Homemade stern tube

Discussion in 'Boat Design' started by Chikokishi, Oct 2, 2012.

  1. Chikokishi
    Joined: Mar 2010
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    Chikokishi Junior Member

    Does anyone have any information on how to build a stern tube type device?
    i attached an image of basically how itll be implemented. I intend to run a power shaft through the keel or just behind it, and then angling back to the propeller. I dont know the terminology to search for what i want. Nor do i really know how the stern tubing (?) works. Ideas i considered? A water tight bearing with an inside diameter that would fit nicely on a rod i chose for a shaft. then using a sealer of some sort to sear the rod to the bearing. But thats about all i have for ideas.

    Or if anyone knows where i could buy one for cheap ( <50$) that would be great. This is a homebuild boat that is to cost less that 600$... so dropping hundreds on a little tube is a bit out of question.

    Lastly, I would imagine that there are devices that are basically a full prop assembly. I also attached an image of this, does this look like anything someone recognizes?
     

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  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    looks like your intention is to have the prop shaft make a 90 deg turn? that is rarely done, requies a gear box. you want to put the engine mid ship and have a straight prop shaft at a slight downward angle go through the keel. The boring through the wood keel will get packing and a bearing to keep the water out. not sure if there is a cheap way to do this, perhaps use a block of nylon as your bearing, and than seal it with an automtive type seal pressed into the bore from the prop side? do not know how that would hold up, but the prop shaft has the potential for harmoic vibration unless it is fairly stiff (the prop shaft winds up like a spring and unwinds, prop acts like a flywheel, wrong harmonic and you will fail the shaft).

    A small outboard, even if homemade from a weedeater motor (there is a thread on this forum about converting a weedeater into an outboard), would be easier and safer since you do not have to penetrate the hull with a propeller shaft.

    good luck.
     
  3. Frosty

    Frosty Previous Member

    Its not a water tight bearing it is anything but . Water cools it and is important. This can be made --it came up first hit on a google search Marine stuffing box . I reckon this could be made from a plumbers store from a copper pipe compression fitting. Of course remove the compression olive and use greasy string.

    http://www.pbase.com/mainecruising/stuffing_box

    The bearing in the tube can be made from wood as they do here in the East even on 6 inch trawler shafts. Its not that they can or its cheaper it is the best material and they ALL use wood. But it has to have swollen to its max by soaking on water for as long as possible --months.
     
  4. Chikokishi
    Joined: Mar 2010
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    Location: Montana

    Chikokishi Junior Member

    I forgot to mention that this will be powered by pedal, not engine! I origianll was going to post this in the pedal forum, but as its over a hundred pages long with technical stuff, id hate to ripple water thats probably been churned plenty times somewhere in that forum. (i dont know of an efficient way to go through 115 pages of posts...?)

    I also dont quite know what a "stuffing box" or a "stern pipe" are. I just have seen the words enough to know they are related to what im trying to figure out.

    Without going into a list of details, this boat has plently of restrictions that are making it such that i do not want an outboard, or anything of the matter. It is to be foot powered, and it is to be compact. Iv thought about the idea of taking a pipe up above the waterline, dropping the prop shaft through there and not worrying about water sealing it, but that would be rather difficult given my circumstances. And not only is it foot powered, but itll only be used when the boats not being sailed, or paddled around. so it doesnt even need to be something that can withstand huge amounts of work. Just within reason. I wouldnt mind spending a few dollars every few months to rebuild it or whatever, is what im trying to say. Thanks for the reply, im reading the site that you suggested.
     
  5. Frosty

    Frosty Previous Member

    I gave you a link of what a stuffing box is with big pictures.
     
  6. Chikokishi
    Joined: Mar 2010
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    Chikokishi Junior Member

    Yes, haha, yes you did. If im not misunderstanding, a stuffing box is basically a tube packed with grease to let the shaft rotate and mostly hold the water out? like in the pic? (grease is pink....)
     

