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Thread: Mud Motor, RPM AT THE PROP

  1. #1
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    Default Mud Motor, RPM AT THE PROP

    Since there are already many hundred threads covering freighter's and mud motors I feel no shame starting one that simply get's my question answered.

    One thing the mud motors have in common is engine type, the good ones are 4 stroke and run up to the mid 3400 rpm range, regardless of the horse power.

    What I want to know is the RPM at the prop, are these motors stepped down or are they running engine RPM? The manafacturers are a bit vague on this point.

    Thanks , Mike

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    Depends on the pulley sizes that are used to spin the prop. Diffrent companies use different ratios. Depends on how the power is trying to be used, bottom or top end. I don't know formula that would work in a broad sence, like jet loosing x power vs. prop. Maybe someone else has better info, that is my understanding though.

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    generally speaking, across the entire spectrum of surface drives used on smaller boats, the motors are under-driven. That's why surface drives are able to push a bigger prop. For example: a 27 hp go devil long tail pushes a 9 inch diameter prop directly off the motor rpm's. My copperhead 12 hp pushes an 11 in. diameter prop. How does it do this? Torque. It's what canoe hulls like.

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    The math, or so i was just told is motor rpms divided by reduction equals prop rpms. Hope that helps.

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    Member mainer_in_ak's Avatar
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    Hey there Don, long time no see! If you're the Don I remember, I think I flagged you over to my hooligan dipnetting spot when you guys weren't connecting? The following year, you stopped by the house to checkout the copperhead? Anyhow, if this is the Don I remember, happy holiday season to yah!

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    The reason the manufacture is being vague is it may depend on the engine and the prop. If you're wanting to know about a engine you have, pull the spark plugs, find the compression stroke at TDC, mark the prop, rotate the engine to the next compression stroke TDC and count the turns.

  7. #7
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    that way no questions you have your prop RPM

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