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Can GPS be fooled?

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Old 11-11-2002, 09:06 AM
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Question Can GPS be fooled?

cpqtim and I were out on my boat this past Saturday for some prop testing. Boat is a 1988 Baja Force 235 with a warmed over 454/Bravo (1.36 ratio), 21 Mirage. We had originally planned to switch out Tim's 1.5 ratio with my 1.36, and add a 23 Mirage Plus to try to see if the 1.36 was costing me some MPH, but we ran short on time, so we had to stick with some prop testing. Good excuse to run the boat and have a beverage, since temps were close to 70 that day. Unfortunately, steady 20-30 MPH winds out of the south were stacking the water up pretty good - steady 2-3 foot "confused" chop, with some 4+ thrown in to remind us who's boss.

I took off for a baseline run with my current setup (as described above), with Tim's GPS along for the ride. In the past, on a day with similar temps (around 60*) with three people on the boat and a half tank of gas, I had managed about 59 MPH @ 4800 RPM with a different GPS. That was last fall, and nothing significant has been changed on the boat since then. When I ran Saturday, it was too rough for me to watch the GPS, but it had a field that captured the max MPH. I was getting beat up pretty good by the steady chop, but I found one stretch where it calmed down enough where I felt comfortable wringing it out. I was running downriver, into the wind, and I'm pretty sure I saw 4800- 5000 RPM on the tach. Didn't look at the speedo - that's what the GPS was for! However, when I slowed down and looked at the GPS, it registered 65.9 MPH for the max MPH!

My question: Could I have actually attained that speed, with the factors of the chop decreasing the drag on the hull? I was also running into a steady 20 MPH+ head wind. Could the wind had the effect of lifting the bow up out of the water enough to gain 6 MPH? The boat has always had a tendency to plow the bow somewhat, and I had suspected that I might need a prop with more cup to carry the bow. I never got better than 56 MPH with the other prop (23 mirage plus), but I think it was too much prop for my setup - I never got more than 4300 RPM it. I'm going to try to test again on a day that is a little calmer.

What do you guys think? Is it possible that the jolting around with the rough water caused the GPS to register a false reading? If nothing else, I guess I can say that it hit 65 on GPS!
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Old 11-11-2002, 09:28 AM
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Your GPS is measuring speed over land. When running down river, the speed of the current is being added to your speed over water.
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Old 11-11-2002, 09:35 AM
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And yea about the wind helping lift the boat. Some of my best speeds have been running into the wind and chop. But Im a little pessimistic about GPS speed numbers for short runs. Alot depends on how many satelites your hooked into and how long you run.
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Old 11-11-2002, 09:39 AM
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The wind probably has a negative effect over a certain speed (80-90??). I think chop would always help with a conventional v-hull.
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Old 11-11-2002, 09:40 AM
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Yes. GPS can give inaccurate readings due to slight variations in satellite orbits. The coast guard also released a warning to GPS users that strong television transmission and radio signals can also cause inaccurate GPS information. I've even had reception issues when the radar on my boat was in operation.
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Old 11-11-2002, 11:02 AM
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Max GPS speed doesn't mean anything, especially in rough condidtions. The device is measuring the speed of a movement, so any rough conditions could cause this. In our first race we had the boat propped for 92 mph since it was extrememly rough. The gps said max speed 104 mph.
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Old 11-11-2002, 11:08 AM
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mr_velocity;
Why would rough water increase the GPS speed.
I believe what your saying and have witnessed GPS's giving incorrect speeds. It gave readings very similar to what you saying, a 90 mile and hour boat w/ max speed reading 100.
Can anyone explain this?

River current was no where near 10mph.
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Old 11-11-2002, 11:16 AM
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Sure the boat goes up, goes down, goes sideways and basically beat in all directions in the rough water. The GPS measures all these movements.
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Old 11-11-2002, 11:30 AM
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That's kind of what I figured too. Too bad I didn't have a co-pilot at the time to watch the real-time reading. I would like to know if the extra bow lift had anything to do with it. I have always suspected that I need more bow lift, and this would help to confirm it.

Would more cup in the prop help me to carry the bow?
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Old 11-11-2002, 11:59 AM
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OHHHHH AUDACITY!!!!!!!!!!!!!!!!!!!!!
Where are ya on this one bro?
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