Given that size boat and twin motors your looking at a current fuel economy
of 1 mpg or very close to it. Going to the 5.7L 300 hp EFI is not going to
make any real difference in economy. It will improve acceleration and top
end. Those engines are not fuel economy engines though. Your best bet
would be to prop for wide open at the very bottom of the max rpm range.
This will hurt your acceleration but will help some on fuel economy giving
you a little more speed per rpm. As you know it would take a huge fuel
savings to cover the well over $10k you will spend on an engine swap.

If you really want to improve economy look at twin diesels.

Otherwise - reprop and save your money.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
wrote in message
oups.com...
Hi fellow boaters,


I'm a NEWBIE to this group, so thanks for your patience in case I don't
quite fit in yet.


Most of you are lucky not to have my problem: fuel economy - gas
where I'm boating costs $6/gallon.


So you'll understand why I'm seriously considering swapping 2
carburated engines for thrifty 300hp I/O Mercruiser Magnums. However,
their cost is high enough (x 2 engines) that I am thinking of trying to
keep the stock Alpha One outdrives hoping they won't die.


I know that the Alpha One outdrives are not nearly as solid as Bravo I
drives. Their horsepower rating is maxed at 300hp so it is cutting it
close. Would there be a way of changing their gearing and propeller
size to make sure that I get both maximum fuel economy as well as keep
the outdrives from busting from overstress?


FYI the boat has a 31 foot semi-planning hull weighing 10,000lbs with
cruising speed of 20-25 knots and max speed of 30-35 knots. I was
hoping that with 600 total horsepower on tap, that I could run the
engines at low rpms sparing the outdrives and saving on fuel, while
maintaining a 25 knot cruise speed (boat rated at 20 knot cruise/30
knot max when powered by twin 200hp engines). Let me know if I'm just
dreaming and about to make a very expensive mistake. Hopefully it isn't
necessary to replace the outdrives with used Bravo Ones, just messing
about with gearing and propellers?


Thanks for all your help! :-D

Rich