Speed Accuracy

[Chris]

I am having trouble believing the speed my two separate instruments are telling me.
1. My x-75 is telling me 2.0 MPH
2. My GPSII is telling me 4.0 MPH
I know I should trust the GPS (not affected by wind, waves current etc..) By Im running my Honda 40hp at the lowest speed I can (throttle wise). I havnt adjusted anything with the motor since it was new, does it seem right that it would idle at 4 or more MPH???
Thanks for any advise.

[SUPERTROLLER]

Chris, it has been my general experience that the GPS usually gives you a slower reading. I don't have one in my boat and only go by the paddle wheel. When I've been in other boats though, the GPS has always read about .5 mph less than their paddle wheel is telling them on their graph speedometer at trolling speeds. Not sure how far different they would read on plane.

(P.S. I answered only to get your question back to the front page so someone more knowledgeable might see it and help with better info)

[Hans]

At slow speeds GPS is not particularly accurate in judging speed, since it really doesn't measure speed, but mathematically derives it by measuring distance moved between the last two points sampled (in time). It has to assume that you moved between those two points over a straight path, while your path between those two paths may in fact have been an arc. If your GPS sample rate is low, your speed is slow, and your path not ruler-straight, a little bit of timing jitter can significantly skew the speed calculated and displayed.

Hans

[K Crowley]

My experience is just the opposite. In relatively calm water, at idle, with a 4-stroke 60HP, my X85 paddle wheel runs between 1.0 and 1.4 MPH. Simultaneously, my GPS 12 always reads 1.7 to 2.0. I tend to believe the GPS rather than the paddle wheel because I frequently troll along side charters with more accurate trolling indicators and my GPS readings jibe with theirs.

Incidentally, on plane the speeds agree almost exactly.

[MAGNA19]

the gps will give you a better speed reading than the finder.i have a eagle ultra 2 and a lowrancew global nav 310,and the gps will give me a sped down to .50,t5he finder wont register below 1.0.i troll down to 1.0 and use the gps for speed.i think it is more accurate than the finder.

neighbor has a furnamo color video finder with speed/temp,and a lowrance the same as mine,and he reads the same on both ,on occasion the finder shows .50 lower or higher,depending on if he is going with or against the waves.

[Eyez]

I agree with Hans on this one. My general rule of thumb is trust your paddle wheel at trolling speeds, and the gps at any speed faster. I've seen a gps read 4 mph while running hot n tots when my paddle wheel read 1.8. I do know that hot n tots don't run at 4 mph worth a crap. Not that the hot n tot test really proves anything, but it was interesting anyway :)


Eyez

[Airwave(OH)]

I have found that the GPS reads faster than the wheel. I have Lowrance 350A with S/T & GPS. I also have a FishHawk for Speed & Temp. The interesting thing here is the GPS and the Fishhawk(which has a paddle wheel) are fairly close. The 350A is usaully about .8 behind. You also have to take into account which direction you are moving as moving with the current will give a different reading then going with it on a paddle. the inportant thing is use (one) and adjust your speed to that one. I know by the sound of the motor and how the rods look after doing it for so long.

[RANGER]

Agreed - 100%. The sampling rate and timing jitter are a consequence of s-l-o-w motion. Satellite orientation is a factor as well. Your paddle wheel is the more reliable instrument below walking speed but pales when your speed is over that. GPS is a wonderful technology but it ain't perfect.


RANGER


"KEEP YOUR LINES WET, YOUR POWDER DRY and THE BEER COLD!"

[Stormsearch]

I think Magna19 made a good point. Water current has a dramatic effect on speed pick up devices while not effecting GPS units. It has been my experience to use the speed pick up devices to obtain your desired trolling speeds and use GPS as a reference. Referencing meaning that my GPS should read the same with or against the waves. Good luck.

[mnjimcarp]

cabelas sells an electronic speedometer kit that is made by Teleflex Marine. it has a 2"face dial that fits nicely on many consoles. this speedometer is super accurate up to five miles per hour. the device will indicate your speed through water. the actual speed over the bottom will be affected by tide or current.
the most effective trolling speed to catch fish may vary depending on the baits used and from lake to lake as well as day to day. it probably doesn't matter which device one chooses to use as your speed indicator as long as you use the same device to keep all things relative for the particular bite that day.
good luck.

[jim]

It sounds like you may the the pinning feature set to on. if you turn it off you will get much more accurate speed readings.
Just a thought, good luck.

[I-Man]

LAST EDITED ON May-25-01 AT 12:35PM (CST)[p]This is my 3rd Trollin' season with a Honda 50/4 on a Tinboat 16'er.
3.7 GPS Trollin'
2.8 GPS Back-Trollin'
I use my Kota Auto-Pilot for the slower speeds. The GPS reads good for me down to 1 MPH. With the same boat & the Rude 50/2 I had before, I could get down to around 1.5 MPH.

PS I might add .. I bought a GPS unit for -3- reasons:
1) I have always used Airwave's method & friends asked me what speed I was goin'
2) Fog
3rd & most important .. to shutoff the noisy recorder & troll GPS marked humps without setting out jugs.

[Chairman]

What is it you are looking for? Are you trolling? If so, speed is only relative and needed for repeatability. It doesn't matter if it is 1.5 or 2.5 as long as you can get back to it. Luhr Jensen makes a unit that works very consistant and uses a bell sinker and string. I always look at both units and read the boards. get the same angle on the boards and check both speed units. Then go back to the sppe you caught the fish on. It is difficult for a paddlewheel to be accurate at real slow speeds.

[Chris S.]

I have a Garmin E-map and I have it set so it takes a reading every one second. I also did this to my eagle when I had it and that was very accurate also.

[John Boyle]

Speed indicators for trolling are all relative to the elements that affect them. As pointed out, the GPS does not take into account you may be trolling in a curve.
It sure can't tell if your in current, and that's important to lure action.
The paddlewheel speed indicator off my on my fish hawk 840 definately gives more relavant information as far as my lure goes.
I also have a strike vision camera that lets me see what's happening at the lure regardless of waves and currents, or if I troll straight or in curves.
I don't use it so much to see fish chasing lures, but when conditions get windy, things get a little tricky and it shows me whether or not the baits are at the right speed, and then I know whether the fish hawk paddlewheel , or the GPS are giving me the goods.
Again, I'd opt for the paddlewheel.