How to make snowmobile engine work in a small 11 ft. runabout?

[rvBilly]

I have a small 11 ft wood epoxy runabout built for speed, I want to use a 500cc snowmobile engine inside of boat, I was thinking of building my own stern drive from the lower leg unit of a seized outboard of 40 hp. Would the prop size yeild me a speed of aproximately 40 mph cause this my goal, the hull weighs about 120 pounds+ 325 pounds of passenger,and engine of about 55 pounds, this just a project but I feel it should work, can I run the exhaust right into the water without damage to the engine? the engine has about the same hp as the 40 hp outboard, does anyone have imformation on any other such projects of this type, I`m having problems finding any imfo on the net, I thought I could mount a bracket system on the lower leg, with a gear and shaft setup to run through the transom with a direct hook up to the engine. thanks for any help anyone may have Bill Pickering BC Canada.

[jimslade]

If its air cooled it will never work. if it is liquid cooled you could do it but it would take alot of messing around. The engine would have to be rejetted, its way too rich for mild temps I would look for a cheap 50 hp you can find one for around $200 bucks today.

[rvBilly]

If it is air cooled why would this not work? I`m in Chilliwack BC and a cheap 40 or 50 hp outboard here is no less than $1000 or more, I have the snowmobile engine allready and if it needs to be rejetted that`s cheap enough, it will be a matter of couplling the engine to to a drive unit and exhaust, this is just a toy and stranger things have been built..

[s.marsh]

I think it would work.The snowmachine engine will run in any position, even upside down if the carb is mounted so the float bowl is level. I've built a few similar projects. see attachments.

[rvBilly]

First picture, what engine is that you swaped on it? Second picture, is that a Briggs and Straton engine? What hp and how fast would it get that boat going? What size of outboard engine was the donner leg setup from.. did it have any problem stalling out, thanks for your reply Bill.

[jimslade]

If it's a small engine than it will work but a snowmobile engine of that size should produce 40 hp plus. thats alot of energy to cool.also you have to set up the exhaust.thats why the only aircooled outboards are no larger than 5 hp.

[rvBilly]

Thanks for your input, I have just recently let the sled run outside for 3 hours in 84 degree F. to run the fuel tank empty, did`nt overheat but of couse thats with no load on it, I thought I could run the exhaust down into the water, what do you think? Bill

[Baldur]

I have a old arctic cat 500 motor that I have though about doing the same thing with. You could keep the centrifical clutch . If you have the old cross drive shaft from the snowmobile you could use that as your prop shaft. The only problem might be proper gearing and prop choice. An air scoop that directs cool air into the cooling fins should keep it from overheating. Snowmobile engines can run fine at some pretty warm air temps. I run my air cooled 440 all day long in the spring snow when temps get well above 60 or 70 with only minor jetting changes. (1 jet size),

[s.marsh]

The first motor is a continental 2.5hp lawn mower engine on a martin 6 lower unit.It runs fine but is a little slow, and doesn't burn much gas.The second is a clinton 3.5hp with a 4 1/2 foot driveshaft.It has enough power to move a small boat without any problems.It needs a better prop.The latest project is a michigan marine 2cyl. in a 16ft. mirrocraft.This is a water-cooled engine but the next time it breaks down a 440cc fan cooled Rotax should drop right in.