Small gearbox

Discussion in 'Boat Design' started by Daan, Sep 7, 2020.

  1. brendan gardam
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    brendan gardam Senior Member

    Ditch the seabreacher and build off a conventional plan.
     
  2. gonzo
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    gonzo Senior Member

    A belt will have more losses than gears. You are presenting opinions based on nothing tangible. For example, that a propeller shaft turning at 900 to 1200 RPMs is ideal. There is no data to back you claim. The economy of outboards, even in small boats, is easy to find. The data is all published.
     
  3. fredrosse
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    fredrosse USACE Steam

    "opinions based on nothing tangible"??
    I would think that a direct quote from a Dave Gerr paper is something that is reliable. The Gerr Propeller Handbook is accepted as a reliable authority on this subject. Yes, V-belt drive is a couple of percentage points less efficient than proper gears, but generally that efficiency loss is small compared to the several percentage points that are often available with a bigger/slower turning prop. Exactly what was found by looking at the Gerr website. Running these calculations is available within the Gerr propeller handbook, and I have only done it for my 14 ft O'day electric conversion. The numbers on that design pointed to an optimum prop which is 14 inches in diameter, this is far larger, and far less RPM than you will find on any low horsepower outboard.
     
  4. fredrosse
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    fredrosse USACE Steam

    Quoting from: 2.016 Hydrodynamics, Marine Propellers, Prof. A.H. Techet, Massachusetts Institute of Technology


    “Diameter


    The diameter (or radius) is a crucial geometric parameter in determining the amount of power that a propeller can absorb and deliver, and thus dictating the amount of thrust available for propulsion. With the exception of high speed (35 Knots+) vehicles the diameter is proportional to propeller efficiency (ie. Higher diameter equates to higher efficiency). In high speed vessels, however, larger diameter equates to high drag. For typical vessels a small increase in diameter translates into a dramatic increase in thrust and torque load on the engine shaft, thus the larger the diameter the slower the propeller will turn, limited by structural loading and engine rating.


    Revolutions per Minute (RPMs)


    RPM is the number of full turns or rotations of a propeller in one minute. RPM is often designated by the variable N. High values of RPM are typically not efficient except on high speed vessels. For vessels operating under 35Knots speed, it is usual practice to reduce RPM, and increase diameter, to obtain higher torque from a reasonably sized power plant. Achieving low RPM from a typical engine usually requires a reduction gearbox.”
     
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  5. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Daan,

    Great project.
    I'm a little late to the party here but have you considered a gimbal mounted engine for an "egg beater" prop drive?
    No tranny, on/off neutral, no reverse.
    Lots of kit available for such an install.
    Interested?
     
  6. Daan
    Joined: Sep 2020
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    Daan Junior Member

    I have never heard about that. Can you explain a bit more about it?
     
  7. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Outboards have been mentioned previously

    You mentioned "cheap as possible"

    If this is an important criteria, the inboard is not the way to go. The engine that you have will be a small part of the cost of actually getting it in a "small boat"
    You will have to purchase a prop, prop shaft, a strut with a cutlass bearing, a stuffing box or other shaft seal mechanism, a thrust bearing (something that you will have to design and build) a coupling device, inboard engine mounts,
    an exhaust system, an elaborate air cooling system, heat shields. You will need some type of throttle and shift control. Noise will be an issue. Not done yet. You will have to steer the boat, so a steering mechanism, wheel or alternative, cables, helm with a steering wheel, a rudder (which could be mounted to the transom, if not then another hull penetration, packing, brackets. A rudder system will be slowly responsive at low speeds and if you get a gear box with a reverse, steering in reverse is sketchy.
    You are looking for a gear box, this and the related apparatus is going to cost you money.

    An inboard configuration is going to take up space in your short boat. I probably have missed some items.

    Most, if not all of the above "cost" issues can be cured with the purchase of a used "cheap" outboard. Noise, exhaust, steering, neutral, reverse, but more importantly more space inside a very small boat.

