Ground Speed

[Fillet]

Was out the other day playing around with the new GPS and noticed 2 things. The top speed on the gps(compared to my dash speed and other graph's spin wheel) was lower. At slower speeds, the gps ground speed read higher. Which do I believe? I was trolling cranks and the wheel read 2 while the gps ground speed would read about 3.5. Now im not an expert with speeds but by the pull in the rods it felt like 2mph to me and not 3.5. Which do I believe? Is ground speed different than water speed?

[john.wells]

Although GPS units have come a long way in accuracy, all readings for civilian units are approximate. Military, aviation, surveyors, and the like have access to better capabilities and equipment for pinpoint precision.

[jwhjr]

I've noticed that they don't seem to be as accurate at slower speeds. In my truck it matches pretty much exactly what's on the speedometer.

[PerchJerker]

First question - which gps? The antenna and receiver will make a difference. A related question, depending on your gps, is how often is it updating its position (in power-saving modes it won't update as often or be as accurate).

Second question - was it consistent, or a blip? Satellite position and other things that cause interference can cause your position reading and therefore your speed to jump around. Is it something that only happened once, or was it consistent? Did it clear up after a few minutes, or if you changed your direction of travel, or was it for a long period of time/

Finally -- your dash speed (water pressure speedometer??) and the speed wheel on your other graph are probably not very accurate. I would believe the gps.

[Fillet]

Its a LCX-27. It was consistently higher that I noticed. Im about 95% sure that as my wheel read 2.0 and the gps read 3.5 that i was not going 3.5. I can see that the wheel wouldnt be 100% accurate but the gps ground speed just seemed way to fast at the slow speeds.

[PerchJerker]

The LCX has as good of a receiver and antenna as you can get, and I don't think there's any power-saving adjustments or anything like that. The only other suggestion I have to monitor on your gps is the EPE, estimated position error. It should be around 20-30 feet most of the time, the higher it gets the less accurate your position and speed will be. You can display it on the screen right by your ground speed if you want to keep an eye on it.

Another suggestion is to run slow beside a different boat and compare to their speed.

What boat / motor / prop se-up were you running when the gps said 3.5 mph? Is that as slow as you could get it to go?

[Deitz Dittrich]

Keep in mind, in dash speedos are not very accurate at all, it goes on water pressure going into the eng. Which changes quite a bit with the change of water temps.. I would believe a shady GPS before I would believe a good spedo. Just my MHO.

[Valv]

Usually a GPS is way more accurate than boat spedometer, especially at slow speeds.

Have you tried to take along a handheld unit, like a Ho2, etc. and check the 27 accuracy ?

Water speed on Lowrance is measured with the addon paddle wheel, if you don't have it installed on back of boat sonar won't show it, while ground speed is measured with the GPS.

[Fishinguy40]

Maybe you can help me with a problem. I took my boat out yesterday to make sure it's ready for Saturday and noticed that the dash speedometer reads 55 mph when dead in the water. When I did accelerate the gauge would peg out past the max reading of 65 mph. So it seems the gauge is getting a signal.

Do you know what type of signal it works on (such as 0-10 volts or milivolts) and where the the signal originate from. I have a 2006 Monark 185.

Thanks for any help.

[Valv]

Boat speedometers don't get signal from motor, they are water activated.

Your motor has a small input which runs water to the gauge through a small black hose. Water pressure changes with speed and runs the speedometer. It still need power for lighting, but not for the signal.

Check your motor, there's a small black hose coming from the front of it, routing through the center section and all the way to the gauge.

I never liked them, whether is water deposits, winter freeze or else they all have a problem here and there.

You can also test it by blowing air into the hose.

[Fishinguy40]

I pulled the speedometer out of the dash last night and found the hose like you said. The wires are for lighting only. So the speedometer is actually a pressure gauge and a cheap one at that, at least in my boat. It's a sealed unit so I can't even tell if the bourdon tube is damaged from freezeing. I can just imagine what the dealer wants for a new one. I guess I'll just have to depend on my sonar/gps.

Thanks for the info.

[BobT]

I don't know about GPS accuracy other than the specifications of mine. Mine is a Garmin Rino120 handheld. It is specified to hold a .05 meter/sec velocity. That converts to .11mph accuracy. Seems it should be relatively accurate.

Bob

[hhguide]

I know that we have an eagle and then a lowrance and when he comes down to trolling speed we use the eagle becuase it can go down to speeds are .2 or .1 while the lowrance cant quite make it that low but they I think are both very accurate and always trust my gps!

[Ralph Wiggum]

 Originally Posted By: Fishinguy40

I pulled the speedometer out of the dash last night and found the hose like you said. The wires are for lighting only. So the speedometer is actually a pressure gauge and a cheap one at that, at least in my boat. It's a sealed unit so I can't even tell if the bourdon tube is damaged from freezeing. I can just imagine what the dealer wants for a new one. I guess I'll just have to depend on my sonar/gps.

Thanks for the info.

You should be able to get a speedo from any boating accesory store (Cabela's, Overtons, etc) for $40-$50.

[Fishinguy40]

Yeah, more money for the wife to complain about.

[Valv]

Which boat do you have, I have dozens of new ones for almost nothing.

[Fishinguy40]

The boat is a 2006 Monark King 185. The speedometer face is black scale with a white background and brass trim.

Even though it is a sealed unit, I was able to tweak it by removing the light bulb from the back (I can't help myself. I love to tweak plus I am a cheap ba#!@^d...LOL).

I was able to zero it. When I get the boat out this weekend I will see how linear it is. If not, I may take you up on the offer. I appreciate it!