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Re: Great Forum!! Thinking of building an outboard

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Hey CC,
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.


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