Why do they keep on doing it?

Discussion in 'Outboards' started by tom kane, Dec 13, 2015.

  1. tom kane
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    tom kane Senior Member

    If you have a big outboard on the back of your boat (or two or three or more) and you have to swing all that weight left and right to steer would it not be easier just to move the lower gearbox and save that trouble that it entails.
    Have the forward and reverse g/box just under the power head clear of the water.
    Yes it has been done,I had an old Penta 120 that had that arrangement and it had forward, neutral and reverse by moving the tiller up and down but still required to swing the outboard left and right, it weighed 125 Lbs.
    And why not also for I/Os.
     

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  2. gonzo
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    gonzo Senior Member

    The engine turns slowly around its axis, so the momentum is very small. It doesn't take a lot more force to move the whole engine and it is much cheaper and less complicated.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    It works well as is, no need to be changing, imo.
     
  4. Barry
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    Barry Senior Member

    Mercury Marine has a hand tiller control for engines up to 200 horsepower.
    Obviously not a lot of force required to run these engines, as Gonzo stated.
     
  5. tom kane
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    tom kane Senior Member

    Don`t you think it would be nice to have better control by turning the lower gear box 360 degrees thus no need for complicated reversing gear.
    I am glad no one said it would not be efficient.
    Tossing all of that weight from side to side has got to be silly way to steer and destabilizing the boat.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    How does it destabilise a boat compared to what you suggest ? If full throttle was available for the full 360 degrees, things could get very interesting. Or dangerous.
     
  7. daiquiri
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    daiquiri Engineering and Design

    That configuration would have a number of issues related to structural strength, waterproofing of the upper/lower gearbox joint, would require hydraulic or wire lines to extend close to the waterline, and would increase the number of mechanical parts required for the engine construction.

    Translation into street language: it would cost more and would be more likely to break.

    Not a kind of product which an average boater would want to buy.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The small outboards that do rotate 360*, rotate the entire engine, not just the lower unit. Having said that, I do have an antique product that does actually rotate the way TK mentions, it would be 60 years old, imo. There is nothing new in it, but no reason for it to take on popularity as an idea.
     
  9. baeckmo
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    baeckmo Hydrodynamics

    As Always, it has been done for I/O legs without success. The problem is that all the engine torque is transmitted through the vertical shaft, which means that the balancing reactive torque has to be taken care of. That in turn requires a servo-system, electrical or hydraulic, with possible fault ratio rising exponentially.

    Take a look at the Volvo IPS system; it is working on this principle!
     
  10. tom kane
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    tom kane Senior Member

    Yes the Volvo IPS is a good working example of the principle, we will probably see more of it with larger and more powerful motors fitted on the stern of boats,which is a silly idea anyway because that is an important area on a boat.
    There have been successful and efficient tractor I/Os .
    I don`t think your reactive torque explanation has any merit.
     
  11. tom kane
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    tom kane Senior Member

    A competent boat operator should have the sense not to try reversing at any where near full throttle.
    The mass of a very top heavy outboard swinging from side to side should be obvious.
     
  12. tom kane
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    tom kane Senior Member

    You are not designing it the way it should be done and missing the more simple design possible with a lower gear box only required to drive the boat without the need to reverse the rotational direction of the propeller.
    The lower gear box only needs to rotate on the leg which is easy to achieve.
     
  13. daiquiri
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    daiquiri Engineering and Design

    As I said above.
     
  14. baeckmo
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    baeckmo Hydrodynamics

    I'm afraid physical laws don't bother about your thinking procedure, or lack thereof....

    Here an example: We look at an engine with a vertical shaft, shaft speed is 600 radians/second and the power is 120 kW (or 5730 rpm and 163 hp).

    The shaft torque is then 120e3/600 = 200 Nm (or roughly 147 footpounds for those who think "man is everythings measure"). Now if you restrict the movement of the propeller on its horizontal shaft, the lower housing ("torpedo" and propshaft) will try to rotate around the vertical shaft with exactly the torque delivered by the engine, in the example 200 Nm.

    That cannot be controlled by your brainwaves or whatever spiritual forces you have at your disposal, it is simple, basic low grade school mechanics.
     

  15. BMcF
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    BMcF Senior Member

    Thank you, Bodo, Reading through this thread was quite painful...until I reached the end. ;)

    Funny coincidence too...as we're currently knee-deep in the details of installing twin Zeus pods in a demonstration craft. Talk about a very complicated batch of stuff...
     
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