I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this
speed, I get reasonable fuel economy, because it's not until about
3,200 RPM's that the secondaries join the party and start tapping the
keg, if you will.

Max RPM on the boat is 4,100, at a speed of around 30kts. I very,
very rarely operate at this speed, and when I do, it's usually for
less than a minute.


There are several things to consider. As others point out, with a 4100
rpm max., there is a distinct risk that there will be detonation
(knock) in the cylinders. High speed detonation is difficult to
impossible to detect, especially when the engines are well insulated
(as they would be in a SeaRay.) Detonation is like hitting the piston
tops with a ball-pein hammer and can eventually punch holes in the
pistons. Also, carburetors try to do the best they can at providing
the right fuel-to-air ratio for combustion. That said, running the
engine just shy of opening the second barrels tends to run the mixture
lean, which will further aggrevate the detonation problem. I have read
in boating mag advice colums NOT to run that way. You will get "save
money" with better fuel economy, but lose in the long run.


As to tabbing, I have a 1989 30ft. Sundancer with flowmeters with GPS
input that shows instantaneous miles/gallon. I have observed several
times that I get slightly better mpg (approx. .05 mpg) by pushing the
bow down slightly with tabs when cruising at 20-25 mph. I would never
have guessed that, 'cause the wetted area goes up (viscous drag), but
it really does happen.