Small gearbox

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

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

    brendan gardam Senior Member

    Ditch the seabreacher and build off a conventional plan.
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    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
    Joined: Jan 2005
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    Location: Philadelphia PA

    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
    Joined: Jan 2005
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    Location: Philadelphia PA

    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.”
     
  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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    Location: Netherlands

    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 at 11:17 AM
  8. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    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.
     
    Barry likes this.

  9. BlueBell
    Joined: May 2017
    Posts: 1,233
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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.
     
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