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Hi J-C,

sorry to reply this late, as usually this group was not very much "alive" I did not visit it for quite a while.
Unfortunately, on the topic of sloping wire antennas and RF ground on a sailbout, you will find 99 different opinions and views.
Look up on eHam forum, you will find the same comments stating that the impedance matching range of the FC40 is unfortunately a bit narrow, hence it is quite picky and sensitive to wire length.
Even in the home QTH, trying out inverted-L wire antennas WITH a large RF ground system (2.4 m long earthing rod, with several radial wires about 5 cm buried in gravel, with lengths ranging from 20m,15m,10m,7.5m,5m to at elast cover most of the HAM bands, it was still rather difficult to find a wire length offering all-band tuning with the FC40. About 23m was OK, and also around 9.6 m.

On the boat, I've tried a lot. As RF ground I have 4 inch wide copper straps, running from the FC40 ground lug to both sides of the pulpit bases, and from there also connected to the lower S/S lifelines of my Jeanneau 37 ft. From the tuner RF ground lug also 2 sets of radial wires, 10m and 5m long, tied with tie-wraps to inside of the aluminium toe-rails (le rail qui court le long des c?tés du pont, entre pont et cocque). Furthermore copper tape to the bronze underwater strut supporting the propeller axle. Since originally the boat also had a woven tinned copper strap running from engineblock (also the DC negative ground), and to prevent potential galvanic corrosion issues due to ground loops or high voltages pumped into this strut, I blocked the DC with about 0.15 microF of high voltage caps. I do think I have quite some RF ground availbale with this. I never did run copper tape to the iron keel bolts. That would be at least 5m of copper tape which for the higher frequencies is just too long.
Antennas: I tried all lengths between 14m and down for the sloping backwire antenna (of course I do not use the backstay but a sloping wire parallel to the backstay running to the starboard side of my pulpit ("balcon arrière"). No length was ok for 80m-40m-20m-17m and 15m allband, except around 9.6m. Length is very critical. Sometimes adding a little "pigtail" extension at the point where the bottom of the antenna wire is bolted to the isolated through-hull helped 80m tuning.
After that Greg on the SSCA forum talked me into a parallel multiwire system, with 13m, 6.5 and 3.75m parallel wires with 10cm spreaders in between, but all connected together at the base, and with a 1:1 current balun at the bottom (derived from K9YAM antenna). That tunes quite OK on all those freq's,theory says every freq will choose maximal current in the optimal choice of wire, and still offering low-angle radiation. Others critised this heavily, saying that its radiation angle behaviour would be very unpredictable. It tuned well but DX results were bad. So I took it down...

Now I have a wire of about 10.5m, connected to the isolated through hull. Directly connected to the underside of the through hull, inside the back lazarette ("coffre arrièe tribord") I have the HD 1:1 current balun (13 turns of teflon coax around a Amidon T300A core) and 40 cm of teflon coax from the balun to the FC40. I did this to avoid 45cm of antenna wire running inside the "coffre" to the FC40. I now have the RF ground system connected to the balun ground output. I have no idea of losses in this balun + short coax system to the atu, but it does tune well on all my wanted frequencies, even on 12 Mhz marine SSB freq.
I had DX contacts to Japan with this on 17m and 20m, from the sea.
Does that prove anything? Not really....I guess a more standardised test with a fixed receiving station and also field strength measurements and antenna modeling would tell me more.
The 10.5m wire length should give low angle take off from 40m till 17m (really the limit for 17m since 10.5m is just a triffle more then 0.625 Lambda)

So far my story.

Jan
ON3ZTT