Inboard engine with lower unit outboard

[Steve Mitrovich]

I've made a 19' power cat that I want to drive with a 20HP centered motor. An outboard on the back end would be too heavy so can I connect the motor via
shaft/gearing to a lower-end outboard assembly in the back?

 

[FMS]

The lightest outdrive is around 80lbs.
The smallest sterndrive sold was the Mercury Vazer (2007 and discontinued now.) That was an 83lb outdrive coupled with a 377lb 100hp engine.
http://download.brunswick-marine.co...ruiser/2007/gasoline/sterndrive/866327r60.pdf
An alpha one weights 75lbs plus the 80lb transom assembly and trim cylinders plus trim and tilt pump.

 

[Steve Mitrovich]

Inboard motor to drive lower-end outboard.

Assembled units are big, powerful and heavy. What I was thinking of is getting a lower-end of a 20-25 hp outboard (plenty for sale on e-bay) with a long drive. Then connecting that drive via gears and shaft to the center mounted motor. From what I have seen, the lower-end of a 20-25 hp outboard weights about
35-40 lbs (of course I have to make a structure to hold it at the location that I want).

 

[Mr Efficiency]

That sounds like a major exercise, if the boat can't support a 20hp outboard at the stern, it sounds frankly like a dud boat. The time you complete this you will finish up with something no lighter, and don't forget it has to steer !

 

[FMS]

Do you want it to trim/tilt?
Do you want it to also steer?

 

[Mr Efficiency]

It is a completely impractical idea to build a functioning reliable unit from scratch. There is way too much involved, and since a 20hp outboard is quite light, what is the pressing need ?

 

[FMS]

I agree. The easiest solution is to move (yourself) forward a small amount to balance a small boat.

A tiller extension can be used.

Move the fuel tank forward (even though its weight is reduced as fuel is used.)

Move the anchor or any other heavy provisions forward to the bow.

A 20hp Mercury 4-stroke weights 115lbs. Adding small buoyancy pods on the transom could be an easy option.

You could build a light version of Tom Kayne's pivotal drive if you are comfortable in a machine shop.
http://www.boatdesign.net/forums/surface-drives/pivotal-drive-subsurface-surface-all-one-37539.html
Or this one:
http://www.boatdesign.net/forums/at...36966-bob-drive-propulsion-uk-patent-1967.jpg
It could be simplified if you only need to trim it up manually to go in shallows or beach and can do steering with rudder.

 

[PAR]

Welcome to the forum Steve.

A 19' power cat should be more than capable enough to hold a 20 HP outboard, assuming it's a real powerboat, not an attempt to convert a beach cat, which doesn't have the volume to support an outboard on it's transoms.

Let's level the playing field a bit, can you post the make, model and year power cat or the plans you're working from? This will let us know what you have to work with.

An inboard on a power cat has some inherent issues to contend with, most not easily solved, especially if light weight is a primary goal.

A simple solution might just be mounting a 9.9 HP outboard one each hull. Typical weights will be about 60 to 70 pounds each. Again, a power cat of this size should easily be able to support this configuration, if it can't not much of a powerboat is it.

Lets find out what you're working with and go from there.