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Outboard shaft length?

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  • Outboard shaft length?

    How do you determine the shaft length on an outboard? What would happen if a motor with a 25" shaft was rigged on a flats boat with a 20" transom?


  • #2
    Re: Outboard shaft length?

    Without a jackplate it wouldn't be possible to get the cavitation plate of motor even with boat bottom .This would result in motor running too low in water and lead to operational problems.

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    • #3
      Re: Outboard shaft length?

      Measure from inside the clamp (the part that sits on the transom) down to the cavitation plate. Boats with short shaft transoms and long shaft engines normally don't perform very good. Too much engine in the water.c/6Hooty

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      • #4
        Re: Outboard shaft length?

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        • #5
          Re: Outboard shaft length?

          that picture contradicts what everyone onb the board says......the shaft there is longer then the transom height....everyone seems tro say 20 inch trransom = 20 inch shaftthen you get people sayin the plate must be even with the bottom of the boat.....if thats the case then u need a 25 inch shaft on a 20 inch trransom

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          • #6
            Re: Outboard shaft length?

            sorry, but you are the confused one friend..the shaft lenghth is the measurement from transomto cav. plate... see the 2 red arrows in Bear'spic.hope this clears up any confusion.regards,M.Y.

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            • #7
              Re: Outboard shaft length?

              The shaft goes between the power head and the lower unit, so the measurment from the top of the clamp to the cavitation plate is roughly the bottom of power head to the top of the lower unit.

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              • #8
                Re: Outboard shaft length?

                Too long a shaft motor will result in a significantly slower boat due to drag. The best is to match the boat's need for shaft length. If you measure a conventional hull from the middle of the transom where the top of the motor bracket sits to the bottom of the hull ... this is the dimension you want to match to the motor shaft length (15", 20", 25" and 30" are the standard lengths).
                Jim

                ===============================
                I don't respond to Private Messages PM's that are motor questions.

                For basic information on a wide range of topics, see Top Secret File
                Link: http://forums.iboats.com/showthread.php?t=299680

                OEM shop manual: outboardbooks.com or Ebay

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                • #9
                  Re: Outboard shaft length?

                  Bear's description to answering the original post is very explanitary in basic terms.However, I will add that not all modern outboards come in exact 15, 20, 25, and 30" shaft lengths. Although they are sold as such, many are actually shorter or longer when actually measured.Likewise many boat transoms don't always match up to to the outboard's shaft length for optimum performance. This is because most transoms are at an angle of 12-15 degrees. For example on a 15 degree transom, an exact 25" shaft outboard would require a transom length (measured along the hull transom) of 26" to line up the bottom of the hull with the cavitation plate. I'm curious as to how red18 measured the 20" transom.Additionally not all outboards are designed to run even with the bottom of the hull. And not all boats are designed to have the outboard run even with the hull.So...it's a combination of the particular outboard's rigging specifications and the boat manufacturers rigging specifications, and proping. This will get you close. Then it's all about testing and making fine-tuned adjustments.That's why jack plates are so popular.....

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                  • #10
                    Re: Outboard shaft length?

                    I measured 20" from the center of the transom, however, there is a step where the bilge drain is which if measured all the way down to the keel would probably add about 5". Which measurement do you use. The boat is an 18' Hewes redfisher.On the motor do you measure the upper inside of the mount or the top of it?Thanks for all the help

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                    • #11
                      Re: Outboard shaft length?

                      Upper inside, the part that sets on the transom.c/6Hooty

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                      • #12
                        Re: Outboard shaft length?

                        ...and use the measurement from the top of the transom to the keel.c/6Hooty

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                        • #13
                          Re: Outboard shaft length?

                          Red18, Yamaha Motor Company calls for this model engine for the 18ft.Hewes redfisher.F115TLRZ Boat specs top rig out with 150hp. If this looks like your rig you will surely need a jackplate to accomodate a 25" shaft motor.

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                          • #14
                            Re: Outboard shaft length?

                            ok well my 15 hp johnson is a short shaft.....the measurement of the transom is 20 inchesthwe measurement from the clamp braket to the very end of the shaft is also 20 inchesno my boat is a very old design and the engine does not come up high out of the water i have a very narrow stern on my boat...is my setup wrong or right.It is the same setup everyone uses here

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                            • #15
                              Re: Outboard shaft length?

                              digimortal777,There is nothing wrong with the picture or what it says. Maybe just that the cavitation plate is not exactly level with the bottom of the boat, but never mind. The standard setting is level or .5 inch below the keel. Then you can raise the engine a hole to raise it an inch for less drag, more speed at wot and better fuel economy. If you don't get any problems with water pressure, cooling, ventilation or cavitatation in sharp turns.In your case digimortal777, is the propeller above the bottom of the boat? Do you have a picture of it so we can see it? What about the hull shape, like a sailboat, rowboat or a... banana. Don't confuse the way shaft length is measured in this case, with the actual length of the shaft itself. (From crankshaft to propeller shaft.)I really want to understand what you have and what you mean. I picture would be nice. Is there perhaps a web site with one for sale?

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