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  1. #21
    Registered timucin's Avatar
    Join Date
    May 2002
    Can GPS be fooled?

  2. #22
    Registered HyperBaja's Avatar
    My Boats:
    1990 Baja 190 DSR
    Join Date
    Sep 2002
    Lewis Center, Ohio
    Originally posted by Tantrum
    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.
    I run faster in rough water.

  3. #23
    Registered mr_velocity's Avatar
    Join Date
    Apr 2002
    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.
    Last edited by mr_velocity; 11-11-2002 at 04:21 PM.

  4. #24
    Registered Tantrum's Avatar
    My Boats:
    Stray Cat Statement Marine
    Join Date
    Oct 2000
    USR, NJ
    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 dont want to bet the pink slip on it though

    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.

  5. #25
    Registered Tantrum's Avatar
    My Boats:
    Stray Cat Statement Marine
    Join Date
    Oct 2000
    USR, NJ
    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

  6. #26
    My Boats:
    Velocity 41'
    Join Date
    Oct 2001
    Lake Norman, N.C.


    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.

  7. #27
    Platinum Member Platinum Member mcollinstn's Avatar
    My Boats:
    1991 F311SR1
    Join Date
    Sep 2001
    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).
    I see London, I see France...

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