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Pounds thrust to HP conversion

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  • Pounds thrust to HP conversion

    Does anyone know how to correlate the ratings of a Minn Kota electric trolling motor to HP?

    I use a Minn Kota Endura 55 as primary propulsion on a 12' jon boat, and want to convert to a small gas powered outboard.

    What does the 55 pounds of thrust equate to in HP? It probably isn't much, maybe 1 or 2?
    Steve
    Livin' the Dream
    The Black Pearl - homemade pontoon boat - 1959 Johnson 18HP
    On A Whim - 1990 ChrisCraft Concept 198 - 4.3L HO, Cobra outdrive


  • #2
    Re: Pounds thrust to HP conversion

    yes there is, the formula is in the forums some where, i searched did not find it.
    FLORIDA GATORS
    TEBOW Country



    Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
    That is what these forums are for. Post your questions, in the appropriate Forum.

    Comment



    • #3
      Re: Pounds thrust to HP conversion

      Thanks guys!

      I looked all over the Minn Kota website and couldn't find the current draw. They also have an answer in their FAQ, but it basically tells you there is no correlation and they are measuring different things. They size the prop for all motors to get an approximate 5MPH top speed under a typical load for the motor, so they could definately be propped higher if you wanted to give up acceleration for top speed.

      The trolling motor has been very dependable over the past 2 years getting the 400' up the channel to my island cottage. Last weekend, 25MPH winds made it impossible for me to get back up wind and I was forced to spend an extra night out there and that got me to thinking that maybe I should use a small outboard instead. After playing with some of the old smaller ones I somehow accumulated while getting my 18HP for the pontoon running, I think they are as light to carry around as the trolling motor and battery are.
      Steve
      Livin' the Dream
      The Black Pearl - homemade pontoon boat - 1959 Johnson 18HP
      On A Whim - 1990 ChrisCraft Concept 198 - 4.3L HO, Cobra outdrive

      Comment



      • #4
        Re: Pounds thrust to HP conversion

        I use a Minn Kota Endura 55 as primary propulsion on a 12' jon boat, and want to convert to a small gas powered outboard
        I would look for 3 to 5HP with a build in tank like a lightwin it should do more than your curent setup
        Get wet. Get dirty. Get bounced around a little. Go slow. Go fast. Go fishing.

        Comment



        • #5
          Re: Pounds thrust to HP conversion

          OK, thank you, that is really what I needed to know!
          Steve
          Livin' the Dream
          The Black Pearl - homemade pontoon boat - 1959 Johnson 18HP
          On A Whim - 1990 ChrisCraft Concept 198 - 4.3L HO, Cobra outdrive

          Comment



          • #6
            Re: Pounds thrust to HP conversion

            Found it. Each 1HP of gas engine theoretically makes about 75# thrust at full speed.
            FLORIDA GATORS
            TEBOW Country



            Please, NO PM's (Private Messages) regarding boat/engine problems. they will not be answered.
            That is what these forums are for. Post your questions, in the appropriate Forum.

            Comment



            • #7
              Re: Pounds thrust to HP conversion

              One is a measure of work the other a measure of energy. It's like comparing apples to Oranges. More specificly ( and relivant) is that electric motors are propped LOW for pushing/pulling at low speeds vs making a boat plane. Gas outboards are not. (Look at the two propps and note teh HUGE pitch difference.)
              Having said all that, MK used to figure approx 20#/hP equivilent when they marketed their HP rated models. ( DieHArd did the same.)
              In MY practical experience you can never get a good correlation between the two by any standard becasue the electric feeds on a steadily weakening electric supply/ output while the gas motor gets constant energy input and delivers constant energy output.

              My advice would be to go with no less than a 3.3 hp. It will give you considerably more power than your 55# electric AND alow you to get into a motor that has at least F& N gears if not F_N_R. This is a biggie IMHO.

              Comment



              • #8
                Re: Pounds thrust to HP conversion

                You guys are basically all correct...
                A horsepower is defined as 33,000 Lbs-Ft/Minute. It is the ability to move something with a force of 33,000 Lbs at a speed of 1 Ft/Minute, or 1 Lb force at a speed of 33,000 Ft/Minute... you need force and linear speed, or torque and rotational speed, to have mechanical power... hope this makes sense.
                On an engine, you can measure HP because you can measure torque, and speed, then calculate HP. There is no exact correlation when thrust is give only.
                It sounds (I have not searched this, simply quoting a post above) like these motors are rated at 5 MPH, so now we have force and speed, so we can come close to a HP number...
                55 Lbs thrust at 5 MPH calculates to 0.73 HP. Thus the SWAG above of 0.8 HP is very close. Keep in mind that we're talking HP at the prop, so we need to allow some losses from motor to prop. Not sure how much those losses are, but I would think a 1 HP might give simliar performance as a 55 LB trolling motor, a 2 HP much more performance.
                why take chances though... plop a 10 HP and you'll never have to worry about headwinds again!
                Sounds like you're on the right track.

                Jorge

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                • #9
                  Re: Pounds thrust to HP conversion

                  Thanks everyone.

                  Based on all this, and doing an inventory of all the little outboards that have followed me home and never been worked on, I think my best best is to get a Martin 60 working well. It is rated at 6HP, and already has confirmed spark and compression. I think a carb rebuild will take care of the rest and put me in business.

                  Thanks again, all!
                  Steve
                  Livin' the Dream
                  The Black Pearl - homemade pontoon boat - 1959 Johnson 18HP
                  On A Whim - 1990 ChrisCraft Concept 198 - 4.3L HO, Cobra outdrive

                  Comment



                  • #10
                    Re: Pounds thrust to HP conversion

                    What is really interesting is that on the Minn Kota website they list the Thrust of their electric outboards as 2HP!

                    Minn Kota E-Drive
                    Steve
                    Livin' the Dream
                    The Black Pearl - homemade pontoon boat - 1959 Johnson 18HP
                    On A Whim - 1990 ChrisCraft Concept 198 - 4.3L HO, Cobra outdrive

                    Comment



                    • #11
                      Re: Pounds thrust to HP conversion

                      Well, with 4 bank chargers as accessories I guess it runs on 48 volts and makes about 150lb of thrust.

                      That fits the 75lb per HP estimate.
                      sigpic

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