Speedometer reading off

[DaveM]

My paddlewheel speedometer on a 2108 Craz reads slow compared to a GPS handheld. At 10-12 mph its rather close, but at 25 mph its .5 mph slow, and above 30 mph its 1 mph slow. Is there a way to adjust it? I see in the menu some adjustment for speed, but is that just to synchronize the digital and analog dial speedometer?

[KnoxMojo]

GPS and paddlewheel will not be the same. One is measuring speed over land, the wheel is speed in the water. I think if your readings are that close, call it good. Are you on a lake or a river with current? If you want gps speed control, you can buy the Nautic-laugic unit. Mine was off, so I picked what was more important to me, wakeboard speed. So I dialed that in using the menu offsets, then I knew my surf speed was 2mph off. I am ok with that.

[gregski]

Traditional boat speedo's (pitot tubes or paddlewheels) always had an adjustment mechanism so I expect they still do. There is most likely an adjustment knob hiding under the dash. Mine was kind of dangling but it was zip-tied to some of the wire harness. There's nothing wrong with it, that's just how they work. But do keep in mind KnoxMojo's point about current if you are on a river.

[korey]

The adjustment in the menu is what you're looking for. Paddlewheels require a little adjustment from time to time - it will be worse if you trailer your boat frequently. And yes its relatively normal for it not not be the same offset through the speed range. Calibrate where the speed is most important to you - probably surf speed.

The "analog" gauge is actually an electronic gauge driven from the same signal, so I'm pretty sure any adjustment you make to the calibration will affect both - not that you can really see 0.5mph on the needle...

[5:00]

I have been in the GPS business since 1988. Here is some explanation of what might going on between the hand held GPS and the boat. Not solving your problem but just giving an idea about what you might be seeing. Getting down into the 1 MPH weeds with a non calibrated system such as on a boat can be tricky. SC doesn't take each boat out and calibrate speedos to the sub MPH degree then certify it. Have to keep that in mind also.

GPS SOG (Speed Over Ground) is good to about .1 knots. You have account for dynamics of the boat. If you are using a phone, up and down motions and accelerometers in phones can mess with the actual speed slightly. The lag on the cellphone apps can be pretty long as well. There is also a sub second lag as the handheld updates at 1Hz or one time per second. Within that time the boat also has dynamics and is not going to hold a perfect constant speed even on glass, it just can't. You have wind, the up and down motion of the bow changing the volume of water the hull is displacing, prop going through inconsistent water, slightest movement of rudder, rocking of boat etc. All of this will cause speed fluctuations (remember you are wanting to calibrate to sub MPH). That combined with the 1Hz update will change depending on where the speed is reported within that second. Bottom line is GPS on a hand held going to be pretty dead on but the boat will change.

Also speedos are going to have a percentage bias as well (planned or not) unless perfectly calibrated. If you have a speedo with a 10% bias (like Honda cars tend to have) it will magnify as you go faster. You don't see much bias at 10 MPH because it is only off by 1 MPH. At 100 MPH you notice because it is off by 10 MPH.

Like Knox said if you are that close you are good enough. Like Korey said adjust for the speed that is most important to you. I put the Nautic-laugic Knox was talking about in our XLV and I have it within about 1MPH at surf speed and thats good enough. You will always program for the same speed and it may be off from GPS but it will hold that speed every time for the most part. It may be off from GPS or off from the next guy's boat. When the next guy says surf at 12 MPH your's may be different than his. That is why I fixed mine so I could try speeds others are saying. Mine was off by almost 3 MPH before calibration, now it's good enough.

Sorry for the long post, I need a life. Ok, I have a good one, just a GPS geek though.

[mattsask]

The Speedo calibration is in your menu. I have a 16 so I'm not familiar with the 18 dash menu.

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