Approaching speed?

[gcapehart]

In one of Coonts "Flight of the Intruder" series, he describes a near miair collision between A-6's and F-4's. The approach speed was over 1200mph. When I got that speed we were going up the Penobscot. The tide may have been running out. It is reported with our tides that drop 10-15 feet here that she'll flow out at 4-5 knots. That speed though was on my GPS. So maybe approach speed doesn't factor since GPS measures the distance you cover? Gary

[John Jones]

GPS is speed over ground.  No effect from the current.  A paddlewheel or pitot sensor will vary with the current.


An off topic tip from my granddad on boats.

"Never approach the dock any faster than you are willing to hit it."

[JimCt]

JJ, please repeat that again...???   You're saying that current has no effect on indicated GPS speed-over-ground???

If he's bucking a 5 knot current those five knots show up subtractively on the GPS speed-over-ground.  Those same five knots of current will be transparent to the paddlewheel since the paddlewheel only senses speed through the water.

[pete]

JJ I know what you mean,The GPS measures the distance traveled not how fast the water is going by your boat.

[John Jones]

Jim,

If the gps is showing 10 kt, that is true speed.  It does not matter how fast the current is or how much the wind is blowing, you are making 10 kt.

If you are heading into a 2 kt current, the paddle wheel will show 12kts IF THE GPS IS STILL SHOWING 10.  You are still making a true 10 kt.  The current does affect the paddlewheel reading.  You can anchor up in a channel with a current and the paddlewheel will indicate the speed of the current even though the boat is not moving but the gps will read zero which is correct.  Heading downstream with the current will make the paddlewheel read less than the true 10 kt indicated by the gps.  Drift free with the current with no power or sail and the paddlewheel will indicate ZERO speed but the gps will indicate your true speed.

Like Pete said, the paddlewheel is only measuring how fast the water is going past your boat whether the boat is truly moving or not.

I'm probably not being very clear.  It's late.  Ask me again tomorrow ;)

[RickK]

The gps gets a signal from the satellites every second and calculates your position - and then it calcs how long it took to get from the last spot to where you are.  So wind, water current, etc doesn't affect GPS - like they're saying how long it took to get from point A to B is your speed.  JJ gave a good example of being anchored in a current at 0mph vs what the paddlewheel is doing/showing.