Small gearbox

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

  1. gonzo
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    gonzo Senior Member

    All that is necessary to make it legal and safe. Check the EU standards for inboard engines. Isolating an exhaust is not so easy. It generates a lot of heat that needs to be removed from the engineroom. Further, the engine is air cooled with need even more heat removal.
     
  2. Daan
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    Daan Junior Member

    The boat I want to build is not for long trips. The build will be only 3 meters long and about 105 cm width. Only 2 people will fit in it.
    My main goal is just to have fun building the boat and make it work and not hours long trips over the water. Maybe later in the future I will expand it a little and put a better engine in it. eventually I want an electric inboard engine but I can't afford those yet.
    Still thanks for your insight on it and your tips.
     
  3. gonzo
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    gonzo Senior Member

    You don't need a long trip for your boat to go up in flames and to die in the fire.
     
  4. Daan
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    Daan Junior Member

    I have used this idea on a previous project of mine. Me and my brother built a smal "train" and it works amazing. I was thinking about this solution but it is not as compact as a gearbox.
    I am now thinking of maybe using a centrifugal clutch.
     
  5. Daan
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    Daan Junior Member

    Do you think a 5 hp outboard engine would be faster/stronger than a Honda 10hp engine? To me it sounds a bit strange.
     
  6. Daan
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    Daan Junior Member

    Can you explain to me how that works? Now I am getting curious.
     
  7. gonzo
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    gonzo Senior Member

    A 3 meter boat, with an inboard, will hardly have enough interior space left for two people. That is the reason millions of boats have outboards.
     
  8. Daan
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    Daan Junior Member

    Ah that makes sense. Thank you for the explanation.
     
  9. fredrosse
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    fredrosse USACE Steam

    Simple belt drive can work very well. Robust enough for 10HP. Reduction to drive a proper propeller can perform better than an outboard with which has a too small propeller. Belt drive can get proper reduction. My 14 ft converted O'day daysailer has a 13 HP IC engine, 3:1 reduction, swinging a 14 x 17 prop. 4 people aboard works fine. Better than an outboard in terms of propulsion efficiency because of the relatively big, relativly slow turning propeller. Belt clutch works well. Not a big technical challenge.
     
  10. gonzo
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    gonzo Senior Member

    Can you show any data to prove your claim of better efficiency?
     
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  11. fredrosse
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    fredrosse USACE Steam

    Without going into details as presented in the Gerr Propeller Handbook, it is accepted knowledge that propulsion systems are always a compromise. Faster boats tend toward smaller propellers to lessen parasitic drag, especially with regard to excessive tip speed. Other economic and arrangement factors tend to favor smaller propellers, as the smaller propellers cost far less, have less clearance issues, have smaller less expensive gear trains, and are better suited to engines which generally run at high RPM. This is OK for higher speed planing boats, which are generally the norm.

    In the case of this forum thread, the propulsion is for a modest performance boat, not really a "speed" boat. The optimum propeller size is generally larger, and slower turning, than outboards which have the several economic and arrangement issues that tend to make the technical/economic compromises favor the smaller than optimum efficiency propeller size. I will present some specifics, according to Mr. Gerr's well respected book, soon.

    For this forum thread, with a simple belt type clutch with reduction, is adequate for the power involved. The engine, rated at 3600 RPM, can have a reduction ratio of 3 or 4 to 1, thus turning the prop shaft at 900 to 1200 RPM, which will be close to ideal. Of course, to calculate the actual "ideal" design basis conditions, we would need to know far more about the boat's hull form, displacement, allowable prop tip clearance, etc.
     
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  12. fredrosse
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    fredrosse USACE Steam

    Found a direct quote from a technical paper presented by Dave Gerr, on his website:

    The Efficient Powerboat, Part 1
    Dave Gerr, 2011

    "Except for high-speed boats, the larger the propeller diameter and the slower the shaft RPM, the more efficient the propulsion. Remember you need low shaft RPMs"
     
  13. fredrosse
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    fredrosse USACE Steam

    Note: This reply is to another person's post, where he stated my post, answering Gonzo's question, was worthless in many ways. That person has now deleted his posts on this subject. However the statements I posted are true and correct:

    "a load of rubbish" ??
    Are you stating that the larger, slower turning propeller would not be more efficient? That is a simple fact here, not rubbish.
    Yes, the Yamaha 9.9 hi-thrust can push large slow moving loads, as a matter of fact, I use a Suzuki 2.5 to push my steamer, at over 1 ton displacement, when I want to, and yes, it does push the steamer, although clearly not the efficient size prop.

    "The fact is a small outboard will push ..... better than any bitza contraption..."??
    In response to that, it is clear that you make a statement withhout proper technical knowledge. Yes, you could make a 10 HP belt drive system incorrectly, however with simple belt drive design standards one could make the Honda engine and belt system work correctly, at far less cost than the price of a Yamaha 9.9 hi thrust. Something any modest mechanic with proper design could accomplish.

    As a matter of fact, I can present such a design, certified by a registered professional mechanical engineer, who has many years of design experience in power transmission, for the subject belt driven design. The comments were about propulsive efficiency and simple power transmission options. Sorry you are so riled by the presentation of some simple true statements.
     
    Last edited: Sep 13, 2020
  14. Daan
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    Daan Junior Member

    Thank you for the usefull information. I think I can work a lot out with this information.
    The only information about the hull of the boat is that I am sort of copying the design of a Seabreacher. It is not exactly the same and I am building it with wood but the width is the same and the bottom is also the same, the length is 330 cm and I have yet to figure out the rest of the design.
     

  15. Daan
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    Daan Junior Member

    I know it sounds a bit stupid that I want to copy the design of a seabreacher but it is only the design that I want to copy and not his capabilities. I am not planning on making it submergible because that is too complicated. The only thing that I want to achieve is getting the knowlegde of making a boat.
     
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