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Old 14 June 2016, 08:36   #1
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Correct speed

Bit of a noob question, im sure but........

When looking a the speed of a rib, which is (generally) more accurate the tach or the speed reading on the GPS.

Mine seems to show quite a difference.

I will run it against another boat, but at the moment I am just a little curious.
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Old 14 June 2016, 08:47   #2
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GPS will be speed over ground and likely quite accurate.

Tach will be speed through the water.

If you're running in a 5 knot tide it'll be 5 knots out from the speed over ground or gps.

Plus tachometers on boats in my experience are often a bit vague anyway.
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Old 14 June 2016, 09:15   #3
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Eh, how does a tachometer give you speed other than the engine speed
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Old 14 June 2016, 09:28   #4
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Generally know as a speedometer, taken from water speed.

But for the pedant, tachometer : noun : any device for measuring speed, esp the rate of revolution of a shaft.

That shaft could be linked to the water and the info used to measure speed. So not technically wrong. ;-)
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Old 14 June 2016, 12:45   #5
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Thank god I'm only a layman, a speedometer measures speed of distance a tachometer often referred to as a rev counter measures the revs of something usually and engine
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Old 14 June 2016, 12:47   #6
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Somebody's got it all wrong 🙄 gps gives speed over ground- SOG. Tach gives engine speed in RPM, the 2 are vaguely proportional but only in a perfect world. You can't compare the RPM of one boat to another unless they are identical in every way. GPS is usually very accurate at giving SOG. A correctly installed & setup Tachometer is also pretty accurate, especially if it's a NMEA2000 unit running off the engine ecu.


Sh1t happens
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Old 14 June 2016, 13:51   #7
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I would use your GPS reading which will be accurate for speed over ground. If you want to know your speed through the water you would have to compensate for the tide, which is somewhere between very little and 11 knots (Swinge).

For your particular boat in similar conditions you could equate speed to RPM but it's probably not worth the trouble.
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Old 14 June 2016, 23:50   #8
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Speedometers on most boats use water pressure from forward motion through water to pressurize air, which is then converted to a speed reading. When working correctly, they are surprisingly (well, to me, anyway) accurate. Problem is, they aren't always working correctly, and give no other indication that they aren't (aside from a reading of, say, 4 mph when on plane with the throttle whacked forward.)

GPS uses satellite signals to read the displacement in a given unit of time to generate a speed reading; will be very accurate assuming you are getting a GPS signal (which should be all the time on the water.)

GPS will only read SOG; speedometer will read speed through water, which will be affected by running up or down currents.

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