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Briggs & Stratton Lawnmower/Go Cart Engine to Power a Boat Answered

I've heard of people mounting small (5 hp or so) Briggs engines in boats to power them. I have a couple of warn out 2-stroke outboards, but they are smelly, smoky, and unreliable. Does anyone know the best way to mount a Briggs (or Tecumseh, Robin, etc) as an inboard? I know there'd have to be shafts and seals, but I'm kind of looking for ideas before I start cutting on the old flatbottom. I have a couple of vertical shaft motors, and one good horizontal.


You could do it, using a sealed shaft and bearing, but it mightn't be the best idea, you'd still risk buggering the hull. You could take the transmission axles of an outboard, run the engine from inside the boat by meshing two transoms together to make a big upside down U what would be simpler though is to simply mount it like an outboard or if you prefer fixed prop boating you can mount it permanently at the back like an outboard that doesn't turn, it's just easier and a lot less risky than attacking the hull, unless you find a scrapper with the transmission shafts in place, take the bearing and seal assembly and seal it in to your boat...

I may go with the outboard route. Everyone seems to be against running it THROUGH the transom. The are probably right. I'd hate to ruin a good boat on this project.

Actually if the transom is flat and not made of any really exotic materials you could just risk it, it would be a cooler project, it wouldn't be any harder to get the parts, or if the mood struck you make them... I have seen boats with an outboard style assembly that goes through the bottom spine, for deeper water, they handle nicely, because the thrust is a bit further forward you can turn in your own length...

Transom's flat and made of aluminum. I could patch any unwanted holes pretty easily. I could run it a foot out the back so it sat down in the water. A rudder for directional control wold work once I was up to speed. I also have a trolling motor I could steer with, even if the main prop is fixed.

. IIRC, most inboard designs go through the hull, not the transom. I/O's go through the transom to a "lower unit." . If you go through the transom with a straight shaft, it seems to me that you will have to put the prop so far behind the boat (to get it in the water) that you will introduce all sorts of steering and stability problems (you'll have a lot of leverage). You will also need to mount the engine at an angle, which may (or may not) present a lubrication problem. . But if you can weld Al, then none of that should be a big problem to fab or repair.

I'll look into that. I gues mine would be more I/O. It just seems like, once it was done properly, it sure would be nice to run the thing on a cheap, non-smelly, (reasonably) quiet engine that I can actually repair while sitting on the river-bank. I could even replace it with a brand new one for like $275. A LOT cheaper than a regular boat motor.

My dad made a fanboat out of a 250cc bike engine, from his still in use bike, he got a spare prop from a plane... Motorbike engines aren't half bad for the purpose, you keep the revs down a bit... Unless you have a tiny propeller...

Hmmm, I do have a 185 Honda engine from an od trike. Starts everytime and runs well.

That'd have some more grunt to it and may well be more reliable, plus you could keep the gearbox, so if you felt the need you could get more speed... The other big plus would be having a self contained motor...

I actually have a Jetski I could sacrifice if I decide to get serious! I thought about grafting it into the rear of a canoe.

I hope it happens!  I've still got the jetskis and canoe, but never have the time to tinker!!

Take the impeller assembly and hook it up to something more powerful.... make a super jetski... Sorry I'm just thinking of what I'd do with all those things lying around...

I just have so much junk, and sooo little time. (I'm looking at a new job that may solve that.) My kids sure do enjoy the benefits of living with a mechanical-packrat!

Hmm I had the benefit of having a job in the work yard where the skip guys where, I once found a big red puegot 406, 2.0L with tax I made money by selling the tax back to the DVLA.... It was a bit rough on it though, skittery comes to mind... However mechanical pack rat....

I just aquire projects MUCH faster than I can finish them. And much,MUCH faster than I can dispose of the leftover spares.

Ah right, that's what my drawer is for, that and the carport usually gets a wreckage clearing once a month...

Actually you could go with twin rudders either side of the prop, about the 3/4 marks and one in behind the prop, lined up directly and covering an inch of length above and below, that'll give you pretty nice control... I have both sailing and power boating experience... The best one being making on the fly mods to a renta-boat in wales, it had a simple screw on lockout to stop you going over 1/4 throttle, I had a leatherman, turns out they're not that wimpy but they're not the easiest to handle at full kilter...

Thanks for the double rudder idea. If I go through with this project, I'll definitely go that route.

it's also very easy to engineer. Its a bit like a rack and pinion... The rudders have two guide bars inside the hull, these are attached together with pivots and can be turned any number of ways, usually through rack gears, for using a steering wheel.

