Radar Or Gps
 

What is the accurate speed of the boat. Is the Highway Patrol radar the true speed or the GPS in the boat. Had a customer tell me his boat does 96 mph by radar, I checked the GPS recall it said; 92mph. The background on the boat is known to run 94 mph in the summer @ 5500 rpm, cool fall 96 mph. This is with a 30 Bravo One. Today we ran 93mph @ 5100 rpms Bravo One 32. Peak HP old engines was 734 @ 5600, now we have 800 @ 5900. Brett @ B Blades told me we would be chasing a number. The 32 Bravo Lab finish would give use a true 96.5 mph @ 5400 then pick-up as the temps cooled. 2004 Top Gun Sportmaster lowers std length. I dont' believe the said; speed by radar vs gps. Any input on hard facts. Thanks Mark

Radar guns have to be calibrated regularly with a tuning fork to make them accurate.

There's no question that a properly calibrated radar gun, used in the proper way, is the most accurate method for velocity measurement.
The average GPS unit computes speed based on a series of trackpoints with every trackpoint have a certain degree of inaccuracy. Maybe 1%.
A modern radar gun uses a crystal oscillator with the accuracy of a high quality digital watch. Say .001% worst case.
That being said, it's much easier however to misuse a radar gun and get it wrong. GPS is foolproof.

A hand held Garmin GPS vs a hand held Magellan GPS always shows .6 - 1.2 MPH faster on the same run.

Th Radar is more accurate as long as the unit has been properly calibrated and is used at the proper angles.

Also, if you are on a river, radar will take the effect of river current has on the speed reading as long as the boat with the radar gun is floating with the current. GPS is speed over ground.

GPS tracks speed over the face of the planet. If there is a current in the body of water it can either add or take away from true speed. That said, Calibrated Radar used properly is most acccurate. Radar also affected by current, these are the reasons why speed runs have to be backed up from both directions.

The average discrepancy between a calibrated radar system, used by a competent operator who understands cosine error, and a high quality 12 channel gps unit will typically be less than 2%.
The only advantage radar has, if it meets the above criteria, is that it updates almost instantly under hard accellaration. GPS, particularly analog units, update slower, and therefore may need a longer run to accurately read top end speed.
I used to drag race boats. The Garmin digital handheld I used was never more than 1 mph (usually spot on) + or - the chrondek
computer system used to run the course. I have a Garmin GPS plotter (with digital speed) on my current boat, and a Livorsi analog GPS speedometer, both with recall, and they use seperate antennas. On full throttle acelleration the Garmin usually shows 1 to 3 MPH faster speeds than the Livorsi. However, at peak speed they show nearly identical #s on recall.
We've come a long way from the old pitot tube "dreamometers".............good for people who really want to know how fast their boats are, bad for bench racers and bullsheetters :)........
And, current can fool both radar and gps.
Regards,
Steve

I think GPS is very accurate you do have to take in account river current. I always use the Max Speed feature you have to hold the speed for several seconds before the number is stored. I have seen innacurate readings before, mostly in the first run of the season when the GPS had been turned off for a few months.

Looks like labbed 30's would get the motor rpm closer to peak hp. It doesn't look like you can pull 32's. IMO the increase in hp was not big enough to increase pitch. Go with the 30's and spin the motors higher which will be easier on the drives.

I remember a bunch of upset people a few years ago at the shootout. They were complaining that their onboard gps units were reading much higher than the radar guns that the officials were using. I believe they have corrected a lot of it over the past few years. They moved the location and angle of the radar guns to get a more accurate reading.
Eddie

The radar gun only detects the component of the velocity in the direction of the gun. The error starts getting big fairly quickly as the gun gets off the track of the boat or you shoot from the boat at an object not straight ahead of the boat.

I calculated the error based on the distance from the boat course once after I got a miserable speed in a radar run but I can't find the spreadsheet. If I find it I will post.

Thanks for the post on this subject. I called livorsi, Todd, says; really accurate. I talked with a S.C. State Tropper, that instructs radar. He explained how the radar would usually show lower speeds but, not higher. If in question they shot you from the front radar and than the rear. There are incorrect readings you can get, but they train as you may well know for proper operation. They have GPS in their cars that reads exactly what the vehicles speed is on the radar gun. My wifes Gps in her SUV reads about 3 mph slower than the speedometer. Maybe the Radar from the State Tropper is for real and the GPS is reading 92, I guess I'll just wait until the props are lab finished and see if we get the 5400 rpm and 96 mph, then the Spoiled Brat, can take the boat and have it shot again by radar. First time I ask him about GPS recall, he was LOST. Thanks Mark