How much efficiency do you lose by having so many (5 or 6) props so close together? I'd think that you'd get some drag from all of the vortices forming right next to each other, at least in the center.
So I'm going to get into some guessing. Prop tip clearance on inboard and semi-planing hulls is around 10 to 15% of the diameter of the prop. So if that holds true for planing hulls then a 18" prop needs a minimum of ~20" on center. Most outboards I know have a minimum of 25-26" on center for mounting them.
A lot of the drag on a boat comes from the running gear, not just the prop. A lot of the fast inboard boats are either made with prop pockets to tuck all of that up tighter or even out of the water. Or run surface piercing drives so that only half of the prop is in the water at any time.