On Saturday, July 17, 2004 at 1:14:51 AM UTC-5, Per wrote:
I presently have a 4.3LX V6 Mercruiser with an Alpha drive in my 21 foot
boat, and cruise at

25 mph. However, fuel costs are going through the roof, so to make the boat
more affordable

when I am retired in the near future, I want to replace the engine with a
3.0L Mercruiser, which

I will buy new, bobtail, and hereafter go slow. (I love the 3 litre because
I had it in a previous

boat.) From tests I have done, I get 3.7 mpg (imperial gallons) on flat
water with the V6 and

a speed of 5.5 mph at 1400 rpm.

What speed and fuel efficiency can I expect from the 4-cylinder under the
same conditions?

By rough estimates I figure it to be about 6 mpg and a speed of 6 mph with
the same prop

and gears. This is the minimum I hope to get. My other question: if it
should't be that good, is

there anything I can do to the engine or the boat to improve things? The
engine of course is

never going to plane the boat, but run all day, for days at 1400 - 1500 rpm,
and be relatively

lightly loaded, putting out something like 10 or 12hp. I don't mean to do
anything too drastic;

just tweaking things for max efficiency at low rpm.

I need a lot of reserve power in heavy seas and strong currents.

max efficiency is measured on plane. Usually 3200 to 3500 rpm why would you want to plow water all day. lol The more boat out of the water the more efficient it is.