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Hello all. I just recently came across this forum, and I'm so excited about it. There is so much useful information on here, plus plenty of friendly faces. I'm a college student that lives in Southern Louisiana and I just love to fish. For the past few weeks I've been thinking of building an outboard for my family's 12' jon boat so we can access more of our small lakes. I'm thinkng of using our old 3.5 hp briggs & stratton classic vertical lawn mower. Any opinions or thoughts would be greatly appreciated.
I agree with tail gunner, the easyest best way would be to use an exsisting outboour minus powerhead.
Now if your determined to actualy build it all from scratch,, thats where it gets complicated.
If and when you get scans of your concept online ill try to get some more imput for ya. using the outboard lower end is really he best way to go with the motor you want to use
That sounds like fun. You want to talk to Ken for sure he helped me big time with my outboard project, our discussions are located further down the message boards. I used a horizontal shaft 2hp briggs with a belt driven prop. Never tested it on a boat but did test it in a garbage can full of water, it worked pretty well.
Since you have a vertical shaft, maybe consider buying an old lower end with the prop attached and attaching directly. Even better buy the whole lower end with midsection and you might even be able to use the forward/neutral/reversre gears on it. You might want to find out about the wot rpm @ full load of your briggs, the gear reduction of the lower end to determine the prop rpm. Once you have that try to buy a lower end that will allow the briggs to reach its maximum rpm. I dont think you have to worry too much about those motors over revving. I am not sure on this, but I think most outboard 2 stroke engines have a higher rpm at wot then briggs 4 strokes @ wot full load. I also read that an average gear reduction for the lower end is 1.57. for example if your 2hp johnson outboard has a wot at 6000 rpm, then the prop will be spinning at 6000rpm/1.57 or 3800 rpm or I could have this backwards but lets assume its correct. So that particular prop will be consuming approx 2hp @ 3800 prop rpm. If you were to put a 2hp briggs on that same lower end/prop and lets say the max rpm of the briggs is 4000 rpm(usually the 4 strokes have a lower rpm max), the the prop would be going at 4000/1.57 or 2500 rpm. from above if the prop uses 2hp @ 3800 rpm for the 2 stroke engine, then roughly it uses 2500/3800*2 or 1.3 hp for the 2hp briggs engine at wot-so there is leftover horsepower that you are not using because the briggs will not rev fast enough. What this says to me if any of the above is correct logically is that if you use a 4 stroke briggs that runs at a lower rpm, then the outboard lower end you want to use should have come from a 2 stroke motor that had a higher horsepower so that you can use all the horsepower that the briggs has. From above you can maybe estimate the horsepower of the lower end you want to install. using the same rpm ratios(you have to find out the exact numbers from the manual of the motors) above you can say 2500prop rpm briggs / 3800 rpm 2 stroke * lower end horsepower = 2 hp briggs..or lower end hp = 2*3800/2500 =3 hp lower end.
So in order to get a briggs 2hp outboard to deliver about 2 hp at the prop, you should install the briggs on a 3hp lower end/prop. Sorry for going on and on. Let me know what you are going to do and your approach, post your thoughts here and photos would be great. Hopefully Ken will stop in and give some input.
Thanks for the great advice. I've checked out you and Ken's previous discussions and pictures, and it's been a great help to me in visualizing my plans. I'm looking forward to hearing his input also. I do have sketches written out. I just need to scan them, and i'll post them to be seen. I really like your idea of using an outboard lower unit. I just hope i can find a good one i can afford. You're right, I spoke with and friend who's an outboard mechanic, and it's very unlikely for the motor to over rev. I'm trying to figure out the wot rpm at full load, I'll let you know once i figure it out. Then, the lower unit size can be figured out. Thanks for the formulas. You're math looks pretty good to me. You're also right about the 1.57 gear reduction. I'll also be posting pictures as I begin. Since my brother is a welder, I'll let him tackle the welds. I can tack up some pretty uly "buggers" on my attempts :) I'll fill you in with more information soon, as I figure it out. Thanks a bunch!
CC,where are you!?
Lets get this show on the road, we want to see this thing develope and work and are here for you if we cant help in anyway. Get yourself a digital camera and post the progress here, or zip your photo files, email them to me and I will post them for you. Cant wait to see this project.
Wow, you guys are on the ball! I apologize for acting so slowly on my project. I've been doing a lot of fishing with my dad to help him recouporate from several surgeries. Good news. . . it's working :) The fish in our area are really biting right now, and we're enjoying it. I've been trying to locate a small lower end at an afordable price and I'm having no luck. For the price people around here want for a small lower end, I can get a whole outboard for the same price. I'm looking at this working older model 9.5 Evinrude that is affordable to me. Hum, to buy or to build? I'd much rather build :)
I'm also still trying to figure out the wide open throttle rpm at full load for this 3.5 hp birggs. Briggs and Stratton is supposed to contact me with the info. Is there a way I can find out this info myself? Can I hook up a tach to the motor? Sorry, if these questions may sound stupid . . . this is one area I'm not familiar with. One more question guys. How do I post pics on here? Or would it be better just to link the pics to my website. I'll be back soon with the pics! I just got a digital camera :)
Ebay is a great source to find used outboard parts. here are some that I found for $10 to $30,
Briggs does not seem to make thier tech data readily avail on the internet but if you ask enough people you should find out soon enough. Better yet call some repair shops and ask them.
You can get a tach for those motors, its called "tiny tach", I think its like $50 though. You might be ok just to assume 4000 rpm for the briggs and 6000 rpm for the 2 stroke. If you do for your 3.5 hp it would be 3.5*3800/2500 = 5 hp lower end from the earlier calcs if they are valid. The way I see it is that you may not be able to buy a lower end with hp that exactly meets your calcs. Maybe anything from 3-10 hp lower end would work good enough.
When I posted photos I would just past in a link to my webpage.
Wow! Thanks for the ebay links. All of these lower units look pretty good. I really like the mercury one. I have some bad and good news. Looks like I'm going to have to use this briggs to build a generator instead of the outboard. Hurricane season is upon us, and the generator will be more useful to keep the freezer running (Last season we lost all of our perishables due to hurricne Lily). So, I've put my outboard plans on hold for now, but I still want to build this outboard. I want to try to make the motor interchangeable to be used as the outboard and generator. So, I'll keep you posted as my progress goes on both projects. I know this is a boat forum, so I'll post info about the outboard and post links to get to the generator info at my website. The good news is, I found a little 97 6 hp mariner for our little flat boat. So, now we'll be able to fish in better areas :) Thanks a whole bunch machnumber2. You've helped me a ton! :) Oh yeah, I'll be calling our local briggs shops tomorrow morning to find out the wot.
Well have to say that I am disapointed but this generator could be pretty neat. Post the details when u get a chance.
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