A couple low dollar FNR ideas for inboards

Discussion in 'Inboards' started by swmn, Feb 28, 2017.

  1. swmn
    Joined: Feb 2017
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    Location: Interior Alaska

    swmn Junior Member

    I have my own reasons for getting away from tiller steer outboards. Not willing to give up boating because of my new friend Mr. Arthur Itis...

    I had a thread a few days ago about parallel offset shaft coupling that got no actionable response, but I was still thinking about the 'problem.'

    What I want to do is is build an FNR transmission around an air cooled 4 stroke engine running more or less 1:1 ratio to the prop. An inboard mud motor.

    First I let go using a horizontal shaft engine (like a snow thrower), and opened my brainstorming to using a vertical shaft engine (like a lawn mower) if I could make that work more easily.

    I stumbled, literally stumbled, onto the kinetic model library at Cornell dot edu school of engineering. Lots of good stuff there, I really should write them a check for all the bandwidth I used.

    I read just about everything they had for belt driven mechanisms and cussed out loud when I figured this one out:


    How cool is that? I rushed to grainger dot com and found some splined connectors and shafts pretty easy, and some jawed connectors, but no double jawed dog clutches.

    Then I found this on youtube, less then 2 hours after finding the model "By this model, the student Carl Benz learned together with his Professor Redtenbacher." https://www.youtube.com/watch?v=hYPuB9nmS8E

    Now my head is spinning. I have a perfectly fine Johnson 25 out in my shed. I am looking at 21hp BS Vanguards. Paid $200 for the Johnson as a parts motor. The lube dripping out of the lower unit looks great. Online price for just the prop shaft, the two F/R gears, the double jawed dog clutch and the drive pinion - almost $1500. Plus I got the shift rod and the driveshaft in the complete motor.

    So first idea for low dollar FNR inboard transmission:

    Mount a horizontal plate to both motor bunks behind the engine. Drill some holes in the plate so I can bolt the lower unit to it, using the factory holes on the lower that bolted to the leg. Cut the skeg off the lower so I can mount it deeper in my boat. Put a friction wheel on the lower where the prop used to be on the donor outboard. Line it up so the friction wheel on the lower mates with a friction wheel on the real prop shaft in my boat.

    At the other end of the donor lower I can use the existing shifter rod for FNR. All that's left to do is mate the factory driveshaft on the donor lower to the crank on my engine.

    With a vertical shaft (lawn mower) engine I could mount a sheave on the donor lower driveshaft a few cm above the metal plate spanning the bunks I just made. Matching sheave where the blade of the lawnmower used to be, correct length belt, Bob's your uncle.
  2. swmn
    Joined: Feb 2017
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    Location: Interior Alaska

    swmn Junior Member

    As above but with a horizontal shaft engine, my best idea so far is two pairs of beveled crown gears. One on the crankshaft of my engine, one on the tip of the recently cut down drive shaft on of the donor lower.

    The other two would go on both ends of a shaft whose length would have to be pretty precise, and that shaft would have to ride in a couple pillow bearings mounted pretty precisely on a vertical plate between the engine and donor lower unit.

    Or a fella could Rube Goldberg up collection of pulleys as demonstrated by the Cornell Kinetic Model Library in leather belts that may or may not work with modern rubber V belts.
  3. swmn
    Joined: Feb 2017
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    Location: Interior Alaska

    swmn Junior Member

    A few pros and cons with mounting a lower unit from a donor outboard into an inboard engine setup for FNR shifting.

    Pros: the engineering is done. Extra points for running the fool thing in what used to be reverse as the new forward, and what is likely the more heavily worn forward gear as the new reverse. Got a 12hp engine? Look for a 15hp donor. Wanna run a 35hp inboard? Look for a 50hp donor lower.

    Cons: Changing the lube is going to be a Pain in the Neck. Drilling all the mounting holes in the new plate so they all line up with the factory holes in the donor lower is going to be a Pain in the Neck.

    The more hp you want to run, the higher a vertical shaft engine will have to be mounted in the hull to keep the sheaves in one plane that is above the donor lower that you don't want sticking out through the bottom planking, cause outboard lowers get big FAST as hp goes up.

    You might could pull the mechanical bits out of a donor lower and re-assemble them into a cage between the engine and prop shaft, but once you have it all bolted together with bearings in all the right places you'll need a leakproof box around it to hold the lubricant.

    I'll try posting a test jpg here directly of idea number 2.
  4. swmn
    Joined: Feb 2017
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    Location: Interior Alaska

    swmn Junior Member

    My other idea so far is to dispense with gear and belts entirely. See if I can attach a .jpg here...


    So look at the bottom half of the image first, the side view.

    The thick pencil line is the transom and hull. I got a prop shaft sticking through there, with a fixed friction wheel on the end of the prop shaft and no thrust bearing drawn in.

    On the right edge of the drawing is the vertical shaft motor, I drew the cooling fins on the near cylinder, a piston with connecting rod, crank shaft, and a big fat rotating friction disc mounted to the output end of the drive shaft.

    In between those two bits is a fixed length shaft with a fixed friction wheel at each end.

    As drawn, its in neutral. If you stomped on it (or mounted a lever) between the fulcrum and the prop shaft the intermediate shaft would engage. That is, the front end of the intermediate shaft would get pushed up against the (spinning) friction disc under the engine, and the back end of the intermediate shaft would be touching the friction wheel on the end of the prop shaft, viola, the prop starts spinning.

    Now look at the top half of the .jpg, the "top view". I got two intermediate shafts in there now. When both shafts are engaged, the drive train will self destruct. If shaft number one is engaged, the boat moves forward. If shaft B is engaged, the boat moves in reverse.

    Just an idea, a little too much time behind the snow thrower this weekend maybe.

    Attached Files:

  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The problem with running garden tractor motors on lower legs is the RPM variance. Garden tractor engines spin up at about 3,500 RPM as max, but the outboard powerhead is spinning typically over twice this, so the gear ratio is way off what you need. You'll have a 20 HP garden tractor engine geared down 2:1 or so, but at half the rated (and needed) RPM.

    Belts work and the dog clutch approuch is about as simple as it gets. These can be homemade, with various clutches available for different shafts diameters. A splined shaft segment, a couple of dogs, some pillow blocks, thrust washers/bearings and either a double sided ring or two ring gears on the engine output shaft. The gears can be swapped out for pulleys for V belts, but this setup tends to be noisy (belt slip). To remove the noise an idler and tensioner can be used, but now side loads are being setup on the shaft, without careful arrangement.

    GoCart racers use a "Comet" FNR transmission that can solve you problems. They can be found used, but a clutch is still needed. These are not rated for a lot of HP (usually around 15 HP), but for the money, you have an appropriate 1:1 forward, a 2.5:1 reverse and they're rated at the speed of most garden tractor engines (4,000 RPM). ATV boxes are also adaptable.
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