RD33 not recognizing Simrad rudder angle PGN



I have a Simrad AP26 autopilot that is connected to the N2K bus via a Simnet adapter. It transmits PGN 127245 (rudder angle). Unfortunately, Simrad uses an instance number of 255 when it sends this PGN, and there is no facility to alter this hard-coded instance number. This product has been obsoleted by Simrad and is no longer supported.

I can view this PGN on the N2K bus using multiple viewers, e.g., Maretron N2KAnalyzer and ActiSense NMEA Reader, so I know that it is available on the N2K bus. My problem is that the RD-33 will not display it. As far as I can tell, there is no facility within the RD-33 to specify an instance number.

Other than finding a means to convert this PGN to a NMEA 0183 RSA sentence, is there any other option?

Bob Breum
Nordhavn 43 "Wanderlust"
Good morning Physics Teacher,

First off, thank you for your service as a teacher!

From my understanding the RD33 does not really care about instance numbers when referencing PGN 127245. The instance numbers will come into play when there are multiple engines or multiple tanks that you are trying to get information about. When you go to the Data Source menu option and you scroll down to Rudder, does it give you the option to choose the AP26 as the source for the Rudder Angle?

If you think the device instance is causing and issue you should be able to use the Actisense NGT-1 to change the device instance. Please follow the steps below.

Device Instance..JPG
On the Actisense NGT-1, the AP26 shows up as instance 1 (or 0, I have two). The problem is that the actual PGN that the AP26 transmits for the rudder angle uses 255 for the rudder instance, not the autopilot instance.

I have circumvented this problem by installing an Actisense NGW-1-ISO between the AP40 autopilot computer and the NMEA 2000 bus. The AP26/AP40 were transition devices that offered both NMEA 0183 and NMEA 2000 (SimNet) capability. The AP26 autopilot control head outputs NMEA 2000 PGNs, and its mate, the AP40 autopilot computer, outputs NMEA 0183 sentences. The rudder PGN that is translated from the AP40's RSA sentence by the NGW-1 is properly constructed, and the RD-33 reads it without issue. It's just frustrating to have to buy and install two additional pieces of equipment (the NGW-1s) to make this work, but that is Simrad's fault, not Furuno's. It would be nice, of course, if the RD-33 could handle the original PGN.

Have I mentioned how much I love Furuno tech support, both here and by telephone? You are amazing.

I have the same problem and I have been trying to find a solution for the past two years. I am using a Simrad AP28/AC42 autopilot (their latest equipment). I am not able to see rudder position information on my RD33 even though my Actisense shows that PGN 127245 is available on the NMEA 2000 bus. Simrad indeed uses an instance number of 255 when it sends this PGN. I spoke with the engineers at Actisense and Maretron (the data looks the same using my Maretron reader) and as they explained it, an instance number of 255 tells the bus to ignore the data being sent. Perhaps this is why rudder position is not displaying on the RD33. I have had extensive discussions with Simrad and they will not modify their software to output a PGN that a Furuno RD33 can read. They actually told me that this is intentional and if I want full information availability I should be using Simrad equipment exclusively (so much for the concept of NMEA 2000 standards). The AP28/AC42 does not have a NMEA 0183 output so I can't use this as a work around. If anyone has found a solution I would love to hear it. BTW, I agree, Furuno customer service is excellent....180 degrees from the awful support Simrad provides.


In my conversations with Maretron, they told me that their DSM150/250 displays will display the malformed Simrad rudder PGNs, even though the N2KView software does not. They told me that they intend to modify the N2KView software to recognize the Simrad PGN, also.

I do not have a DSM150 or DSM250 display, so I cannot verify this, but I know that N2KView does not recognize the Simrad PGN.

Also, it would be interesting to place two NGW-1-ISOs back-to-back between SimNet and the NMEA 2000 bus. Let the first one convert the malformed PGN to a NMEA 0183 sentence, then let the second one convert it back to NMEA 2000. I think this might work in your situation. It depends on whether the Actisense product will process the malformed PGN.


Thanks so much for your quick response. Your idea of converting to NMEA 0183 and then back to NMEA 2000 is a good one. I need to find out if Actisense can read the Simrad malformed (good word) PGN. I may simply purchase a Maretron DSM150 and install it at the helm if it will indeed read the Simrad PGN. I will let everyone know how that works out.

Krogen 44
I went through this too a couple of years ago with Simrad.

First, it's important to distinguish between a Device Instance number and a Data Instance number. What you can change with Actisence, N2KAnalyser, etc. is the Device Instance. This is only exchanged between devices during address claims, or when queried specifically.

A Data Instance is only part of select PGNs, and 127245 is one of them. Data Instance is used to make individual PGNs self-identifying as to which device they are representing.

In the case of Simrad, its the Data Instance inside each PGN 127245 that is 255 and is causing the problem.

I ended up connected to a Simrad engineer in the UK who was one of their N2K people. He said their interpretation was that using 255 for the Data Instance meant "not used", as in they don't use Data Instance, i.e. it should be ignored. But he acknowledged that others interpret it as indicating the PGN data is invalid. I think this is an example of how poorly specified N2K is. Otherwise companies wouldn't be interpreting it so differently.

As a work around they gave me firmware that would make the Data Instance the same as the Device Instance. Make the Device Instance 5, and the Data Instance becomes 5. It solved the problem of N2KView and others not being able to display the rudder angle, but it totally screwed up the auto pilot components in a variety of ways, so I had to back it out.

The Furuno guys will be glad to hear that my ultimate solution was to replace the Simrad AP with a Furuno Navpilot.

BTW, using back to back converts may well work. I've had to resort to that before....
Thanks TwistedTree for the tip. I used back to back converts which worked perfect!

I'm glad to see that my idea worked out for you.