BoatBanter.com - I/O OR OUTBOARD

Hi Jim,
Opinions, everyone has one! My opinion is outdrive are best used in
heavier boat applications. When in the size range you speak of, the
advantage of an outboard is not having to share your very limited deck
space with an engine compartment. I do recommend a 4 stroke outboard as
they are much quieter and fuel efficient then 2 strokes.

Outdrives are great for larger boats with v-8 engines.

Regards and luck,

Capt. Frank Hopkins

Jim Stallings wrote:

Hello All,

This will probably be a painfully obvious question for many of you,
but I wanted to get some advice and I'm a novice powerboater (plenty
of sailing experience though!!)

Anyway, I'm in the market for a small runabout boat - perhaps 17 to 19
foot. It's to be used in both fresh and salt water here in the NW USA.
I plan to pull skiers as well as just cruise and maybe fish.

My question is - what are the advantages of an I/O vs outboard, and
vice versa. I've been leaning toward an I/O, mostly from the
standpoint of noise, simplicity, and looks (no unsightly motor in the
way....). What are some other considerations I should be aware of or
take into account.

Thanks VERY much for any help you can provide.

Jim

Capt Frank Says:

I do recommend a 4 stroke outboard as
they are much quieter and fuel efficient then 2 strokes.

Not necessarily so. Read below.....

Two-stroke Conventional Wisdom
BY RALPH LAMBRECHT

Everyone in the marine industry and most of its environmental critics are aware
that there are now at least four manufacturers of two-cycle outboards with DFI,
direct fuel injection: systems that put the fuel directly into the combustion
chamber after the intake and exhaust ports close. Such systems eliminate loss
of some of the incoming fuel charge out the exhaust ports along with the
scavenged products of combustion that occurs with carbureted or EFI manifold
injection systems. Nevertheless, conventional wisdom tells most of the critics
of the two-cycle engine that it will never be as clean as a four-cycle engine.
For this reason they would eventually advocate banning the two-cycle engine
from the waterways on environmental protection grounds.

There are even more manufacturers producing four-cycle outboards, including the
same manufacturers that make the DFI two-cycle engines. They must seemingly
follow down both roads for self preservation, as part of the outboard market is
definitely leaning in the four-cycle direction, driven that way by hype,
environmental concerns, and certain perceived advantages. We have already
considered the ramifications of the increased engine weight for the
four-cycles, the potential effect on boat trim, and the possible inability to
float the boat level when swamped, as required by federal regulations for
outboard boats less than 20-feet long. Then there is also the increased cost
and complexity involved with four-cycle power, to be offset by savings realized
in fuel consumed and elimination of smoke and oil slicks.

This may be the price of progress, they say. But, is it possible to "have your
cake and eat it, too?" Some recent tests run comparing 2002 model two-cycle DFI
outboards with four-cycle outboards of equal power rating, mounted on the same
boat, would seem to indicate such things are really possible. Comparison tests
of two brands of four-cycle 225-hp outboards were made with a current state-of
the-art DFI two-cycle 225. On identical 20'7" boats one four-cycle brand
produced a best mileage of 4.7 mpg at 27.7 mph while the two-cycle gave a best
4.5 mpg at 28.6 mph. Very close. But, the two-cycle had a top speed of 59.8 mph
against 52.4 mph for the four-cycle. At the same 52-mph speed the two-cycle
gave better mileage to the tune of 3.2 mpg to 2.7 mpg for the four-cycle. The
two-cycle produced better fuel mileage at every speed from 34 mph up and was
also better at trolling speeds of 4-7 mph.

When tested against the other 225-hp, four-cycle brand on identical 24' boats,
the DFI two-cycle again prevailed overall, delivering a matching best 3.15 mpg
at 32 mph. This outran the four-cycle 49.3 to 45.7 mph, getting better mileage
(2.58 mpg) at its top speed than the four-cycle (2.44 mpg) at its top speed. It
also produced far better mileage in the trolling speed range from 3.5-8 mph.

