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Old September 30th 10, 07:09 PM posted to
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Default Converting a 30' Pontoon from outboard to I/O

In article , keillorp135 says...

On Thu, 30 Sep 2010 08:02:06 -0700, "Bob La Londe"

wrote in message
.. .
On Wed, 29 Sep 2010 21:01:12 -0700, "Bob La Londe"

wrote in message
On Wed, 29 Sep 2010 09:16:44 -0700, "Bob La Londe"

"Tim" wrote in message
I went to a high school homecoming game/reunion last night and had a
blast. I ran into my cousin who I had gone out on his pontoon boat
last week and we did some talking about doing a river run next year. I
have previouslygone out with he and his family on the
Wabash river launching in at Vincennes In. I find out that when the
river is up you can go all the way up to to Hunnington and a bit
beyond which is near Ft. Wayne Indiana

Vincennes is the nearest launch to the Wabash for the both of us, and
if you consider Vincennes to be "0" on the miles, you can consider
that to make the full run north would be approx. 200 miles. We're
very well considering doing that with the pontoons next year. It would
make the perfect "mother ship" to carry all the stuff necessary to
make a really enjoyable run. Besides, as you can see by the map,
there's several ramps and marinas along the way. So if need be, stuff
can be had with out a great inconvenience.

We also talked about power. Making a run like that with a 1977 Johnson
V-4 would be fine if I hauled a tanker behind it because it loves to
drink the gas. I have in the past bought a couple of clapped out
boats to salvage the mercruiser engines and alpha drives in case I
needed them for future use for my 18 ft. Chris Craft. I know that the
ratings are different and the 85 Sea Horse presently pushing the
pontoon actually would have more torque than a 120 hp GM 3.0, but at
the sacrifice of double the fuel consumption, if not higher. Besides,
I'm more versed on working on an automotive style 4 cycle than a 2
stroke. And I think that eventually by force or by choice, the old 2
strokes are going to be fading out really fast in the near future, so
may as well entertain the thought of doing a conversion this winter.

He said he has a friend in Mt. Carmel who has a 28 ft. Harris who had
his converted over a few years ago, and suggested I talk to him about
his. He did tell me that a local welder in Mt. Carmel has done a good
job at converting several over to even putting 350 GM's in them!
Besides his shop rate being very reasonable, he's well versed in steel
and aluminum fabrication so that sounds like a good recommendation.

Then with the 3.0 GM I can outfit the charging system the way I want
to power the options I want as well, instead of relying on the 10 amp
stator 'battery charger' on the v-4.

Consider this. Are long runs like this going to become a regular (2-3
a year) deal or is this a one time adventure? If it's a one time
the expense of the conversion may out weigh the savings in fuel.

I have no clue what (please hold the jokes about me being clueless until
end) what your fuel economy is with that rig, but given you are pushing
toons, and not a high performance planing hull, I am betting at optimum
throttle settings it will get 2-3 MPG up current. This can vary
and a test run or two might be in order. Figure at 2 MPG it will take
gallons of fuel to make the trip. Go with 125 due to inefficiency and
trips to make it a little more fun. Even at $4/gallon (marina prices)
that's only $500 dollars worth of fuel to make the trip up and less to
the trip back. Figure $900 - $1000 for fuel. Add $100 - $200 for oil
misc. Still seems cheaper to me than a complete retrofit and change of
propulsion system, and that's with a cost safety margin.

Now what might be a good investment is a fuel usage system so you can
compare RPM throttle settings to speed on the water and fuel usage to
the optimum speed to make good progress. Basically the sweet spot in
RPM range of the motor.

Of course if your cousin will go all the costs of the conversion and you
to share the savings in fuel it's a whole different story.

My big concern would be that 33 year old motor. If it runs good, has
compression, carbs are clean and adjusted (not too rich or lean), lower
oil is clean with few metal particles and no water, has a fresh impellor
housing I would probably make the run. If there is anything
about the motor other than its age I might have 2nd thoughts.

Pontoons are basically displacement hulls, like a canoe. They don't go
very fast, but they are VERY fuel efficient. My 5000 pound sailboat is
a displacement hull and gets about 10-15 mpg depending on wind and
current, using a 9.9 hp 4 stroke, high-thrust Yamaha outboard driving
it at about 6 knots.

Planing hulls are only efficient when planing. When not planing they
are bulldozers. Displacement hulls are entirely different from that.

Well based on that then converting his cousins toon would cost more than
or 40 such trips.

My feeling is that he has two basic problems with his present rig.

1) it is a 2-stroke. They burn a lot more fuel than a same sized

2) He has an engine designed for propelling a planing hull on plane at
high speed. For pushing a pontoon boat, you want a big, deep, slow
turning prop, not a little high speed whizzer.

The Yamaha 9.9hp 4-stroke high thrust on my sailboat is geared low
and has a huge prop. Like having my own personal tug boat at all
times. The prop on my motor is probably bigger than what he has on
that old 75 hp two stroke. He's wasting most of his power churning
water without producing much forward thrust.

He doesn't need 75 hp to push that thing at hull speed. And he
definitely doesn't need a car engine!

Remember it's a '77. Its probably rated at the power head, not the prop
shaft like current production motors. It probably produces more like 50-60

Lots of pontoons out there running 50-60 HP Mercs with the bigfoot gear
case. Since I never really had much interest in pontoon boats I've never
really done any research on them, so that's all I have to add.

The thing is if it is a good motor (regardless of how "bad" it is in other
respects), the cost of changing it out at this point will cost more IMO than
several such trips. Now if they use it everyday all summer every year
that's a different story.

Just about every pontoon I've ever seen is overpowered for hull speed.
Pontoon buyers don't want to go 8-10 kts. My father wanted to build
one for salt water and put his used 240 hp Continental radial engine
with airplane prop on it. Never did, thank God. Now that would have
been over powered.

Pete Keillor

Somebody at bought one of these last season iirc. Never
remember seeing a trip report though..

Anyway, have a great day.

OH, I could do the 105 footer, but I would hate to waste the last few
seconds of my life with my eyes closed, screaming like a little