Sounds like you need to drop prop pitch a bit to get the WOT RPM where it
should be. Reducing pitch reduces load on the engines at the same RPM and
you might just be able to run a few 100 RPM more before the secondaries
open. Another option may be to have a performance shop change the "rods"
which control when the secondaries open. I presume you have the old Chevy
standby Quadra-jet carbs.

wrote in message
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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.

I'm wet behind the ears when it comes to propping, so a question for
those in the know:

At 20kts, the boat requires slight tabbing, so it's not quite 100% on
plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at
about 24kts, and will plane without tabs. The first comment will
likely be that operating on a full plane is more fuel efficient, and I
would normally concur, but it's not the case with this boat. I've
done the calculations, and it's more efficient for me to keep out of
the 4 barrels.

My current props are 3-blade. Would it be possible to prop the boat
so that the cruise speed at 3,000 RPM's was bumped up a few knots? My
thinking is that if I can get on full plane while keeping out of the
secondaries, I'll enjoy the best of both worlds, higher cruising speed
while maintaining fuel economy (if that's really such a thing on a
boat). 8)

I'm not concerned with losing a couple of hundred RPM's on the top, as
I simply don't operate the boat in that range. Another benefit is
noise. Above 3,100 RPM's, the engines generate significantly more
noise.

Is a 4 blade prop an option? More cupping on my current prop? More
diameter? Other?

Thanks!