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Thread: Thrust measurements

  1. #31
    Sam Cullis Mark75H's Avatar
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    I agree, we don't "use the longest leg motors you could find". But you are not thinking of the lever correctly. The force is applied to the water at the prop, here the force is the output of the motor and does not change relative to the water. Speed comes from the force applied to the water which does not change regardless of the length of the mid section or transom height.

    What I say changes is the force applied to the boat - at the contact point closest to the prop. The force is not being applied to the boat at the prop; the force is being applied to the boat closer to the pivot than the prop and is amplified by the difference in length between the prop and where the mid section contacts the boat.

    I am not saying there is a lever effect against the water, there is a lever effect against the transom structure (which is the subject of this thread).

    This is the reason thrust blocks are used and why motor/transom brackets are not shorter (just an inch or two from the tilt axis total length). If there was no thrust block and a shorter transom bracket, the transom bracket would twist the top of the transom right off the boat because of the distance between where the force is applied to the water (the prop), and where the boat receives it (the thrust block or bottom of midsection/transom bracket).

    Think of putting a pipe wrench on the transom of a boat and pushing with a 100 pound weight. If the wrench is supported against the transom half way from the top to the bottom and half the length of the wrench handle you probably can't break the transom. Move the support closer to the wrench jaws and you become more likely to break something off the boat. Move the support more to the bottom of the boat and closer to the far end of the wrench and there is less multiplied force on the boat because the length between the 100 pound force and the boat is less.
    Since 1925, about 150 different racing outboards have been made.


  2. #32
    Team Member zul8tr's Avatar
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    Default Thrust

    I believe you may have missed the point of my post? I completly understand the forces envolved, their related moments and free body diagram of the loads as well as the benifit of the design of the clamp bracket used, simple physics. While instructive your analogy of the pipe wrench may help others who need that knowledge. I think without a free body sketch of the forces and moment arms I speak about in my post we will run in opposite circles here. My post was directed to the force experienced at the thrust bracket due to static engine load and dynamic forces of thrust and drag experienced at the gear case and prop while underway and their related moment distances.

  3. #33
    Sam Cullis Mark75H's Avatar
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    Your answer is only right Sam, if the pivot point is attached to a fixed point, not on the boat.
    You are going to have to re-explain this to me then. To me it seems to say I am wrong in all practical application and only correct in one abstract case.

    Is not the pivot point the axis of the tilt bracket? Is the tilt bracket axis not a fixed point on the boat?

    A nutcracker is just as much a lever as a seesaw.
    Since 1925, about 150 different racing outboards have been made.


  4. #34
    Team Member JohnsonM50's Avatar
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    All said & done its a variable set of forces that can go from smooth water steady to violent ruff water hammering. Without tie downs it would hammer all the more every time you let off or turned & then went back on the throttle it would release then smack down good. For the reason of the post you could measure & do the math all day, it sums up to the part[s] to be made need to be strong, real strong & yet not too heavy to defeat its own purpose.. performance. Ive made & run extensively 2 mechanical tilt adj. devises, the 2nd of which worked well with 1 problem. For it to be totally effective.. a variable interactive tie down I didnt make yet... From this minds eye-seat of pants experience I can tell ya.. whatever you make It will take a beating!

  5. #35
    modifiedoutboard OUTBOARDER's Avatar
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    depends on prop, copy something that works and improvr if possible.
    ANTHONY McCULLOCH
    modifiedoutboard@hotmail.com

    Some things never change!

    They Want it cheap..............

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