A third set of tests compared a 135-hp, two-cycle DFI outboard against a
130-hp, four-cycle outboard on identical 20' boats. The two-cycle delivered
4.25 mpg at 20.8 mph against a best 3.97 mpg at 20.4 mph for the four-cycle.
Best economy for the two-cycle was achieved at 27.9 mph: 4.45 mpg. It also
bested the four-cycle in the 3-8 mph trolling speed range and beat it in top
speed 43 mph/3.54 mpg to 37 mph/2.97 mpg.

"Bah, humbug!" you might say. But there are sound engineering internal
combustion engine principles for this surprising result. It is true that the
typical four-cycle engine may have an inherent advantage in fuel consumed per
horsepower. But not when the engine must be designed to produce very high
horsepower per cubic inch of displacement at high engine speeds, as it must to
achieve even the already heavier weight seen when compared to its two-cycle
competitor.

In order to achieve this high-power output, while firing only every other
revolution of the crankshaft, the camshaft valve timing must develop
considerable overlap between intake and exhaust valve openings and closings,
which means it begins to suffer some of the same raw fuel loss out the exhaust
problems as the carbureted or manifold injected two-cycle engine. It only has
manifold injection, so the fuel and air must mix in the manifold and enter
together past the intake valve into the combustion chamber while the exhaust
valve is still partly open. The result is Some loss in fuel economy.

Since the four-cycle engine has the same radical valve timing at low engine
speeds, it suffers even more when compared to the two-cycle DFI engine at
trolling speeds. The only way to fix this problem in the four-cycle engine is
to go to direct fuel injection into the combustion chamber after the valves
close, like the DFI two-cycle, or have a system providing variable valve timing
with engine speed, conservative timing at lower speed and radical timing at
higher speed. Such systems are now being developed for future automobile
engines. Such things would add complexity, cost and weight, to an already more
expensive and heavier product.

Then there is the factor of acceleration from idle to planing speed. On the 241
boat the 225-hp, two-cycle DFI went from zero to 150 feet in 7.06 seconds while
the four-cycle took 7.76 seconds. On the 20' boat the 135-hp, two-cycle DFI
went zero to 150 feet in 6.2 seconds while the four-cycle took 8.7 seconds for
the same distance. Acceleration from zero to 30 mph on the 20'7" boat for the
225-hp two-cycle DFI took 5.77 seconds compared to 10.7 seconds for the 225-hp
four cycle. This demonstrates the better low-end torque and fast-rising power
curve of the two-cycle, firing every revolution of the crankshaft. The
four-cycles are quieter at low engine speeds, but this advantage goes away at
the higher engine speeds.

So, the conclusions are that the state of the art two-cycle DFI outboard can
match or beat the four-cycle in fuel economy, top speed, and acceleration. What
about exhaust emissions, which brought on the whole move to four-cycle
outboards in the first place? These two-cycle engines can match or beat the
four-cycles there, as well. It matches pretty much with the fuel economy story.
The more fuel the engine consumes at a given boat speed, the more exhaust
emissions that come out the other end. With precise microprocessor control and
direct injection of the fuel into the combustion chamber after the ports close,
the two-cycle DFI can better the most stringent exhaust emission requirements
now proposed out to 2007. The four-cycle can do no better.

After more than five years of testing and field experience the 2002 two-cycle
DFI outboards have been developed to have quality durability, economy and
environmental friendliness to match or beat the four-cycles, and at lower
weight and cost. Both can exist and be successful in the marine market but no
one should sell the two-cycle engine short on its ability to survive and
prosper long into the future. It just has too many good things going for it.
You might even see it on some future stern drives.

Ralph Lambrecht is an engineer with more than 50 years of experience in the
marine industry and marine safety standards development.

Lambrecht, Ralph. 2002. â€œTwo-stroke conventional wisdom.â€? Boat & Motor
Dealer. April. 34-37