Thanks a lot, everybody, what a helpful bunch you are.
I noticed a few questions, so for clarity : My boat is an MGC27 with the self-tacking headsail (she has a normal genny too but that's for sea sailing), I foresaw the problem of tacking single-handed along a fairly narrow river ! They are reputed to tack (and sail) like witches, I'm looking forward to that ! Believe it or not (long story, won't bore you here but happy to send an email to enquirers !) I've not yet sailed the boat and have only seen her in photos ! Yes, I know !
The fin swings up under the keel, that's what gives the slightly-over-three feet draught. So all the weight is still outside under the hull. I shall very tentatively see what happens when I try her with two slabs down and the fin up, and if she feels as if she's about to fall over I shall chicken out VERY quickly and drop the fin !
Thanks to those who sent me the links to the BA depths maps (not really charts ?) which are very useful, I've downloaded the lot and converted them from .pdf's to .jpg's for easier sliding between them on my computer screen. I hope and trust that they're on the pessimistic side, and they do say their depths are at Mean Low Water so there's hope for my <6ft draught with fin down. Sad about shallow Rockland and Bargate (Surlingham) though, I'd like to overnight there sometimes. Maybe I'll let the fin do the soundings, I haven't much faith in the depth sounder over soft mud. I believe that the fin swings up when grounding, so the stodgy tiller should give due warning that it's time to go somewhere else ! Deep fin = higher pointing and better tacking ; can't have everything.
The mast is a lofty (for the loa) 38ft tall which, plus the 5ft of height from water to step, gives an air draught of about 42'. That's the boat's only feature which isn't Broads friendly. But it really makes little difference, once you have a mast you have to lower it for the bridges whatever its height. Would have been nice to be able to slip under Postwick, though. I've designed a fairly simple deck-mounting collapsible sheerlegs which should enable me to deal with lowering the pole singlehanded (VERY carefully and slowly, but who's in a hurry ? Bring your beers and come and watch/laugh, I don't mind !). Sure, I'm nervous ! However, being stuck in france due to Covid, I'll only have half of the summer for sailing and I'm not going to waste it on boatbuilding. She's in a marina at Brundall, so I'll be satisfied with toddling up and down the Yare until next year. If 'They' manage to fix the Haven and Mutford bridges, I'd be able to 'go round outside' and explore the Waveney too, but of course all the 'proper Broads' are 'up north' ; pleasures to come next year, when I should be able to drop the pole and sneak under bridges at low tide. Anyway, they tell me it's more peaceful 'down south'.
As for the cost of pumping-out ; well, there are other ways involving plastic bags to avoid pumping a loo, and as I'll be on my own I can put up with them. I'll find the cheapest, become a 'regular', and pay up when I must. Well, that's the theory ; I've lived in some primitive parts of the world ! Reasonable conduct, respect for others and the waterways, and general discretion seems to be the way on the Broads, as it is everywhere else.
Thanks again everyone, looking forward to 'bumping into' some of you !
Rob.