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Phantman wrote:
I've always wondered why the designers
didn't just get the cooling water through the bottom of the boat
like
any normal straight inboard setup.

Jim wrote:
I suspect that the engineers felt it
unnecessary to reinvent a proven design

JamesGangNC wrote:
The inside water pumps are basically the same design rubber vaned
pumps. All of them will self prime if needed and the distance is not
far.

Jim wrote:
I don't think the internal pumps are self priming. I would consider
changing
my mind on that if I could see some proof.

Phantman wrote:
Are you familiar with inboards? I don't mean sterndrives. I mean
proven design straight inboards that have been around since long
before sterndrives were dreamed up (and still common everywhere). They
get their raw water through the boat's bottom via a thru hull fitting.
Whether or not they use a standard automotive pump or a special marine
design that's self priming, I'm not sure. But whatever it is, it sure
looks like a standard auto water pump and bolts right into place.

Jim wrote:
Rick, the pump under discussion is the raw water pump that brings water
into the boat, not the circulating pump.

Phantman wrote:
Well, lets get on the same page then. My question was, "why wouldn't
the designer of a sterndrive use the same less complex method of
cooling water intake that Inboards have always used (and still use).
It's a time tested and proven design, no hauling the boat for impeller
maintenance, and less expensive to build. I see no advantage to their
more complex, more difficult to maintain design. That's not to say it
doesn't work at all, obviously it does. But it's one of several
complexities of standard sterndrive design that could easily be
simplified imho.

Jim wrote:
Who knows why they designed the IO the way they did. I don't know if it's
more complex. The main difference is the IO makes 2 90 degree power train
turns vs the outboards 1 turn.
Cheaper to build, maybe? Less work and expense for the boat builder(no
bronze thru hull, screen, hoses, sea strainer, etc.)
How would you simplify the design of the IO?

Most importantly, I would eliminate the boots. All of them. Which
means I would have to start over from scratch with a design. This
forum probably isn't the place to get into that. But eliminating the
outboard impeller and transom intake system would be a step in the
right direction as far as I can tell. I've asked this same question
of mechanics, engineers and some fairly knowledgable people, but so
far I haven't found anyone that can explain why the outboard impeller
is a better idea. I'm sure I'm not the only one who's thought about it
though, including whoever designed the I/O in the first place. I wish
I knew what he was thinkin'.

Rick
Staright inboards also have a raw water pump. It is just mounted on the
engine block and driven by a belt like the other accessories. I think you
are confused about how inboard engine boat cooling systems work.

All of them have a pump that supplies water from the outside. It is a
rubber vaned. In merc alphas it's on the outboard leg. On the rest it's
usually mounted on the front lower side of the engine. The impeller needs
to be replaced from time to time because if wear. They also have the
conventional automotive type recirculating water pump mounted on the upper
front of the engine.

The thermostat is more complicated on a boat. Rather than simply blocking
the water it switches the paths. When the engine is cold the thermostat
allows water in but causes it to continually circulate inside the block.
That circulation is done by the original automotive pump that is mounted on
the front of the engine. The rest of the water supplied by the raw water
pump is diverted to the exhaust system. When the engine is hot the water
from the raw water pump goes into the engine and then goes to the exhaust.
That way no matter what the position of the thermostat new water is always
going to the exhaust.

The original design is because the early i/os really did use the lower half
of an outboard. Early big merc outboards actually have some parts that are
interchangeable with the early i/o legs. As to the reason, it let them
build a boat with some of the advantages of an outboards but without the
finicky, easily damaged, aluminum 2 stroke engines. Instead they used
cheaper existing automotive cast iron blocks. The automotive block is
engineered with a lot of excess strength. It can stand more abuse or lack
of care than an aluminum 2 stroke. Outboard engines run a lot closer to
many of the theoretical limits of the engine. That lets them have a much
better hp/weight ratio. But it also means that mistakes and problems don't
have to push them far before they break. Outboards are much better for
trailering and the ability to trim the prop position and use the prop for
steering are advantages over straight inboards. Those are the features that
they were trying to leverage with i/os.