    You could sell the Honda, throw in less cash that the list above will cost you, buy a used 10 hp outboard and have a nice "little" boat which will be much much "cheaper" than your plan.

    If you are fixated on using your Honda, you could buy an outboard with a blown engine, and mount your Honda in place of the blown engine to get your shift, steering, thrust, etc components looked after but then really all you have
    created is a noisy outboard.

    If "cheap" is a major focal point, an outboard is the way to go.
     
    Last edited: Sep 26, 2020
  8. gonzo
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    gonzo Senior Member

    You need to add a permanently marine approved fuel tank (required for inboards) and fuel hose, fuel fill, fuel vent and a bilge blower.
     
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  9. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Thai longtail boats.
    I'm not suggesting you build one but the drivetrain is intriguing.
     
  10. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Daan,

    Not interested?
    Have you abandon Your thread?

    BB
     
  11. Daan
    Joined: Sep 2020
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    Daan Junior Member

    Sorry for the late response but I do think your suggestion is interesting. but this way I will have forward and neutral "gear" but no reverse. Me and my brother came up with an idea to make our own "gearbox" with a chain, some sprockets, pulleys, belts and a centrifugal clutch. We still have to work it out but I hope it will work.
    Thanks for your suggestion anyway and if you have other suggestions let me know.
     
  12. fredrosse
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    fredrosse USACE Steam

    Having made several belt / chain drive systems in the past (go-karts, small tractors, and one electric boat), I would like to make some suggestions:

    The centrifugal clutch, either belt or chain drive, may not be suitable for the continuous application of 10 horsepower at 3600 RPM. Check the specifications for the clutch and the belt to assure slipping/heating will not be a problem.

    An alternate that is far more robust is to use a simple V-belt tensioner with a movable idler pulley. A simple lever arrangement on the idler will allow engagement/release reliably. As a first "guess", perhaps a 100mm diameter engine pulley, driving a jack shaft pulley 150mm to 200mm, with a 75mm external clutching idler, spring loaded to engage the belt, or to dis-engage.

    Roller chain final drive will work well, however it will be somewhat noisy, and the chain requires lubrication/maintenance. Use O-ring type motorcycle chain if available, this chain has lubrication sealed-in.

    One better alternate is to use toothed belt for final drive, with 2:1 reduction readily available from millions of automobile engines found in junkyards, at very low cost for the 2:1 belt sprockets. Some simple machine work will adapt the automotive sprockets to your shaft diameters.

    For a small boat, under 6 meters, reverse is not really necessary, and one can maneuver the boat at the dock with common canoe paddles or oars.
     
  13. brendan gardam
    Joined: Feb 2020
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    brendan gardam Senior Member

    as others have said, its a lot of machinery to fit in 3meters. i can picture the op burning his arm on the exhaust while trying to shut the motor down after his big toe got stuck in the drive chain. :eek:
     
  14. fpjeepy05
    Joined: Jan 2010
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    fpjeepy05 Senior Member

    The OP doesn't need to do anything you say. If he wants to avoid a fine from USCG, reduce his insurance liability, or be safer those would be good things to do, but it's his decision whether or not to do them.
    Converting the carburetor, ignition and charging system to be sparkproof fall under the same category, but there are lots of places in the world where people run them without.
    Run a dry stack for exhaust cooling. Wrap is with fiberglass header wrap. It's not difficult.
    And yes larger propellers are more efficient than smaller ones. That is common knowledge no data is necessary. The reason outboards are more efficient in some applications are; they can improve LCG, have a better shaft angle, less appendage drag, lighter, and they are rated for HP at the prop, not at the crankshaft like inboards.
    Your negativity isn't helping anyone, and it doesn't make you look as smart as you think it does.
     

  15. brendan gardam
    Joined: Feb 2020
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    Location: east gippsland australia

    brendan gardam Senior Member

    Your post doesn't make you look very smart. What gonzo said is correct. It doesn't matter how small the boat is it still has to be safe. He could listen to you and build himself a floating bomb I suppose. Won't matter to you if he gets injured will it.
     
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