Unusual GPS speed display today...

matt_unique

Well it's Spring launch time and this is when we will be chatting about unusual readings!

I was running Napoleon up to speed today and I happened to notice whenever I went above about 35 knots my GPS speed reading would blink to 0 knots, then it would show the actual speed (i.e. 40 knots), then blink to 0 knots, etc. It did this over and over again whenever I was about 35 knots.

Below 35 knots the speed reading was steady, no blinking or anything out of the ordinary. I have a 1 year old Furuno 1834 C/NT with the BBWGPS GPS receiver. My kneed jerk reaction is a loose wire or something but it was no less "bumpy" at 30 knots as it was at 35 knots. I fired off an email to Furuno technical support but I wanted to bounce it off you guys here.

I was following a route based on GPS waypoints I had previously entered. I did not appear to loose my position or chart display it was just the speed reading.

By the way, I topped out at 41.5 knots on the GPS today. Not bad considering I was fully fueled (900 lbs of gas).

Thanks

starcrafttom · Posted: Fri Apr 10, 2009 9:04 pm Post subject:

Sounds like a loose wire or a sensor going out. Its the same drill that I hate- chasing down a loose ground or wire.

ppriest · Guest

Do you have a paddlewheel-style through the water speedometer and how well does it synch with your (calculated) GPS-based speedo?

Paul Priest
Sequim
J.C.Lately

matt_unique · Posted: Fri Apr 10, 2009 10:31 pm Post subject: Speed

I only have GPS speed. The speeds today matched the speeds that I recorded last season with the same RPM.

I mentioned I was thinking a loose wire but if that were the case it should show erratic readings at 30 knots as it does above 35. It does not make sense that it only happens above 35 knots. In fact, I bounced over some larger waves at the end of my trip today cruising at 22 knots and there were no erratic readings.

It will be interesting to determine the cause/solution....thanks for brainstorming.

journey on · Posted: Fri Apr 10, 2009 10:47 pm Post subject:

Let's see. A GPS doesn't measure speed directly, but computes position, differencing successive positions and divides by thyme. Remember dead reckoning? Average speed = change in position/thyme.

Anyway, I assume that for speeds over 35 knts, there is something in the computational algorithm that burps. Or the processor runs out of time to compute the speed in one display cycle. If you can, try it in your car.

Boris

Pat Anderson · Posted: Sat Apr 11, 2009 9:36 am Post subject:

Boris - have you ever tried DR with parsley, sage or rosemary instead of thyme? Laughing

matt_unique · Posted: Sat Apr 11, 2009 9:52 am Post subject: GPS

Ha ha - well the only way I could try it on the road would be under tow. I need a volunteer to ride in Napoleon and take readings while we cruise down the highway Wink

Mine is a fixed unit with external antenna. Taking a look at the owners manual I see there is something called a "speed/course smoothing" adjustment for the GPS...I'm throwing pasta at the ceiling until I hear from Furuno at this point...maybe it's an adjustment issue. I can also run a GPS system test.

I will figure it out eventually and the ideas are helpful!

JamesTXSD · Posted: Sat Apr 11, 2009 10:46 am Post subject:

Pat Anderson · Posted: Sat Apr 11, 2009 10:50 am Post subject:

I feel another thread hijacking coming on!

Nope, "Scarborough Fair" by Simon and Garfunkle.

TyBoo · Posted: Sat Apr 11, 2009 12:41 pm Post subject:

Jim might know his FD dolphins, but he don't know his Simon and Garfunkle.

Once in a while my Garmin will go screwy on the speed and I turn it off and back on to get it to work. Must have some Windows software in it. The Furuno guys will help you out, I'm sure. They seem to be the best for support.

matt_unique · Posted: Sat Apr 11, 2009 1:19 pm Post subject: GPS

journey on · Posted: Sat Apr 11, 2009 5:39 pm Post subject:

Come on, Pat, you're pulling my leg. Everyone knows that:

Max speed = Avg speed* parsley

Change in elevation = height / rosemary

Depth = fathoms ^ sage

Thyme is the correct unit to use.

Boris

JamesTXSD · Posted: Sat Apr 11, 2009 8:08 pm Post subject:

Wefings · Posted: Sat Apr 11, 2009 9:14 pm Post subject:

important info
Marc