Can I get away with saying it depends...
Mat started to look into this for two main reasons, firstly the age of the equipment on Allegra means its getting to the rip and replace stage, as once one piece of equipment is upgraded it gets more difficult to make that system to talk to something else without additional hardware. The marine navigation equipment is all perfectly working and serviceable it just isn't easy to update. Second, Allegra has a navigation table that at one point had space for a plotter or even might have had a plotter but was now covered up with a piece of plastic.
During the winter of 2020-2021, Mat started to research what OpenPlotter could bring to the table. You need:
and that's it... You have an out-of-the-box working Chartplotter. There are going to be a lot of things you want to add to this, such as sensors and other software (UK Charts for one!) but for relatively a small amount of money you have a working system.
The above diagram shows how the instrument network is set up on Allegra. The Pi will connect to the SeaTalk network to get boat data into the system using an optocoupler listed below. This means that the two systems are isolated from each other.