There are numerous references
problems with Dynaplates. The problem is not with corrosion
of the bronze itself, and perhaps "corrosion" isn't the right word, but rather with growths on the Dynaplates in seawater which work to degrade its initial advantage, i.e., of a very large surface area in contact with the water.
The construction of Dynaplates involves a honeycomb of "beads" in contact with seawater. Once these beads become covered by algae and marine
growths -- as they inevitably will -- the bronze is no longer in direct contact with the water. Yes, there's no doubt some capacitive coupling going on, but this is not as effective as direct contact.
Here's what the folks at yachtfunk had to say recently with respect to the ARC
: "Traditionally, boatyards
have installed dynaplates, which allow for a direct contact to seawater, which is a good electrical
conductor. At first glance, dynaplates seem to be a clever solution, looking at the way they are built; they consist of little pellets making the contact surface bigger than the holes to be cut into the hull for their installation. Unfortunately, this apparent advantage turns out to be their biggest drawback, because after about six months of sailing algae and shells will have started to colonise the pellets, so they need cleaning
or replacement to do their job properly." (from the Noonsite website).
There are many other such references
Gordon West did some testing of traditional RF ground systems a few years back, and published his results in SAIL magazine. IIRC, he found, inter alia:
1. There's no need for 100 sq ft of copper;
2. A direct connection to the nearest bronze thru-hull is adequate;
3. Large ground plates are unnecessary, and the porosity of some plates effectively reduces their contact with seawater.
However, in the absence of really good empirical data on Dynaplates and other forms of ground plates, and their effectiveness in improving transmitted HF signals -- particularly in comparison to other RF ground systems -- I think we can't say for sure how well they work when new or after prolonged immersion in seawater.
What I CAN say from personal experience, and from professional experience installing HF systems on boats, is that I absolutely agree with points #1 and #2 above. I also tend to think that #3 is substantially correct, leading to my working belief that groundplates and DynaPlates are an unnecessary expense in most cases.
In the past week I have installed three HF systems on cruising sailboats: a 33-footer, a 42-footer, and a 40-footer. None used Dynaplates or ground plates for the RF ground system. All systems work perfectly, tune quickly on all ham and marine bands, and received excellent on-the-air signal reports over daytime distances of 500 to 1,000 miles. This follows my personal experience with HF radio
aboard my own boats over the past 40+ years.
When it comes both to antennas and, especially, RF ground systems on boats, some thinking "outside the box" can yield excellent results :-)