz jr. got a straight bottom top gun to go 108? NK
I guess perception is reality.
Last edited by Emaginashun; 12-02-2005 at 12:48 PM.
I drag raced boats for the past 20 years before going "offshore" style boats.
In the early years, our timing systems consisted of 2 cops with radar guns on a pontoon boat beyond the course, in the shutdown zone. Pretty crude stuff. Some boats, particularly those with large inboards did give a decent radar target and reflection. Some, particulary streamlined boats, like a STV or Mirage outboard did not. We ended up putting metal duct tape on some of these boats to get a "reasonable" reading.
Radar can be accurate, but it requires a highly reflective target. There is also an issue with cosine error if not shot directly (0o) at the boat.
Later, and much better, most sanctioning bodies put into use automotive timing equipment like Chrondek systems. These are very accurate and give RTs, ETs, and top speed measured over the final 100 feet of a 1/4 mile course, just like car drags.
Like any electronic device, these weren't perfect, and did occaisionally give some bogus #s, but by and large were very accurate. I'm sure that's why UIM uses a similar system for the official kilo records.
I began using 12 channel handheld GPS about 10 years ago in my race boats to test. The earlier 4 channel units updated so slow, particularly on a drag boat, they were useless. The 12 channel units were usually within a few tenths MPHs of the Chrondeks that we used. The GPS was usually slightly slower, which could have been calibration of the Chrondek system, or
even with the 12 channel units, a slightly slower update.
GPS can be fooled by flowing bodies of water, since they are measuring SOG/speed over ground. Other than that, I have found the 12 channel units very accurate.
Last edited by Steve Zuckerman; 12-02-2005 at 01:25 PM.
Question; how can the GPS determine the difference between water and land?GPS can be fooled by flowing bodies of water, since they are measuring SOG/speed over ground
It is simply measuring distance traveled with a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites.
The speed is smoothed and not instantaneous speed.
It can't; therein lies the problem.Originally Posted by Emaginashun
So the rule is 1 MPH per beer, and 1 foot per shot. NOW I get itOriginally Posted by birdog
Last week was my birthday, and I swear I was doing 140 in 15 footers
For a real world GPS speed, just backup the run going the other direction and average it like the big boys do in the kilo runs. I'm always up for 1 more high speed run.....that's a perfect excuse for the girlfriend.....
Horizontal speed remains constant and surface conditions do not factor in on GPS measurement.Originally Posted by CigDaze
Last edited by Emaginashun; 12-02-2005 at 03:17 PM.
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