VHF antenna - insulate or not?

[Beausoleil]

I have some stray RF issues on the boat, but before I track them down, I'd like to know the recommendations for the mounting of VHF antennas and their electrical connections with respect to bonding/grounding.

I have marine VHF antennas atop the main and mizzen - a Metz Manta 6 and a Shakespeare 5215. Both appear to be shut-fed, as their DC resistance between shield and center-conductor is much closer to zero than infinite. The Metz 2m ham antenna, as measured from the coax, seems to be series-fed, and also insulated from the mast (no connection between coax shield).

If this is the case, then, unless there's a serious problem, then instinct tells me that I should have a single-point ground with respect to each VHF radio's chassis to the boat's main DC ground point - yet my Raymarine Ray240 radio internally connects the chassis and SO-239 connector to the power input's negative lead, as do both my West Marine VHF 550 and my Icom 706MkIIG HF/VHF/UHF ham radio. So I basically have a multi-point ground for my marine VHF radios (at the radio and at the antenna), and a single-point ground for the ham radio.

My RF theory is rusty, so pointers and explanations would be appreciated. Once I know the preferred/proper method (e.g., insulating the marine VHF antennas on the mastheads), I'll be able to begin tracking down the real gremlins in the system...