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RedDog
08-30-2002, 12:17 PM
The other day went for a ride in a friend's boat and made a WOT run. His GPS read top speed at 68.x MPH. My GPS read 66.x MPH. We came to the only logical conclusion a performance boater could reach - something is wrong with my GPS! That is good news since it means I have been running faster than I thought.

Both units were Garmins - his a Model 10? and mine an eTrex Legend. Why the difference? Has anyone else experienced this?

As Ben Franklin once said: A man with 2 clocks never knows the correct time

MOP
08-30-2002, 01:12 PM
I think my GPS gets odd readings now and then, I run my boat up it turns 4600 and pins the bogus speedometer but my GPS will show two to three miles an hour diffrence on different runs. I have a buddie that borrowed a Falcon radar gun, that also gave three different speed reading same day all within an hour on both our boats. I am to the point that the only acurate way is still the measured mile and a stop watch.

Gary
08-30-2002, 01:43 PM
Hummm, how do you know that his isn't the wrong one :p

Woodsy
08-30-2002, 02:10 PM
The most accurate way to determine speed is a calibrated radar gun. Usually, GPS & radar are within 1-2mph of each other. The GPS, because of its positional error, and depending on the amount of satellites it picks up is usually off a little. Because your E-Trex is newer, and probably has the DGPS or WAAS correction alogorithm, I would bet that your was more accurate. Also, boat speed depends alot on conditions: temp, waves, wind, fuel load, etc. a 1 or 2 mph variation is not that uncommon.

Woodsy Von Donzi
Navigational Hazard

Lil Grizz
08-31-2002, 12:41 AM
Some GPS's have a slower refresh rate than others. My fathers GPS will climb up smoothly when mins seems to jump 10mph at a time during a hard accel. Every so often I will make a WOT run and pull off the throttle but the GPS will linger and sometimes even climb up while I am actually almost off plane. Dont forget that holding the gps in your hand can affect the way it reads, try securing the units and testing them side by side. I bet the one reading higher will also show speeds sooner than the slower one. by that I mean one will lag behind the other.

HyperDonzi
08-31-2002, 09:33 PM
mark off 2 bouys, a set distance apart and have someeone time you from 1 to the next, then i cant remember the next way to multiply it out to but if someone needs to know, i will find it for you.

Rootsy
09-01-2002, 08:47 AM
Average velocity = displacement / time...

Average acceleration = Average velocity / time...

the 4 equations of motion for instantaneous velocity (acceleration is 0) and or constant acceleration are:

V^2 = Vo^2 + 2AS

S = So + VoT + 1/2AT^2

V = Vo + AT

S = ((Vo + Vf)/S)T

Vo = Initial Velocity
Vf = Final Velocity
V = instantaneous Velocity
S = Displacement
So = Initial Displacement
S = Displacement
A = Acceleration
T = Time

if acceleration is not constant you get to enjoy some wonderfully more difficult calculus and differential equations...

HyperDonzi
09-01-2002, 09:44 AM
that is to complicated, find out how many feet per second you are traveling. lets say you cover 100 feet in 2.7 sec (100/2.7) = 37 fps.
But how many MPH is that?
There are 60 seconds in a minute and 60 minutes in an hour, so there are (60 * 60) = 3600 seconds in an hour.
This means you were going (37 feet/sec * 3600 sec/hour) = 133,200 feet per hour.
Almost there...
There are 5,280 feet in a mile, so you were going (133,200 / 5280) = 25.2 MPH.

i know no one on this board topps out even remotly close to 25 but this is an example.

mattyboy
09-01-2002, 09:46 AM
Jamie,
please you're making my head hurt :D wink :D

Matt

HyperDonzi
09-01-2002, 09:50 AM
this one wont make anyones head hurt, looking at it,
do this FPS*.682=mph

Rootsy
09-02-2002, 12:02 AM
HYPER,

only problem with the measured distance in a given amount of time method is that you never know your true instantaneous MAXIMUM velocity... you only know an average velocity over that given distance. This is perfectly fine if you are at WOT and the boat is neither gaining nor losing speed... me personally, i havn't encountered this... i am either gaining or losing a few tenths with each wave or correction of the wheel... etc... but its a good guestimate..