I had seen these about, over the years.
This info was posted by a long standing and very reputable Drascombe forum user who is also a radio ham. (Mr Ole Henderson) in response to a query there.
"A very easy way to double Tx/Rx range is adding an antenna tail, giving the radio a full 1/4 wave dipole arrangement. the tail; a downward hanging wire electrically attached to the radio's ground side provides balance for the shorter helical "rubber duck" antenna on top. Relatively easy diy for land-use handheld radios, more difficult for marine handhelds.
A solution: As it happens, an outfit in British Columbia makes a tail, "Rattail Antenna Booster" tm, for terrestrial and marine handhelds. Cost post paid; ~$US40. It attaches to the back of the radio case with velcro. Electrically, it uses capacitative coupling. Yep, I hear incredulous laughter in the background.... But, fact is, they do seem to work: Not using the marine handheld, but an 5W 2 meter amateur band handheld with tail, I connected with a radio net via a repeater about 50 miles down river in Portland OR three times (Government Cove protected calm waters, open water and ashore). Surprised me and them! The Rattail antenna wire is soft, flexing easily and does not seem to get in the way."
Ian Soutar • bodyworksvictoria2@gmail.com..the Manufacturer, says...
The device amplifies both transmission and reception as long as it's allowed to dangle free from contact with your body.
https://rattailantenna.com/I found Mr Soutar very helpful. Be sure he knows its to be fitted to a Euro spec handheld VHF marine radio. Apparently different radios require different "tuning" of the tail.
The device arrived promptly from Canada, delivered to UK for $47 Can, £28ish. Simple transaction. PayPal.
Appears Well made, easy to use.
From adhoc radio tests (Thanks NCI) it seems to work well, and is very easy to use.
It travels with the radio, no aerial connections to make, nor antenna to hoist. Not that I'm saying it as good as this type of alternative aerial.
No connection with seller. I am not a radio ham.
Pic is their marketing, not my radio.
