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In Reply to: Re: Homemade outboard motor using briggs 2hp posted by KenOhki on March 02, 2003 at 14:34:56:
I like the sound of that throw-out gear, like in a car starter. I will look for that as I am building. lol about about our conversation taking over the first page, we should probably be taking this to emails I guess.
Your previous question on a different post brings a problem to my attention here. (I think I screwed up on the calculation on your other post, will look at again) Plus this guy emailed and said that I was missing a thrust bearing at the lower end, otherwise the shaft will slide thru. The bearings do have set screws in them, if I cranked them down would that be enough to hold the shaft in place. From your last question I was wondering then if an estimate could be made on the shaft force involved. Couple of assumptions, say max speed of this outboard and boat is 5 mph (2.2 m/s) and it requires the full 2hp (1500 watts)to push it a 5mph then from power = work/time or power=(force*distance)/time noting that distance over time is speed then power=force*speed or force=power/speed
f=1500 watts / 2.2 (m/s) = 681 watt seconds/meters = 681 newtons
since 1 newton=.224 lbs
f = 152 lbs
jeez that cant be right
If I have two bearing each with 2 set screws each screw will have to hold 40 lbs thrust or so.....but that 152 lbs seems really high I dunno
that 152lbs cant be if that were true then lets say you have this 250 hp ( 190,000 watts) motor and it pushes the boat say 55 mph (25 m/s)
from f=p/s then f = 190000/25 = 7600 newtons = 1700 lbs
there would have to be 1700 lbs force on the prop and on the back of the boat, if you were to apply 1700 lbs in the small area where the motor hooks to the back of the boat doesnt seem like it would hold it
I do not think the bearings I saw will be able to hold the shaft without some modification. Question, Are those bearings designed for taking stress from a left to right perspective on the shaft in your diagram? If they are, I would try drilling the shaft, to let the set screws actualy enter a so called set screw hole on the drive shaft, Might also look at stonger material, becuse now,, thay are more acting like shearpins then set screws.or perhaps, grind little flat stops on the shaft where the screws contact the shaft.
Thrust bearing, Yeah that would probably make it much stronger, Might be some other way you could get around that besides drilling the shaft or grinding as I sugested. If they are cheap, Its probably the best way too go.
In your math you mentioned it you would require all of the 2Hp of the motor to acheive the 5mph.. But isnt Prop horspower somewhat lower then actual Engien Hp on most outboards? or do I have this backwards. Also, the weight of the boat gear, motor etc dont seam to be in the equation. I supose it could be irelivant when trying to figure stress on the shaft.thrust is thrust, but does how much it is pushing affect the stress? I guess I estimate more then reli on math in most circumstances ;p And I would estimate that about 2 Hp would produce about 55 pounds of thrust, But mind you that is indead a estimate. I dont think you have told us yet how big the boat is you are using. Are you planning to pipe the exhuast underwarer?
Think I posted the first one to the wrong place..btw the bearing are not really desinged to take a sideways force, but they look pretty rugged and might handle a couple hundred pounds without too much trouble
Making the set screw holes might work fine, these are like industrial grade bearings, the spec. sheets on them actually gets into the 1000's of pounds capacity. The reason I got them was because they were the only ones I could find. So I am thinking I will try the holes first rather than deal with a thrust bearing adding to this design. As for the math, I dunno I am just thinking back to high school physics I think. They numbers could be way off and it was kind of odd that it did not account for boat mass. The truth is, I do not have a boat lol. I plan on putting this in a garbage can full of water to test it and if I am lucky maybe I can rent a skiff to try the motor out on or maybe somebody else will test it out for me on thier boat. I figure the boat plus people (up to four) and everthing might be like 700-1000 lbs, pretty heavy for that little motor but then again it doesnt have to push it that fast. (the reason i came up with 5 mph is because that is the harbor speed limit, I want to be able to keep up with traffic so we dont get run over by some huge yacht). Piping the exhaust to the water is a great idea, the motor is going to be louder than your normal outboard. I wonder if piping it to the water would make it quiter. Are exhaust threads on these things pretty standard? I wonder if a threaded copper or plumbing pipe would screw right in where the muffler goes.
well, a boat that can hold 4 with a mass close to 1000 pounds, Yeah, thats allot to ask of a home made 2 hp, but what the hell, A prodject is a prodject hehehe. Figure out a portable padlewheel device and simplify the whole process hehehe. But It appears you are aware that you are building a motor that is way underpowered for the aplication its being applied too. But it will move the boat, I am getting the impresion that you are gonna run this in a Bay?. Might not be strong enough to fight current. I was thinking this was gonna be a pond lake aplication..
I am not sure of the threading for the muffler, But I am sure a simple adaption can be had, ANd it will definitly quiet your noise level allot. When do you think you might actualy have this together? It seams that most of the problems have been addressed?
I think everything is about a done deal and its ready to build. I am going to home depot in a bit to buy some threaded pipe, hopefully not too hard to find what I am looking for. Hopefully be welding later tonight, if all goes well I hope to have a running motor outboard in a couple of weeks or less. Btw, this harbor isnt really like a bay, its like a small city where houses back right up to the water and have a dock in the back yard, the channels being organized like streets-the water is always flat. But some places you go out into the main "streets" and meet up with larger traffic where you have to be more careful. Thanks so much for your input and advice Ken. Wish me luck, I will post photos maybe tonight of the progress.
Thats cool, good luck with your prodject.Np on the advice. Making things do what I want them too as oposed to what they are designed to to is a hobby of mine.
Hey there guys I just wanted to chime in here. I have read all of the messages on this discussion. I have been searching for a way to make an outboard out of my old lawn mower engine. I have a 3HP briggs & stratton. I have an old flat bottom jon boat.
Sketch up what you have planned I would love to see it. Do you have a vertical or horizontal crankshaft motor to work with?
Maybe you should start a new thread if you are seriously looking to do this. Wouldnt want to get the prodjects confused by starting a new one in the middle of this one. So start a new Post from scratch, and tell me a little more. how big a boat, weather you want inboard, outboart or something Unique. How much money you have to spend, and what parts you might have on hand.
Finally started some welding tonight, a bit concerned about a few things..
1. my welding is pretty bad hehe
2. the pipe that I am using is galvanized something welded to steel, hopefully these things weld ok
3. my welder is for small hobbies stuff like that so it may not be as deep as it should be.
Anyway I was happy the see a general form start to take place. The link below shows some photos of the progress-double click the small photos to see larger ones...
Looks like your on your way. I think your hobby weilder should be enough to handle it, but I am not the one to give welding advice,, I gorrilla weld myself, but my welds do seem to hold.
From your pictures. The only warning light that went off in my head was about the motor mount.With such a small motor, it might not be a concern, However it appears to me that the weight of the motor will have a good deal of leverage against the transom. All outboards hang over the back, this is true, But they also are much closer to the transom, and in some cases even overhang into the boat some. Your mounting plate looks like it is overhanging the rear too much. The further away from the transom you mount, the more you will amplify any stress created by weight and motor thrust.. I realize its a little late for magor design changes, But I think this could be a Important design flaw you might want too address before you go much further. I bet you have a formula kicking around someplace in your head to figure how much leverage 50 pounds or so will gain at the end of a 1 foot arm. Is there any specific reason it is so far out?
I think I am going to start a new post because we are getting kinding far to the side here...same name outboard motor
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