Just so you know, I've heard that gravity fuel fed motors are illegal in boats in the US.

I guess you could use a motor to run a alternator and power your boat with a trolling motor, might not be as quick, but if you where careful it could be very quite...

Interesting, I have like 4 trolling motors. That would be fine for flat water and slow rivers. I did have an inadequate motor disaster a couple of years ago, so now I'm a little leary of being underpowered.

You could use a larger electric motor, there is lots of 24 and 36 volt models now, some are several HP... (( I just seen a 3/4 HP 36 volt used selling for only $25 so they can be had for fairly cheap )) And what was your disaster ??

You remember Gilligan? "A three hour tour"? A friend and I put in for a couple of hours of trout fishing. The water was high, and the crappy 2-stroke wouldn't start. We were already out in the current, and the trolling motor wasn't strong enough to get back to shore. We washed through a bit of whitewater over a low-water dam, through some trees, etc. Fortunately, I had a cell phone and called my wife. She picked us up late that night in the next town. We were starved, scratched, sunburned, and a lot wiser!

Ya that is something old 2 strokes are bad at is crapping out when you need them, thats the only time my chainsaw stops working, is when there is tree on the road, but any other day you could cut down a forest..... But I think a 3/4 HP electric motor should have enough power, ok I won't say enough, people are pretty crazy, I know I like more the bigger the waves get .. But it has more power then a weed wacker engine has and they should have more torque if you had a few deep cycle cells on the boat then even a 5 HP Briggs and Straton...

In that case I suggest taking an old V8, souping it up and running jet impellers...

Really? I can see why, but I'd never heard that. They used to be plentiful (I still have a couple), but I guess times change.

I read it in a George Buehler book (the yacht designer), I've done the weed whacker motor mod myself. It's only illegal if you get caught ;-)

. Did it say why? I'm having a hard time imagining why gravity feed would be any different.

Could be the cut-off. Free flowing petol, above heat and flame, makes for "How not to" instructables. There's a right and a wrong way to do gravity feed, and I'm sure some legislators kid did it the wrong way.

It might also be an issue with the Oil you mix with the fuel, I know you shouldn't use outboard or ashless oil in weedwacker, but then again you don't run your exhaust into the water so who knows ... just a though ....

BTW- I'm pretty sure it's not illegal in al states, and probably 'grandfathered in" on old motors.

Lord, it was a decade ago ;-) Shoot, someone just call the Coast guard, they'll know.


10 years ago

It might be a lot of hassle but could you mount a horizontal shaft engine onto the "transmission" of the old outboard? Presumably that has its own way of taking a shaft rotation from an above-water engine and powering the propellor. Unless the original 2-stroke is vertical shaft, in which case... use a vertical shaft :)

The original 2 stroke is vertical, so probably the shortest method is to graft a vert-shaft briggs onto the existing bottom-end of the old 2-stroke.


10 years ago

Typically in the third world you see non-marine engines (air cooled) and shafts mounted above the transom, at a steep angle (see photo.) This one looks coupled in some way, but I've seen the motors directly on the shaft.

Here's a similar commercial design.


I saw something like that in the canals of Bangkok years ago. They had a NISSAN (out of a car!) engine on a swivel with a long shaft out the back. Now there's a commercial engine like that. It's built for running in the shallows. I believe they are called "go-devils" or something.

I knew they must have a name ;-) It's got "simple" all over it, at least...

And if you used a small automotive engine, you could cool it with the lake water, so you lose the fan, and rad... (( now I'm wondering how much of a rooster tail you could make with the water pump on a engine ??? ))

??? Inboards are water cooled, too--something similar? Did they fix the Nissan engine, and use a universal joint for the shaft? The smaller one's I've seen, the whole engine pivots...

The whole engine pivoted! They had some real slick bearings underneath. It was impressive to watch a guy swiveling a car engine around in the back of the boat.

. Unless you plan on going with an inboard or i/o arrangement, I don't think the horizontal shaft will do you much good.
. I think I'd try to find the shaft/lower unit (with the transom mounting gear) of a small outboard (something like a British Seagull ) and adapt my motor to that.

I thought about that, and may go that route. Briggs actually makes one like that now. I could attach one to the bottom end of one of my worn-out outboards. I'd heard though, of people mounting it inboard, with a a low prop at the back, folloed by a rudder for direction. Any thoughts?

. I don't like the idea of cutting holes in a perfectly good hull. Seems to me that it would be fraught with trouble. But that's just me. . I would think that a "directional" prop (a la outboard) would be more maneuverable at slow speeds than a "fixed" prop and rudder, but that's just a gut feeling.