That should work on SeaTalk already. What brand is your chart plotter? That may be the issue.
Jim...
No, RADAR data is not carried on SeaTalk or SeaTalkhs(NMEA2000). Only SeaTalkng carries RADAR data. Its too high a bandwidth for the other two.
I'm not trying to troubleshoot anything here James, I know how it works and what is wrong. I was simply commenting on my implementation for the OP.
Raymarine suggests two methods. One is point to point with a chartplotter and then optionally point to point from that chartplotter to another chartplotter, effectively using the 1st chartplotter as a repeater. This is why the chartplotters all have two SeaTalkhs ports. This is ok for simple 1 or 2 plotter installs with one major issue. If the intermediate chartplotter is not on/fails then RADAR data cannot be sent to the 2nd plotter. In my case the RADAR comes to the plotter at the nav station and thence to the help. So, a failure of the nav station prevents me getting RADAR where I need it most, at the helm. Additionally this plotter-as-a-repeater method prevents sharing the data with other devices such as, in my case, OpenCPN on a Pi.
The second method recommended by Raymarine is to use their expensive SeaTalkng hub. Well SeaTalkng is basically ethernet cat 5 with special plugs. In fact, in the 1st method there's a component (can't remember what RM calls it) that swaps the polarity of a couple of lines, effectively making what is in ethernet terms a "cross over cable". I need to figure out where it is and remove it, make up a new cross over patch cord, or something similar, and then I'll be able to connect my RADAR, both chartplotters, and my Pi to 12v switch and I'm good to go. I may need to prevent one of the chartplotters acting as a DHCP server, unsure of that yet.
fwiw I have 2xRM C90W