An alumium hull will act as a great ground so there is no need for a separate grounding plate.
You need however to prevent a DC connection to the hull. This is usually done by connecting the SSB
ground to the hull via a capacitor this allows the SSB
ground to function but stops the electrolysis
that would result from a direct connection.
If you Google
it I think you will come up with some detailed instructions.
Sorry I cannot do the search, but my internet
connection from the boat is very slow.
If you get stuck post again and I will try and dig up some more specific information. I think I saved some articles, but I have had no luck finding them so far.