1) The really short answer is: Yes it would work
, but you'll not notice any difference between a "6db" and "3db" antenna, except for any height difference between the two, which can also be done using an antenna mast
2) The basic answer is: Height is king!
No question, unless you have a very long run (> 100 feet) of very lossy cable (such as RG-58), there is no substitute for height in VHF Marine
communications! (see below, for links to detailed info)
Line-of-sight from 23' - 25' high (or masthead height?) is approx. 6nm....line-of-sight from 7' - 8' high is approx. 3.5nm - 4nm....and, of course your typical handheld "range" is going to be one of these numbers added
to the line-of-sight of the other boat's / other station's antennas, plus about 5% to 10%....
So....so, who/where are you trying to communicate with? And, can you install a fixed-mount VHF radio
? (which, even if using a low-mounted / rail-mounted antenna, will be MUCH better than your handheld VHF radio)
Whatever antenna you can manage to get up higher will be your best option.
Although, Jammer is correct....you're not going to be able to notice the difference between a "3db" vs. a "6db" antenna.....as anything is going to be better than a rubber-ducky!
Although, using the desires/criteria you write, and the boat
you have....using a handheld with the rubber-ducky is a darn good choice!
3) But, you asked about external antennas.....and, while we all know that a masthead mounting would be best.....I understand that you do not want to mount an antenna at your masthead....so, okay then, most sailors would be better off getting an antenna mast
extension and mounting a good quality antenna on that, securing this antenna system to your rail/pushpit, etc., as high up as you practically can manage, all the while keeping it secure and out-of-the-way of your running rigging
, docklines, etc...
But, on your boat, I think the boom extends aft of the stern a bit?? And, hence your boom is going to hit any antenna / antenna mast you mount there...
securely mounting a 3' SS whip onto your boom gallows might just be best for you? (and protecting / securing the antenna coax, as well as making provision for the adapter cable to be protected/secured) As this antenna can be whacked by the boom many, many times, with no issues....
Please choose a good quality antenna, a short length of coax, and a secure mount....and you'll be much better off than using your rubber-ducky antenna....BUT..
But, the "handheld radio-to-VHF antenna coax" interface (usually a short length of small / flimsy coax, with SMA connector on one end, and a SO-239 connector on the other end) is going to be the weak link in your set-up, and will fail sooner-or-later, leaving you without any external VHF antenna, and searching your boat for your old "rubber-ducky" antenna...
4) So, unless you want to install a fix-mount VHF radio and/or masthead antenna (which are your best options), I'm back to my mentioning that using a handheld (and its rubber-ducky) is not a bad choice for you / your boat, using your desires/criteria...
Now, on a 17' boat, I understand that having a fix-mount (25 watt) VHF radio is an issue (and I do understand that you do not want a masthead antenna), but the reason handheld VHF radios (marine, ham, commercial
, police/fire, etc.) all come with "rubber-ducky" antennas, 'cuz they last....and usually last longer than the radio....and while are inefficient as all heck, they do their job well....
So, Bertinc, as you might now be seeing, the answers to your questions are not as simple as you thought?
5) Of course, there are other antenna options / choices, such as the 6400-R, 6396-AIS-R, etc...extension masts, etc....but, I think the above answers should suit you well....(although, if you can mount a rigid mast extension and antenna, then you may want to have a look)
FYI....Here, you can see my TWO 4' Galaxy fiberglass
mast extensions, and TWO 1' SS mast extensions, and their two HD SS rail-mounts....getting my main GPS
antenna above my solar
array, and my AIS-VHF antenna (spare VHF antenna) / my Bullet2HP wi-fi system, about 10' to 11' off the water
So, I do have experience with using rail-mounted antennas and antenna mast extensions....and if you do it right, they are wicked strong and will last a long time! And, work well!
And, if wish great detail of VHF radio range, etc...have a look at this thread here:
Hope this helps?