Speed difference-RM depth/speed vs. GPS

[brackish]

Out yesterday for a great sail (4.5-5.5 knots downwind with nothing but a reefed main, caveat to follow) and had both the Raymarine depth/speed instrument on and my Standard Horizon fixed GPS.  The Raymarine gets it signal from a hull mounted paddle wheel, the GPS, well from that magic satellite in the sky.  I noticed that there was one knot + or - difference between the two, the paddle wheel unit the slower of the two.  My lake is in a lock system so current is not a real factor, but I checked both up lake and down lake to make sure. 

Anyone else have both and notice this difference?

So my reefed main was getting me between 4.5 and 5.5 knots as measured by the GPS; 3.5-4.5 knots as measured by the Raymarine Depth/speed.

[atrometer]

Paddle wheels are notoriously inaccurate - slime, friction, seaweed, placement, water temp, etc. - all make a difference

[NateD]

By "one knot + or - difference" do you mean that sometimes the GPS was higher than the paddle wheel and sometimes lower? Your other comments suggest the paddle wheel was always low.

GPS speed calculations usually have a dampening or smoothing function that keeps it from jumping around too much, which might cause it to be above or below the paddle wheel (assuming Raymarine hasn't written a similar smoothing function into their display).

[brackish]

By "one knot + or - difference" do you mean that sometimes the GPS was higher than the paddle wheel and sometimes lower? Your other comments suggest the paddle wheel was always low.

GPS speed calculations usually have a dampening or smoothing function that keeps it from jumping around too much, which might cause it to be above or below the paddle wheel (assuming Raymarine hasn't written a similar smoothing function into their display).

Yes the Paddle wheel was consistently low even during gusts the difference remained approximately the same.  I've also noticed that the paddle has one reading when on a close reach starboard, tack over to port same sail settings that is close as practical and it can vary by a half knot or better.  The GPS reads approximately the same, albeit different in both cases than the paddle wheel.

[nies]

I have the same experience and believe its the location of the paddle wheel on the stern making 1 to 1 1/2 difference...........nies

[skip1930]

Sure, Brackish is correct. There is a difference in two ways. 'Speed' and 'Time to Display. Both are adjustable.

The Raymarine ST-40 paddle wheel speed sensor was registering speed changes slower then the Raymarine 435i Chartplotter's GPS read out..

BUT! On the ST-40's menu there is an adjustment to kick indicated the speed up or down.  And additionally display changes as 'change faster' or 'change slower'.

So the best of both worlds.
What I did was to fire up both instruments and I picked the Chartplotter's GPS as being the standard and correct speed.
By poking + or - on the ST-40 I could drive the speed paddle indicated speed up or down to match the GPS.

I left the time to display longer for a smoother display that was not changing so fast. I'm not driving a speed boat where I'm worried about running up on the shore in a heart beat and need a fast display.

After all we are only doing 1.2 to 6.3 knots and in my clear sailing water, I don't worry about hitting bottom until I can see the bottom.

I guess we'd call that sync'ed up.

skip.

[Bob23]

I always take the higher number. Same thinking when on the scale: If you get more than one reading, take the lower number!
I sail in tidal currents so the GPS number is the one I'm interested in. My paddle wheel lasts about 2 weeks into the season. And I'm removing it this spring anyway.
Bob23