Big Block vs Small Block ref: fuel economy

[magster65]

280 with a Single Gas Hog.
280 with very efficient engines, but 2 - more weight, more drag, less work for each.
300 with V-8's - should use a little more gas, but, depends on injection systems...
300 with bigger V-8's, but the most efficient drives. Should use a little less gas than the 300 above.

I can see them using about the same gas.

I agree with that.
My 290 (5.0 MPI / B3's) is 1.5 MPG at cruise (20 gph @ 30 mph). The MPG doesn't change much as the speed and fuel flow are proportional. At 5000 rpm it's still 1.4 MPG ( 32 gph @ 46 mph). I wouldn't want to putt around in a boat this style but I don't mind putting...

 

[BonBini]

I would think the lone big block would be the obvious choice... Who’s carbed and who’s fuel injected?

On paper it would seem… ie: the 280’s:

1)…496cid, 8 large cly‘s, say 230 net hp (including water pump, accessory belt, internal engine and out drive) to produce 200 prop hp to cruise the hull with one out drive at 30 mph.

2)…265cid x 2, 12 small cyl‘s, with all the extra’s as far as parasitic loads x 2, say 300 net hp to produce 240 prop hp to cruise the hull with two out drives at 30 mph.

Granted my numbers are just estimates: I’m looking at the extra fuel needed to produce the 70 extra hp to move boat 2 the same speed. Could the 496 be that inefficient in converting fuel to hp?

496 V-8 = 29 horse per cyl.... 265 V-6 = 25 horse per cyl.... Air speed through the smaller V-6 intake runners is probably up in comparison.