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  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    There are a number of engineering approaches you can take to address the issues associated with a human powered shaft. As to what's currently available on the market, well the logical choices would be those making these types of boats. Most pedal boats tend to go with a paddle wheel arrangement, which keeps things simple.

    [​IMG]

    A simple search will produce quite a bit of home made brew, some not worth considering, others clever, but not particularly effective, while some have promise.

    As to a $50 unit, well good luck with that. The mechanical aspects aren't hard to figure out, but if you want it figured out for you, you'll have to pay for it. The above unit is complete and ready to mount on a boat, for $200.

    Try here > http://www.recumbents.com/wisil/hpb/boat.htm <
     
  8. CDK
    Joined: Aug 2007
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    CDK retired engineer

    A stern tube is just a piece of pipe around a prop shaft, with bearings to keep the shaft centered and a provision to keep the water out.
    Making it yourself is easy if you have the proper tools, almost impossible if you don't. Another approach is to use something else and adapt it for your purposes, like an old electric garden pump. It already has bearings, a shaft and a water seal; just cut off and discard everything you do not need.

    How will you make the 90 degree transmission?
     
  9. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    A 90 degree drive leg for a low power pedal boat can be made pretty easily with no gears. There is a mass of information on the pedal boat thread, including descriptions of right angle gearboxes, flexible shaft drives, twisted chain drives and my own favourite, the double universal joint drive. The latter is relatively cheap if you have access to a lathe, as it uses off-the-shelf commercial joints, bearings and seals.

    To give you a taster, here's a couple of photos of one of my UJ drive legs, but you'd be far, far better off looking through the pedal boat thread than getting folk to repeat stuff they've already posted there.
     

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  10. Chikokishi
    Joined: Mar 2010
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    Chikokishi Junior Member

    Jeremy,
    Yes! I saw those pictures. i went through that forum for a while looking at things. The question i was trying to have answered in this post was how the water seal works.

    CDK,
    The "provision" that you mentioned is what im trying to get a better insight into understanding. Iv found plenty of online sources explaining where to buy things, how to repack things, and really high tech professional things, but i want to know the very simple, low key, *idea* for how they work. As for the 90 degree angle, i was considering a bevel gear mostly, but i have not got to that point yet.

    Par,
    Indeed iv noticed this myself also. Iv already thought about a few different ideas, this post is the beginning of my own particular idea. You suggestion of buying the premade kit doesnt interest me (although a sound idea) because my design must work, and also be completely removable from inside the boat. Long story short, the same place that the pedal drive will be will also be where the bed is. So i need to be able to use the drive system when needed, and remove it when not needed. (i already have ideas on this idea also)

    Iv been looking at these things for a while, are they really just like... a pipe with a (mostly sealed) seal that (mostly) is snug against the shaft? So its friction closed but not so tight that you cant rotate the shaft? That seems terribly... simple? For some reason im expecting some sort of bearing system or something crazy.. but its looking like its just a tube with something stuffed into it around the shaft to get into the waters way. Am i wrong? What keeps the friction from being a problem?
    Thanks for the replies!
     
  11. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    In my legs I've used off-the-shelf commercial lip seals (i.e. oil seals) on the output shaft. They are dirt cheap and seal very well indeed, especially if the leg is filled with light oil to equalise the pressure as well as lubricate the joint. It's not normal marine practice for big boat shafts, but is pretty much identical to the way outboard drive legs are sealed.

    In the first photo above the oil seal is on the outside of the output shaft at the lower left, with the support bearings for the shaft behind it.
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I've used lip seals (Simmer rings) successfully in the stern tubes of my boat, but like Jeremy Harris states, what is good for outboards and stern drives is seldom used for conventional prop shafts.

    There are a few simple rules: There must be a bearing in close proximity to keep the shaft centered, a lubricant (oil or grease) must be present, the shaft and tube diameters must be within specs for the seal and the running surface must be immaculate.
     

  13. Chikokishi
    Joined: Mar 2010
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    Location: Montana

    Chikokishi Junior Member

    Sweet, thanks for the info. Like i said, i was expecting something more technical i guess? Its silly how simply a lot of technology is.
     
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