BoatBanter.com - View Single Post - pontoon boat outboard question

wrote:
Hi all,

My brother is building a very small houseboat based on a 24' pontoon
boat.

I have calculated the load capacity of the pontoons and come up with
3000 lbs maximum.

The boat is fitted with a 1970 era outboard motor, from the model
number a 60 hp johnson 2 stroke.

This motor was off a small ski boat.

Given he wants to cruise along the Mississippi river is there a better
option to power the thing. Estimates of a cruise speed of 10 knots for
4 or 5 hours a day are being considered.

I wouldn't go much below 60 hp for that rig, unless you go to a high
thrust 50, like a Mercury "BigFoot" model. You will use about the same
fuel at a given speed as a smaller engine, but will have extra overhead
& less W&T on the larger unit. That year 60 hp has the larger V-4 size
gearcase, so it is a good candidate in that regard. The biggest problem
I can see is that all those old 60's are getting tired by now, they are
prone to rod failures in their old age. The parts for the
Hydro-Electric Drive gearcases are, of course, getting harder to find too.

Any comments are appreciated.

specifically - would a 20 hp do the job ?

I wouldn't do that in the river. You might be looking at a one-way boat
at times with the currents.

- should he look at changing the propeller for maximum low
speed operation.

That era 60 3 cylinder(~265 lbs.) should be propped for 5000 rpm @
WOT(4500-5500 rpm range - rated 60 hp @ 5000 rpm). That will give the
best balance of economy, speed, longevity & maneuverability. A 15 inch
pitch will probably come pretty close... a 13" might overspeed at WOT.

He is considering a second outboard for backup anyway.

A high-thrust "sail" type 9.9 or 15 might do the job for backup.

Rob