Homemade outboard

[weldbilt17]

Re: Homemade outboard

Chaindrive. Interesting. I was thinking of running direct from the crankshaft to the driveshaft. I have several old 5.5 - 7.5 hp midsections with full shift lower units I could adapt to it.

This might be a fun winter project. I'll give it some more thought.

My first tought was a direct drive to, a little less to deal with than the chains and gears, but the reason I tried it with a chain setup was the gearing potintial. I really don't think it helped a hole lot but I put a little larger gear on the motor than what I put on the lower shaft hoping that it would give it a few more r.p.m's than the motor,but the motor never opened all the way to full throttle I guess just not enough horses for that lower unit, but I'm not giving up yet thogh THANKS.

 

[weldbilt17]

Re: Homemade outboard

The speed sounds about right for the 8. You may get a couple more mph with a smaller h.p. lower unit by minimizing drag and raising the rpm. I'd guess a lower unit in the 6 to 15 h.p. range would work well. But remember the prop won't ever turn the rpm it did with the outboard powerhead, the rpm of the aircool is far less than that of the outboard.

Good luck! Fun tinkering on these things ain't it?

max!

Loads of fun fiddling junk, my favorite thing to do. I was hoping that the smaller lower unit would let the motor get to full r.p.m.'s. The way it is now it never reaches full trottle because of the load it's under, I guess kinda like a mower bogging in tall grass. I'm afraid that it will lead to the demise of the little briggs do to overheating. But I'll keep tinkering and you do the same .

 

[weldbilt17]

Re: Homemade outboard

i have a spare r1 motor that i want to rebuild and put on a boat...150 hp at 11,000 rpms would be FUN!

A V-DRIVE jon boat sounds cool.

 

[swordfish25]

Re: Homemade outboard

Did you try putting a smaller prop on? If it is the prop for the 35 it would be too big.

 

[weldbilt17]

Re: Homemade outboard

Did you try putting a smaller prop on? If it is the prop for the 35 it would be too big.

No I haven't tried a smaller one yet,that one is the only one I have at the moment, but I may try one a little later if I don't go ahead and get a smaller lower unit. THANKS.

 

[Home Cookin']

Re: Homemade outboard

not to hijack for total redesign, but consider the "go-devil" configuration that eliminates the lower unit; the prop is on the end of a straight shaft and the motor is on a pivot. The home-made ones I saw in Thailand had small-block automobile engines on them! Loud, fast and dangerous--a trifecta for afternoon male fun!

 

[rvbilly]

Re: Homemade outboard

Did you ever get the smaller lower unit ? If not try regearing the sprockets under the motor to allow the engine to get more RPM`s , I have a lower unit from a 4 HP Johnson and a spare 3.5 HP Briggs I`m gonna try rigging up for a trolling motor , I think it`ll be close to perfect , I`m also thinking of running a exhaust pipe from the Briggs down into the water , would still give it a bit of back presure and be quiet , as long as it doesn`t suck up water in through the valves when the engine is shut off ?

 

[ebentraywick]

Re: Homemade outboard

heres my home made outboard getting closer 14hp brigs v twin http://www.youtube.com/watch?v=M6Wbe2dsltY

 

[Mark42]

Re: Homemade outboard

Generally, the riding mower motors are governed to run max efficiency and HP at about 3200 - 3500 rpm. Rather slow for the props/lower units designed to run with 5000+ rpm motors. I bet less pitch on the prop will work well with the Briggs and Tecumseh motors or use a belt drive to create an over drive condition, like a 1:1.5 ratio to get the leg turning at outboard motor speeds. HP will need to be increased accordingly to compensate for the overdrive.

 

[rvbilly]

Re: Homemade outboard

Alot will depend on what HP the bottom leg was from , a 10 Hp lower unit would work good with that 14 Hp Briggs , I think with say a 15 Hp lower unit the engine is still going to turn it decent but only up to the governed 3600 Rpm their set at , would not be able to gear it up to get the 5000 Rpm the original 2 stroke normally runs at for correct prop speed , the other down side to the Briggs compared to the 2 stroke is the weight difference , I think that 14 HP is going to be quite a bit heavier , lol , I`de still build it though , I have a 4 Hp lower unit from a Johnson and when I get some time will put my spare 6.5 Hp Briggs on it , Bill

 

[ebentraywick]

Re: Homemade outboard

Generally, the riding mower motors are governed to run max efficiency and HP at about 3200 - 3500 rpm. Rather slow for the props/lower units designed to run with 5000+ rpm motors. I bet less pitch on the prop will work well with the Briggs and Tecumseh motors or use a belt drive to create an over drive condition, like a 1:1.5 ratio to get the leg turning at outboard motor speeds. HP will need to be increased accordingly to compensate for the overdrive.

the lower unit is a jounson 18hp the mower i have gone thru the hole thing and tooken off all the goveners when it was on my racing mower. but i blew my tranny on the mower so i hade this engine so i will see how it works..

