I have what might be an unconventional layout for my 20MA. No tank under the deck- that is full of flotation foam. I have huge console that would house a head. I put a 20 gallon tank in there. That is fine for Buzzards Bay cruising. I have no T-Top. A T top might help static CG a little when stopped but will hurt underway. I may put one on the boat at some point.
I am about to put a 22-24 gallon Whaler style tank ahead of the console. They are like $250 at West Marine. I plan to tuck a fuel line under the cap so no under floor plumbing required- just a little step over hat channel on the deck for the 1 foot run to the bench seat. That gives me 42-44 gallons. I get 5MPG, so that's a lot of range for me. (I don't think any 20 footers do a lot better than 5NMPG. So I think a full to empty range of more than 300 miles is really unusual for a 20MA.)
If I took the kicker (58 lb) and stowed it forward, that would help with static CG. Because it is behind the transom, that's about like one guy standing in an aft corner.
I still need to pull the fuel tank hatch and take the plywood out and foam core it.
I really think you won't be happy with that Honda. I think it is 200 lb over ideal (under 300 lb), 100 lb over the sort of normal compromise (375ish), and 30-50 lb more than most "performance" or "plus kicker" compromise setups (like mine with the 58lb kicker). And it isn't insanely powerful, so planing out might still be a bit of a slog.
My 20 MA came with a 175 Yamaha and it was 415-420lb, I think. 60 lb lighter and I am pretty sure it planed out well. I never used it. I ditched it for the lightest 4 strokes I could find- 2 stokes make me seasick, it wasn't an option. I also wouldn't run an old motor without a kicker in Buzzards Bay, so the main engine had to get lighter to support that change.
All that outboard weight is, well, outboard. Behind the transom. You aren't floating it 1:1. It is on the other side of the seesaw. You float it, then you float the lever arm it makes again by sinking the transom further.
You can offset it with anchor chain to a degree so you pay the least amount of stern sink by having "ballast" all the way forward. But not when you are anchored. Then that ballast is on the sea floor. Which is when it might be when you need freeboard the most. This isn't to say that anchor chain is a bad thing- quite the contrary. But it isn't ballast at anchor.
At any rate, I think batteries and tanks forward are the way to go. Fore aft fuel sloshing is bad.
I think the comment about CG from fuel burn is a bit overthought, in so far as I don't think tanks can be too far forward on a 20 footer. There isn't the volume in the forefoot to really mess it up. Plus: If your tanks are forward of the console, and you are too light from fuel burn, a 5 gallon pail or two of seawater on the casting deck will fix things quickly. Along with coolers and other gear.
That's how I run my 20MA and do it without tabs. Maybe I don't know any better, but it seems pretty good.
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