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70 hp mercury prop size

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  • 70 hp mercury prop size

    I have a 70 hp merc. on my 18 foot cobia center console. The boat goes very slow and barely gets on plane with two people in it with no fishing gear or coolers. It has those rubber dolphin fins on the motor, I guess to help it plane easier. The serial number on the motor is 6382744. I want to know the year of the motor and what the prop size is supposed to be. The motor runs good. Doesn't miss, doesn't over rev, although I don't have a tach. on it. It sounds like it runs good. I also have a 16 foot bow rider with a 1973 evinrude 70 hp on it. That prop is almost twice as big as the merc.'s by looking at it. The bow rider goes about 34 mph according to my gps and speedometer. The merc. 70 hp 18' boat goes about 15 mph on flat water. I was told by a guy who claimed to work as a boat mechanic who was buying my bow rider that the prop on the merc. is supposed to be 13.5x17.5. The only number on the prop I found on the merc. was 48 85632 11p. It was on the outside of the prop. So I don't even know what size I have. This so called mechanic also said that the 18 footer was lighter than the sixteen footer and should go much faster than it does. He claims all I must do is change the prop. I told him I figured the prop was correct because the motor doesn't seem to over rev. Any input would be appreciated.


  • #2
    Re: 70 hp mercury prop size

    The prop is an 11" pitch. Without a tach you'll never know whats right. There is no "standard" prop. If the motor is healthy that prop should allow the motor to spin to the moon. The holeshot should be awesome. Start with a compression check, then verify the motor is running on all cylinders at wot. After youve installed a tach.

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    • #3
      Re: 70 hp mercury prop size

      I checked the compression and got 150 psi on all three cylinders. Then I checked the spark at the plug with a tool that you lay on the plug wire and it lights as the plug fires. The top cylinder was dead. It was running on 2 cylinders. Next I refered to my chilton's book and did some ohms tests on the coils that the spark plug wires go to. They all checked out good. Then I half *** checked the ignition SCR switch box thing with a test light. I went from ground to each of the three hot terminals on the right side of the switch box where the switch box connects to the coils. Two out of three blinked consistantly while the dead plug wire one was blinking randomly. So I am assuming that the SCR switch box is at fault, but I'm not sure if the trigger coils under the fly wheel are at fault. Also I tried testing the SCR again and I got all three to blink consistantly but the motor is still missing. The dead plug wire is no longer dead. It seems like all the cylinders are missing at random times because I pulled out the brand new spark plugs and some are wetter with gas than others. So now I'm not sure if I have a carb. problem. It is definitly misfiring. The fuel is spotless and I put all new fuel lines and primer bulb.

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      • #4
        Re: 70 hp mercury prop size

        I ran the motor at night and sprayed some water on the spark plug wires and there were lightning bolts to ground and from one plug to another. So now I'm waiting for new plug wires. I know that will make it misfire. Hopefully that will solve my problem.

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        • #5
          Re: 70 hp mercury prop size

          Depending on the year of 70 you have, I believe that's only a 10" +/- 1/2" diameter, but the gear ratio is different than the 'Rude which spins a larger diameter prop slower.I have an '83 70HP merc on a 17ft Basstracker, and a 14" pitch is the best it will do...the 13 pitch does much better but I'm over 3500ft elevation on a light, flat-bottom boat designed for a max of 60HP. The small gearcase merc won't compete with the OMC's larger gearcase for thrust on that size of boat.

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          • #6
            Re: 70 hp mercury prop size

            sujend14,How did you wind up with a 70hp on your 18' Cobia c/c? That boat ought to have more like 125 to 150 hp in my opinion.

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            • #7
              Re: 70 hp mercury prop size

              The plate on the boat is rated for 6 people 125 hp. The guy I bought the boat off of had a lot of boats in his yard. I think he put the crappiest motor he wanted to get rid of on the cobia I bought.

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