Why SpeedOnGround and Paddle Wheel speeds so different on Lowrance LMS320

[cfauvel]

Went our today and noticed that when on GPS window the SOG was signficantly different (slower) as compared to the padle wheel speed I have as the speed increased.

Findings:

RPM Wheel SOG
3200 23 20
3500 27.33 22
3800 33 25
4000 37 28
4800 45 n/a
5050 50 41

The boat with everyone aboard and with about 33 gals of fuel weighs an estimated 3500lb

running a 14.5 X20" 3blade raker on 225 Hp evinrude.

Which is more accurate?

 

[Mischief Managed]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

GPS is more accurate. I'm pretty sure you can adjust the height of the paddle wheel to calibrate the speed. That said, a current will cause a big difference between the two. They should only be the same in still water.

 

[Silvertip]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

If you are operating on a river or moving water where there is a current, you could anchor your boat and the GPS would show zero speed because you are not moving. Yet a paddle wheel speedo would show some reading that would be the equivalent of the current.

 

[Pez Vela]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

I'm pretty sure you can adjust the height of the paddle wheel to calibrate the speed.

That's a menu function. You may calibrate the paddlewheel speed in percentage increments, but the percentage errors reported are inconsistent.

 

[cfauvel]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Yeah I see the paddle wheel giving me the current's speed when I am anchored, but at speed would the current even register, when I ran those numbers I was going perpendicular to the incoming tide in a bay. I'l lhave to re-do the tests with a newer GPS found on an iphone. My Lowrance is nearly 19 years old, no WAAS.

 

[DuckHunterJon]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

GPS will always be more accurate, even with out WAAS. The paddle wheels are notoriously poor for accuracy. Reason is, you don't always have nice smooth (laminar) flow over the wheel. You have turbulent flow over it which can cause it to run slower or faster than you are really going. Try out the new GPS, but I bet it's close to what you found previously.

In the end, I only ever look at the speedo on two occasions. One for top speed runs (a couple times a season to make sure everything is running properly) and when setting up for trolling sensitive fish (think Walleye).

 

[dingbat]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Yeah I see the paddle wheel giving me the current's speed when I am anchored, but at speed would the current even register

Can?t speak for your location but the current in the chesapeake typically runs 1.5- 3.0 knots (1.1 -3.5 mph). Get around an inlet, mouth of a river, or anywhere the water is constricted you can easily see double those speeds. Several of the ocean inlets in the area see current in excess of 10 knots (11.5 mph) when the tide is running

 

[cfauvel]

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

Re: Why SpeedOnGround and Padle Wheel speeds so different on Lowrance LMS320

#DuckHunter Will do ....thanks