"Jim and Becky" wrote in message
...
I saw a comparison in a boating magazine last fall.
I didn't get a chance to read it all but essentially a new boat was
offered
in Inboard or I/O configurations.
It was perhaps a 28 foot Regal or Rinker twin screw.
Same engines yet the I/O had a top speed 10 mph higher than the straight
inboard.
Does anyone remember this aricle? I believe it was on the front page.
Is this difference in speeds common and why?

The inboard has a untrimable fixed prop, pointing about 13 degrees downward.
This causes stern lift, which pushes the bow down, resulting in a higher
wetted surface, hence the lower speeds.

The I/O is trimable. Once on plane, the out drive can be trimmed up, which
pushes the stern down and the bow up, resulting in less wetted
surface...which means higher speeds.

The advantages of an inboard is that they are much less complex, and have
many fewer moving parts verses an I/O. They also (because they run flat in
the water) generate very small wakes...perfect for a slalom ski boat. Big
disadvantage...single screw inboards do not back well.

The advantages of an I/O is that it is trimable, and backs like a car. You
can even use the trim to alleviate a rough ride in choppy water. But even
though it has less wetted surface, the propulsion system has more drag than
an inboard.

--
-Netsock

"It's just about going fast...that's all..."
http://home.insight.rr.com/cgreen/