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Old 30-06-2015, 07:47   #1
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Simrad speed and cog differences

I have new Simrad speed, transducer, NSS8 mfd and autopilot with an AC 42 autopilot computer and AP 24 autopilot control head. I Just installed a replacement AC 42 and calibrated it per the manual. I now notice( and there may have been a difference before that I didn't pay much attention to). That the COG on the autopilot control head , which is consistent with my standard Richie compass, is about 10* different than the MFD displayed COG. Am I correct in assuming the MFD gets its COG from its own internal GPS and that's why? But why such a large variation? I have re calibrated with the same results. Is the internal gps on the NSS8 that far off?

And there is also a SOG difference of about 1 mph between the MFD and my IS 20 Combi readout. Again is that because the MFD has input from its own GPS. Why doesn't the MFD report what these two sources report since they are all on the NMEA network? How can I resolve these differences?


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Old 30-06-2015, 08:02   #2
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Re: Simrad speed and cog differences

Is this a paddle wheel speed transducer or a sound type of transducer?...NMEA STW, Speed Through Water. The paddle wheel transducers are notoriously inaccurate and become worse over time as biological fouling slows them down. Sound transducers can also be more inaccurate the further aft they are placed where the laminar flow is more disturbed.
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Old 30-06-2015, 08:12   #3
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Re: Simrad speed and cog differences

Also, individual instruments may need to have their calibration offsets individually adjusted.

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Old 30-06-2015, 09:05   #4
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Re: Simrad speed and cog differences

This is a paddle wheel speed. It's clean.

I don't think there is calibration for the speed on the com I or MFD. As to the MFD I do know many recommend updating with an external GPS antenna for mor accuracy but would that impact the course heading?
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Old 30-06-2015, 09:59   #5
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Re: Simrad speed and cog differences

The COG difference is probably because the AP is reading magnetic and the MFD is reading true.

The SOG is speed over ground and is based on the GPS, while the paddle wheel gives you speed through the water. The difference can be the current. However, all paddle wheel speed readouts have a calibration--RTFM.
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Old 30-06-2015, 10:30   #6
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Re: Simrad speed and cog differences

Ok BUT my combo gives a separate SOG reading. I know the difference. It's that reading that does not corelate
With the SOG on the MFD.

You are probably right on the course difference and I will check the MFD setting next time on board. Thanks.
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Old 30-06-2015, 10:33   #7
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Re: Simrad speed and cog differences

RTFM.....really!?!?
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Old 01-07-2015, 10:45   #8
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Re: Simrad speed and cog differences

Have you incorporated the magnetic deviation for your area into your COG? Are you set for True or Magnetic? Has the Ritchie Compass been calibrated?
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Old 01-07-2015, 12:24   #9
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Re: Simrad speed and cog differences

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Originally Posted by malyea View Post
RTFM.....really!?!?
Strictly, only when all else fails..then RTFM.
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Old 01-07-2015, 13:06   #10
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Re: Simrad speed and cog differences

I agree that the COG shown on the MFD may be set as true and that could explain the difference.

As to the difference in SOG between the speed instrument and MFD SOG I have RTFM since being reminded and I can calibrate the speed on the Simrad Combi but does SOG have anything to do with paddle whelk speed calibration? I think it's GPS data.
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Old 01-07-2015, 14:14   #11
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Re: Siimrad speed and cog differences

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Originally Posted by pitlaw View Post
I agree that the COG shown on the MFD may be set as true and that could explain the difference.

As to the difference in SOG between the speed instrument and MFD SOG I have RTFM since being reminded and I can calibrate the speed on the Simrad Combi but does SOG have anything to do with paddle whelk speed calibration? I think it's GPS data.
Disconnect your paddle wheel log. You will have a happier life. It will never be accurate.
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Old 01-07-2015, 14:18   #12
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Re: Simrad speed and cog differences

SOG has nothing to do with the paddle wheel. It is strictly calculated through the GPS.

Our paddle wheel log stays remarkable accurate as long as we clean it once in a while. Our previous one did also. So did the one on our old boat.

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Old 01-07-2015, 14:26   #13
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Re: Simrad speed and cog differences

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SOG has nothing to do with the paddle wheel. It is strictly calculated through the GPS.

Our paddle wheel log stays remarkable accurate as long as we clean it once in a while. Our previous one did also. So did the one on our old boat.

Mark
How do you know it was accurate. What precise measurement tool did you compare it to? There is no way to measure the accurate of a paddle wheel log. ... Unless you have an electronic current measures thingy.
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Old 01-07-2015, 16:55   #14
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Re: Simrad speed and cog differences

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Originally Posted by lordgeoff View Post
How do you know it was accurate. What precise measurement tool did you compare it to? There is no way to measure the accurate of a paddle wheel log. ... Unless you have an electronic current measures thingy.
Compare SOG and STW on reciprocal headings ?
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Old 01-07-2015, 17:34   #15
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Re: Simrad speed and cog differences

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Compare SOG and STW on reciprocal headings ?
Yes, and also doing timed distances on slack current water, or just comparing SOG and STW in a no-current, no-wind, no-wave area.

If it is an argumentative point, I will admit that we are probably only calibrated within 0.1kts.

I thought a GPS, compass and paddlewheel log constituted an "electronic current measures thingy".

Of course, I don't plot AIS targets using sextant and paper, so what do I know...

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