Drive Shaft Market Exists?

Discussion in 'Inboards' started by DouglasEagleson, Oct 10, 2016.

  1. DouglasEagleson
    Joined: Nov 2015
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    DouglasEagleson Junior Member

    I am working on a small 14 foot plywood tug design. The question becomes how to power the tug? In general I have a mandate to have a small outboard as backup power. Leaving the inboard as main power.

    I have an old surf boat that was professionally fitted with a 12 hp Briggs and Straton gas motor. The drive shaft was perfectly sized for the flat section of the hull. And sized for the location height of the Brigg's drive shaft. I plan on using a small air cooled diesel from China.

    I do want to publicize my project also. The surf boat is 25 foot long. Meaning that as applied to the 14' tug, the engine would mount in the front of the tug. But all the shaft angle issues would not mandate a new shaft design.

    I am looking for advice on the topic of starting a small boat line of work for my company. I am developing my first amateur tug.

    My question:

    Would a drive shaft of this versatile usage be a viable product to sell. I can copy the shaft I have and usefully sell it?

    It would be in a no gear and clutch drive. Just use a shaft coupling from the Grainger's company. Direct Drive with a rubber coupling. The capacity to not use a drive bearing type engine is an open question. Small vessels can be oared around a gas dock ok.
     
  2. gonzo
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    gonzo Senior Member

    Basically, it is all wrong. As you say, the 12HP was perfectly sized. What you are proposing is not. An engine on the bow of a boat is at a really bad location. It should be around the CG. Without gears, the boat will start moving as soon as the engine is turned on, another bad idea; particularly for a tug. Without a thrust bearing, what you call "drive", the crankshaft will take all the thrust and the engine will destroy itself. Finally, and most important, there is nothing versatile about your project.
     
  3. DouglasEagleson
    Joined: Nov 2015
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    DouglasEagleson Junior Member

    ok I won't make shafts for misdesigned conceptions.
     
  4. mydauphin
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    mydauphin Senior Member

  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I am at a bit of a loss to understand the whole idea, and what it accomplishes, but maybe that is just as well !
     
  6. The Q
    Joined: Feb 2014
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    Location: Norfolk, UK

    The Q Senior Member

    I've no idea what a 14ft tug would be for but these type of boats, a Norfolk Broads River Cruiser Class http://rivercruiser.org/
    18360001.jpg
    often have an engine in the bow forward of the tabernacle / lifting mast foot, with a long shaft up to 25ft to a prop just in front of the rudder.
    This is to allow for more cabin space, it's quite successful. long shafts from the bow are not a new idea.

    (yes I know this one hasn't got an inboard fitted but it's the best picture I could find)

    Copying other peoples designs of propshaft is likely to bring legal action against you
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Calculating for a suitable shaft, can be as simple as looking it up on a chart, on a shaft builder's site.
     
  8. DouglasEagleson
    Joined: Nov 2015
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    Location: Gaithersburg, MD

    DouglasEagleson Junior Member

    Shaft builders seem to make the design desired. The design I believe is a parametric solution. Meaning two variables must be designed. So in hearsay terms, the shaft log angle, the propeller size, the shaft length, the point of the engine output shaft, the distance between the engine and the shaft log, and the clearance of the propeller diameter to the hull bottom, and the placement of the strut must be decided.

    My boat's design used a shaft log and a keel rear bearing. A poor solution because the water gets to the shaft log fitting wood. For a tug boat a fiberglass plate needs to be installed for the shaft log. I would then use a wood strut with a rear bearing. An oddity where the shaft is in water ahead of the wood strut. This wood strut could be sized as a small keel section.

    In general the length of the shaft and the engine shaft location can solve the set of variables.
     
  9. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    Even a tiny engine like a 12 hp puts out an enormous amount of force, if something breaks from improper engineering more things will break. Imagine what would happen if even a 1/2 inch shaft, 3 feet long breaks out at 500 rpm. I have seen 3 inch and even bigger shafts break like pretzels, and the effects in a engine room are not good. That is why the engineering of shafts, struts, thrust bearings, engine alignment should be done with care, precision and knowledge. Otherwise you are sitting next to a bomb.
     

  10. DouglasEagleson
    Joined: Nov 2015
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    DouglasEagleson Junior Member

    Yes, thanks for the warning. My conception could be a likely candidate for such failure. A good reason to not play designer on the internet.

    I blew a shaft on a sailboat once. It was a 50hp diesel inboard and used a truck drive shaft with u-joints. It was so malformed a vessel I did it on purpose. The joints had been underwater once. Full power for fifteen minutes at the dockside. One end flopped around thumping the hull. The hull took it well. One reason to disallow u-joints on boats.
     
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