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Mako 20 Johnson 150 Prop Questions

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  • Mako 20 Johnson 150 Prop Questions

    Been running my boat and overall happy with everything working. Of course I'm getting the itch to find something to do on it other than fish. This got me looking at my motor and prop setup and wondering if I had room to improve. Here are some basics of where I'm at today:

    - Mako 20B

    - Vent plate parallel with bottom mounted on 3rd hole

    - 99 Johnson 150

    - Prop is OMC PN 389924 which is a 3 blade 14.5x19

    - WOT and trimmed for speed gives me 36.2 knots @ 6000 rpms

    - Cruise before falling off plane is 22.5 knots @ 4000 rpms

    Here are some things I'm wondering about

    - After some research, it seems my WOT is about 500 rpm high. Before raising my motor, changing plugs, and running Seafoam, I was getting 5000 rpms with this setup. With this setup, trimming for speed I can get to 6000 before dropping speed, but should I focus on trimming for RPM? Is this reason enough to reprop?

    - Working through the prop slip calculators, it seems I'm getting a 28% prop slip which seems high. I ran the WOT numbers for Sailor's old one which seemed the closest to mine and it looked like 24%. Is this still a gremlin to chase down or a likely characteristic of the 20?

    - Falling off plane at 4000 rpm and 22.5 knots seems high to me. I have no real numbers to back this up, but have noticed that there have been several times where if I could run a bit slower while staying on plane, my ride would be much better. Is this a prop issue or a sign that I want trim tabs?
    [br]\'82 Mako 20B

  • #2
    The first obvious answer is that your 19 prop pitch is too low.

    That is like driving around in 3rd gear on a 4 speed transmission.

    Try borrowing a 21 pitch prop and you will see a world of difference.

    Your hole shot acceleration may decline. Your top end speed may be about 40+mph at 5000rpm and your mileage should improve.

    Your motor operating range should be 4500 to 5500 rpm.

    If you were near New Orleans I could give you several props to try out. But Miami is a far road trip for that!
    Capt Mud Flat[br]\'73 Mako 232[br]Mandeville, LA


    • #3
      are you running an aluminum or stainless prop?
      76 25 \"Aenigma\"[br]73 17[br]Richland/Long Beach, MS[br][br][br]17 project[br]


      • #4
        I'm running a stainless prop. The two extras I have readily available are a stainless 14.5x26 and an aluminum 15x17. Hypothetically the 14.5x26 would drop my rpm by 700-1400, putting me between 5300 and 4600. I think that's a bit too much of a drop but may test anyways. I know my brother had the two props on a 150 Johnson and 19 dusky and the 26 would not get him on plane.
        [br]\'82 Mako 20B


        • #5
          I think you are getting close. The 20's stay on plane at ridiculous slow speeds if you can get the weight up front. Lowest honest on plane I ever saw was 12 mph with four people sitting up front (one adult and three kids). Are your batteries under the console?

          You also need to adjust the trim for your speed. Slower speeds require the motor to be down and tucked under. Higher speeds you want it up and trimmed out. tabs would help tremendously as well.
          1978 Mako 25 - Blind Hog
          1985 Mako 20c - sold
          Fort Walton Beach, FL


          • #6
            Batteries are still in the back and will probably stay there as I have considered the idea of a trolling motor in the future and not sure that 4 batteries would fit well down there. I'm usually running with 1 guest that sits in the leaning post with me. I do keep my cast net and 20 qt RTIC with drinks in the back. Perhaps shifting some more weight forward will help a bit. Otherwise, I'll likely test the 26 pitch I have access to or search for a used 21.
            [br]\'82 Mako 20B


            • #7
              Something is not jiving. I had a 150 Yamaha on a 201 with a 13 1/4 X 17. A quick search indicates that the lower unit has the same gears.

              My top speed was about the same as yours at 5400 RPM. I could cruise at 30 Kts turning 4300.

              I replaced that engine with a 200 and used a 14.5 X 17 prop and the WOT bumped around 5500.

              I'd try the 15 X 17 and see what you get.

              If you want more of a definitive answer, contact Ken at Propgods. He nailed it with the prop selection for the 200.
              2007 Everglades 243 250 Suzuki[br]1988 201 Yamaha 200 - Sold[br]1990 261 T/200 SWS - Sold[br]1978 25 225 Johnson - Sold[br]1975 20 85 Merc - Sold[br]Jacksonville, FL[br]


              • #8
                Hey Dave,

                I checked with Ken and he's thinking that my performance looks decent but my slip is weird. Running your numbers, you were between 9-12% slip which sounds right. His suggestion was to test the other props I had and if buying new, going with a Powertech OFS3, in 16 pitch, expecting about a 10% slip. The thought is the larger diameter (15.25) and cupping should give a better bite, so propping down might actually be in order. I'm a bit concerned trying this one since my 14.5 seems to have just about a finger's worth of clearance and since I actually want to drop my rpms after checking specs.

                One thing I'll work on this weekend is sanding a bit more on the bottom where the transom meets the running surface. Looking closely, I could see some spots where I either left some fairing compound or a little extra glass when I did the transom job. It doesn't seem like much, but it will be interesting if this is either causing noticeable resistance or turbulence which is messing with my slip number.
                [br]\'82 Mako 20B


                • #9
                  pretty sure I was running a 15x17 stainless with my old 150 and I'd cruise all day at 28-30 mph. When fishing I would always have a 120 qt cooler all the way up front and a rope from each handle to the bow cleat to keep it there. I always wanted tabs on the boat but never got to it.
                  1978 Mako 25 - Blind Hog
                  1985 Mako 20c - sold
                  Fort Walton Beach, FL


                  • #10
                    Got my burs sanded out and confirmed I'm just a hair above parallel with

                    Also confirmed I'm in knots so my speed isn't bad. Might test tomorrow.
                    [br]\'82 Mako 20B


                    • #11
                      Put on my brothers 14.5x26 stainless raker prop and got some new numbers:

                      Wot and trimmed out 37-38 knots at 5200 rpm

                      Wot and trimmed down for cutting waves 35-36 knots at 5000 rpm

                      27.5 knots at 4000

                      18 knots at 3200 then it falls off plane

                      9 sec hole shot idle to plane (not sure what my other prop had)

                      Felt like the bow wouldn't lift as much

                      The good news is I got my rpm under the suggested high end, although maybe a bit too much. Found a better wot speed. Better clip at 4000 and lower rpm and speed for plane.

                      Where it could improve is maybe get a bit more rpm trimmed (5500).

                      Now doing my slip calc I'm at 38%!

                      Kept pulling right when trimmed down even though my trim is all the way right.

                      At wot trimmed up it felt "loose" like there was play side to side. Is this chine walk?

                      Based on this, I'm thinking overall to keep the 26 on and look for something in the 23/24 range. Also what I read is the raker is a "bass boat" prop, so not sure if getting a different style will make me happier as well.

                      [br]\'82 Mako 20B


                      • #12
                        That 26" pitch prop is going to suck your gas tank dry...your wheel is a bit too small. My dad is spinning a 14.25x17 on the F150 on his 192.he had a 150 ox66 and I think it had a 19" pitch prop and it ran well.
                        Chris Miller[br]Mystic Islands, NJ[br]1974 17 Classic[br]1988 211 Classic (sold)[br]1990 Grady White 230 Gulfstream (sold)[br][img][br]


                        • #13
                          I think I'll try my 15x17 aluminum next to see what I see. If my slip numbers improve and I can keep performance good I'll look to increase diameter some.
                          [br]\'82 Mako 20B