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. :eek:

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! ;)

Your GPS is measuring speed over land. When running down river, the speed of the current is being added to your speed over water.

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.

The wind probably has a negative effect over a certain speed (80-90??). I think chop would always help with a conventional v-hull.

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.

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.

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.

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.

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?

OHHHHH AUDACITY!!!!!!!!!!!!!!!!!!!!!
Where are ya on this one bro?:D :p

More positive rake helps bow lift. Not sure if cup will have anything to do with it. Generally, the bigger the blades, the more bow lift. Once in a while, Ill run into some of the old ss michigan wheels they used on the old scarab 3's and they have serious bow lift.

My Merc mechanic has a prop that he referred to as a "raker" that I was going to try. He said they were used a lot on outboards, and I think the diameter had been cut down some. Doubt if I can spin it; I think he said it has a 24" pitch - too much for my mill with 1.36 gears.

Anyone have a set of 1.5's for sale? :D

On the river in downtown Chicago I have seen strange speeds and course info. The signal get bounced around when surrounded by sky scrapers.

GPS CAN BE INACCURATE.MY BOAT DOES 90 MPH BUT MY GPS SAYS 80 SOMETHING:D

As long as you have three birds locked in it will be accurate. The problem is mainly with the max speed, which is accurate but it may not be in the direction you think. If your GPS says 80, then my friend you are doing 80.

What?, did you have you're boat on the trailer, being towed by a GM truck?:D And besides any more you're GPS never gets above 48 mph:p

Hmmmmm..
My understanding of how a GPS works in that it locks on a location at set intervals (once a second for arguments sake). The mathmatical calculation of the disctance between the captured locations and elapsed time is how fast your going (MPH).
Because of this current in a river will effect your speed. If your going with the current your traveling at your boat speed+the speed of the current resulting in a higher total speed. The reverse would aply if going against the current.
I also thought you needed a minimum of three satalites to get a position. My Garmin GPSIII+ usualy hits 5 or more. If you were to lose contact and only have two satilites to track from I dont think the unit would have enough data to calculate.
I have seem these units give highly suspect reading (Max Speed).
The error being at 100mph+ I dont believe that its being effected by the boat being towed :eek:

I had a hand held GPS in my suburban that said I was going a max of 125 mph while driving throught the keys. I don't recall going over 60 mph!

Hey brad,
Slow that thing down and pay attention.
Maniacs in their hoped up cars...I tell ya

Yeah, if it came back with a reading of 125 MPH, I would know something was up! Since it was only about 6 MPH over the speeds that I had seen before, I thought there could be some possibility that it was legit.

I suppose the current could have had some effect. Water was up, and current was pretty swift. But I would also expect the 25+ MPH headwind to offset that some, too.

Can GPS be fooled?
no!

I run faster in rough water.

The rough water GPS numbers are not based on the boat running faster in the chop. When a boat is propped for a race it's not always propped with the biggest wheel. Take a look in the back of the WHM truck sometime, quite a collection of props. You need to determine how fast you need to go to beat the competition. Then you pick that wheel. Instead of a 35 you might run a 28. That way you can run the speed you determined to win the race and get the accelleration of the smaller wheel. So the numbers I threw up were based on the calculated speed with the wheel we ran, very low 90. The GPS max said 104, impossible considering the boat was equipped with HP 500s that are rev limited.

Budman,
Sorry we got a little off track from your initial question.
I would think if you were getting a good amount if air under the hul as a result of the chop and wind you could grab 2 or 3 mph.
If you were running with the current 2 or 3 is could be possible.
6mph total is alot but if the conditions were perfect Im thinking it could work......you dont want to bet the pink slip on it though :p

I reply to Tumici; I always thought it was undiputable myself until I saw it with my own eye's

Just for the record;
Garmin GPSIII+ should have read low 90's / max speed was low 100's
Magellan? should have read low 100's / holding the unit I saw 114.9.

Ok
I was scanning a few web sites because now I have to know how accuate these things really are;
Here are the two warnings that one of the sites has up.

Look Out!

Some GPS receivers have speed limits.
GPS receiver manufacturers sometimes program speed limits into the devices, so that if the device is moving above a certain speed, it will not work properly. A receiver meant to be used in a car may not work on an airplane, which travels much faster than a automobile. This is more often the case in car, airplane or boat-mounted receivers than in the hand-held models.

GPS receivers have temperature limits.
Like most electronic devices, especially those with LCD screens, GPS receivers may not function properly above or below certain temperatures. If you plan to use your receiver in any extreme temperature situations, such as mountain climbing or hiking in the desert, you should check to make sure the receiver model can function in those conditions.


Ill be back with more info in a few hours :D

GPS
 

Budman, I have had the same experience with GPS. I also have a Garmin in my dash and took my Garmin handheld with me, while testing props on Saturday. The top speed on the mounted unit was 84.7 and on the handheld 83.6 while running in chop and 10 knot winds. This was for almost a mile , for accuracy. GPS probably can not be fooled , but it is not fool proof. By the way, the speedometer read 92, so I turned off both GPS's and was very happy.

1) CHOP - most, if not ALL, nonstepped hulls will run faster in a chop. Some in a light chop, some in medium chop. It is a factor of breaking up the adhesion layer of the water on the hull. In glass water on a vee hull, the water will stick to the hull and will even hang on out to the edges a bit, keeping the boat from possibly lifting itself onto another strake and riding higher. The chop does the same thing that steps so - it lessens the ability of the water to "cling" to the hull.

2) WIND - there may be some instances where additional wind could lift the hull for more speed, but on a conventional veehull running above 60 mph, I would tend to think that running into the wind would slow you down - strictly my opinion..

3) GPS and satellite locks - a 2D gps lock requires at LEAST 3 satellites to triangulate position. a 3D gps lock (Lat/Lon/Altitude) requires at LEAST 4 satellites to triangulate position. Regardless of what you may think, a 3-sat lock is not as accurate as a 4 or 5 or 6 or 8 satellite lock. The more sats, the more accurate the triangulation becomes. A 4-sat lock is much more accurate than a 3-sat lock while an 8-sat lock is only a teeny bit more accurate than a 6-sat lock (point of diminishing returns somewhere around 6 sats)... Multiplexing receivers (one true receiver that switches channels between all of the locked sats for computation) are in theory LESS accurate in theory than a receiver with 6 true receiving sections all constantly monitoring timing info from the sats) - in actual use, some multiplexing receivers outperform their cousins with a handful of active receivers...

4) topspeed function - I would expect different mfrs to have different math built into this particular function. Some may be optimistic. I don't know...

5) CURRENT - running with the current will always net a higher GPS speed.

When testing, I prefer to pick a day with negligible winds (less than 15mph). Run two directions down the same stretch of waterway. Have somebody watch the GPS and verify a stable (nonvarying) reading for more than 10 seconds. Average the two results. I usually see a 2.5 - 3.0 mph difference from one direction to the other (on a lake with current that you'd never detect unless you were watching for it).