I used a Nav6+ for 5 years in the Med. Initially suffered variable reception but this improved greatly when I connected the ground/drain wire from the antenna to the keel to get a good RF ground. Most of the time I operated without the whip but eventually left it on as I found it did help in marginal conditions. However, the grounding is the real secret.

Back in Aus now we don't have Navtex so I was tempted to sell it on e-bay but I like the data display and logging too much to part with it.