Speed problem

boristhebold · 18 August 2009, 07:43

Need advice, my yamaha speed guage shows a few knots less than showing on my Garmin GPS, anyone have an idea what the difference could be due to ?

doggypaddle · 18 August 2009, 07:49

If it the error is really only a few knots ie ~10% of the reading then it could be down to normal gauge inaccuracies?
car speedometers are terible for this. My vectra speedo said 140mph when I blasted down the runway but the peak speed on the gps was 125mph

it wasnt too far out at normal speeds but still around 5mph fast at 70mph.

Poly · 18 August 2009, 07:54

your gauges are also measuring different thing. the yam gauge is measuring speed through the water (probably as estimated from the vacuum generated by dragging a tube through the water) and the gps is measuring speed over ground. So even if both devices were 100% accurate, tide/current will give rise to a variation.

boristhebold · 18 August 2009, 08:06

Thanks that makes sense, probably why the Yamaha speedo was originally showing MPH and the GPS Knots, I expect the previous owner relied on the GPS for speed and maybe the speedo just as an indication in miles an hour, It certainly feels like the GPS speed is more accurate than the yamaha guage.

JSP · 18 August 2009, 13:27

Your clock is reading speed through the water, GPS is speed over land.

JSP · 18 August 2009, 13:28

Quote:

Originally Posted by Polwart

your gauges are also measuring different thing. the yam gauge is measuring speed through the water (probably as estimated from the vacuum generated by dragging a tube through the water) and the gps is measuring speed over ground. So even if both devices were 100% accurate, tide/current will give rise to a variation.

I should read the whole thread before replying

doggypaddle · 18 August 2009, 13:48

Quote:

Originally Posted by JSP

I should read the whole thread before replying

Doughnut

JSP · 18 August 2009, 14:02

Quote:

Originally Posted by doggypaddle

Doughnut

hhmmm doughnuts...nice

boristhebold · 21 August 2009, 07:54

Thanks, if my speedo is reading speed through water and gps is reading speed over ground, which one would I use for refference to ensure I keep under the poole harbour speed limits ? excuse my ignornace as im new to powerboating .. comments please

chewy · 21 August 2009, 08:17

Quote:

Originally Posted by boristhebold

Thanks, if my speedo is reading speed through water and gps is reading speed over ground, which one would I use for refference to ensure I keep under the poole harbour speed limits ? excuse my ignornace as im new to powerboating .. comments please

Your GPS. The pitot tube will give a slight reading if tied up on a mooring with the tide running under the boat.

jyasaki · 21 August 2009, 15:25

Quote:

Originally Posted by Polwart

the yam gauge is measuring speed through the water (probably as estimated from the vacuum generated by dragging a tube through the water)

Actually, I think it's pressure generated by having the open end of the tube facing forwards. The opening is usually on the leading edge of the lower unit; you can find it by running a fingernail up the front edge. May be somewhat clogged as well, which will add to the discrepancy between analog and electronic meters. [Note: If the tube falls off the guage, you get water spraying around inside the console. Make sure it's firmly affixed if you like your console dry.]

There are also aftermarket pitot speed guages; they usually have a triangular plastic pickup mounted on the transom.

jky