 

[ebentraywick]

Re: Homemade outboard

Alot will depend on what HP the bottom leg was from , a 10 Hp lower unit would work good with that 14 Hp Briggs , I think with say a 15 Hp lower unit the engine is still going to turn it decent but only up to the governed 3600 Rpm their set at , would not be able to gear it up to get the 5000 Rpm the original 2 stroke normally runs at for correct prop speed , the other down side to the Briggs compared to the 2 stroke is the weight difference , I think that 14 HP is going to be quite a bit heavier , lol , I`de still build it though , I have a 4 Hp lower unit from a Johnson and when I get some time will put my spare 6.5 Hp Briggs on it , Bill

yeah the rpm are way difrent now bechase there is not govenners. i will keep u guys updated

 

[rvbilly]

Re: Homemade outboard

>>>>>>>>>>>>>>>> Cool <<<<<<<<<<<<<<<<<<

 

[Mark42]

Re: Homemade outboard

As life gets busier with kids and all, I miss the time I used to spend making hot rod lawn mowers, putting 18 hp briggs on mini-bikes (also made a twin lawn boy 2 stroke motor mini-bike. too scary for words), and lots of other fun and totaly socially acceptable machines.

I think you have inspired me to get this old modified 1970 Rupp go-kart out of the shed and fix the blown differential:

BTW, the mini-bike has a torque converter and 8hp and does about 35mph. With those tires, it climbs anything.

 

[ebentraywick]

Re: Homemade outboard

As life gets busier with kids and all, I miss the time I used to spend making hot rod lawn mowers, putting 18 hp briggs on mini-bikes (also made a twin lawn boy 2 stroke motor mini-bike. too scary for words), and lots of other fun and totaly socially acceptable machines.

I think you have inspired me to get this old modified 1970 Rupp go-kart out of the shed and fix the blown differential:

BTW, the mini-bike has a torque converter and 8hp and does about 35mph. With those tires, it climbs anything.

looks like fun i have so many projects check out my youtube channel http://www.youtube.com/user/ebentraywick

 

[stingertrey]

Re: Homemade outboard

OK im completely new to this homemade outboard thing but it looks so fun.. I have an extra old 5.5 hp evinrude lower end from the sixties.. I parted it out to finish restoring my other 63 evinrude just like it.. But Im thinkin I should try to put a mower engine on top of the extra leg.. How exactly do you mount the motor to the leg.. I understand using a coupling and pins of some type to attach the shafts. but You made a plate with the bolt patterns of the lower end.. and it also mounted the engine?

 

[Mark42]

Re: Homemade outboard

OK im completely new to this homemade outboard thing but it looks so fun.. I have an extra old 5.5 hp evinrude lower end from the sixties.. I parted it out to finish restoring my other 63 evinrude just like it.. But Im thinkin I should try to put a mower engine on top of the extra leg.. How exactly do you mount the motor to the leg.. I understand using a coupling and pins of some type to attach the shafts. but You made a plate with the bolt patterns of the lower end.. and it also mounted the engine?

Looks like these guys took a 1/4 " aluminum plate and bolted it to the lower unit then the motor bolts to the plate. You will have to drill the mounting holes carefully. And some spacers will be needed to get the mower motor to be positions right. Also a flexible coupler will help mate the two shafts. Look at mcmaster.com for shaft couplers.

 

[stingertrey]

Re: Homemade outboard

Ok thanks man.. Im gonna start tinkering with somethin like this asap lol I'll post pics if I ever get this completed

 

[maxum247]

Re: Homemade outboard

I've built one outboard useing a small push mower engine and it would kick back hard and not start when trying to pull start. My brother told me to put a cast iron flywheel on it and that solved the problem in that case.

My question is: Is this a concern with any combination of pull start push lawn mower engine's or might it have just been the piticuliar combination of horsepower and lower unit that I used to build it, anyone know?

The two electric start versions I've built don't seem to share this problem and start fine as are. Thanks!

 

[windsors03cobra]

Re: Homemade outboard

I've built one outboard useing a small push mower engine and it would kick back hard and not start when trying to pull start. My brother told me to put a cast iron flywheel on it and that solved the problem in that case.

My question is: Is this a concern with any combination of pull start push lawn mower engine's or might it have just been the piticuliar combination of horsepower and lower unit that I used to build it, anyone know?

The two electric start versions I've built don't seem to share this problem and start fine as are. Thanks!

The blade acts as a flywheel there and adds some mass and momentum to the light engine in those cases.

Guy by my father in law has the Go-Devil setup like I see being used in craphole countries. This particular one what looks to be a 20 horse Briggs V twin on a Camo'ed jon boat. Probably gets some skinny water with that home made drive.
Also seen the Briggs & Scrapiron mounted onto a normal outboard lower unit, did a double take on that. Some crafty folks out there.