Can you identify this?

[AlanR]

It's not a coil. This type of aerial responds to the electric field, not the magnetic field. About half of the tube is filled with a copper rectangle, which is charged by the electric field, producing a small voltage, which is immediately amplified by the built-in low-noise amplifier. That amplifier needs a power supply, hence the DC fed into the co-ax. I won't go on........

[jrobert890]

Hi Tupaia, I think you guessed it. The boat is equipped with a Munro Sea Me unit. It must be the antenna for it. Thanks everyone for the input!

[MicHughV]

if there is no visible wire leading to the base, another guess might be a "radar" reflector"....I've seen similar sized one's attached to a shroud...

the inside of the unit has a bunch of odd shaped metal pieces....