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14' Mirrocraft & 20hp Johnson prop selection help.

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  • 14' Mirrocraft & 20hp Johnson prop selection help.

    I've picked up a new to me boat and motor and I'm looking to get the right prop to get the most out of this combo.

    Boat: 1978 Mirrorcraft Resort F-4604 14’ x 57” 175 lbs
    Motor: 1993 Johnson 20 hp 115 lbs 2.15 gears
    Gas and gear: 50 lbs
    Current prop: 11x9 aluminum
    Baseline speeds:
    2 people (390lbs) 32kmh 5450 rpm
    4 people (800lbs) 26kmh 5200 rpm

    Typical lakes and rivers I'll be on are are at 3000-5000 feet elevation

    Mostly use it for exploring lakes and rivers with 2-3 people. Currently it's quick to get plane and handles very well. I wouldn't mind a bit more top end speed.

    I've heard a 10.5x11 or 10.1x13 might work well. What would be your recommendations? Things to consider?


  • #2
    If the rec. rpm range is 4500-5500, then I'd go with the 11p prop.Should put you at ~5000 rpm with 2 people and ~38 kph. If the range is 5000-6000, stick with the prop you have.

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    • #3
      RPM is 4500-5500 so I think I'll try the 11. Thanks!!

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      • #4
        Make sure the engine is mounted at the proper height too. Mounted too low can cost you up to 20% in speed and fuel economy.

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        • #5

          From previous posted data :

          2 people (390lbs) 32kmh 5450 rpm
          4 people (800lbs) 26kmh 5200 rpm

          Was it at typical lakes and rivers located at 3000-5000 feet of elevation ?

          If at sea level, posted data will vary when at high altitude levels. In that case prop it right for 3 up for engine to rev at least middle to max wot rpm range factory stated.

          If going for the 11 pitch prop post wot runs at 3 and 5 K Ft of elevation with 2 and 3 up. Would be interesting to see how much wot rpm varies playing with variable given load and height.

          Happy Boating


          Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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          • #6
            I would double check your tach first. Your #s seem off because you doubled your weight and only lost 200 rpm. If your #s are right you are already propped very well.

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            • #7
              Originally posted by WesNewell View Post
              Make sure the engine is mounted at the proper height too. Mounted too low can cost you up to 20% in speed and fuel economy.
              I'm planning to play with this a bit too. The cavitation plate is slightly below the bottom of the boat so I think I can lift it about an inch or so without trouble. Lakes are starting to freeze here so will have to wait till spring to test it out.


              Originally posted by Sea Rider View Post

              Was it at typical lakes and rivers located at 3000-5000 feet of elevation ?
              Yes these numbers are from a couple lakes near home at about 3400’. I'll post some numbers once I get the 10.5x11 and get a chance to get it out again.


              Originally posted by flyingscott View Post
              I would double check your tach first. Your #s seem off because you doubled your weight and only lost 200 rpm. If your #s are right you are already propped very well.
              I was kinda wondering that too. I expected a larger diff in top speed with double the weight. I tried it twice, once before I had the tach installed and the ear RPM gauge seemed about the same as when I had the tach installed. This was one of the tach’s with the wire wrapped around the plug wire. I’ve got one of those optical tach one too I’ll test out next spring.


              Thanks for all the help! I’m going to pickup a 10.5x11 prop over the winter and give it a test in the spring. I think I’ll keep the 11x9 handy as well and swap depending on intended mission for the day. Planing to do some work on the boat over the winter too, reconfigure the rear seat into side seats instead of single bench as well as clean it up and paint it.

              Comment


              • #8
                Post a picture of the rear of the hull and I can give you a better estimate of where the av plate should be in relationship to the bottom. Here's mine. Yardstick is sitting on top of av plate.
                Click image for larger version  Name:	engine-mount.JPG Views:	1 Size:	993.3 KB ID:	10676095

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                • #9
                  Originally posted by WesNewell View Post
                  Post a picture of the rear of the hull and I can give you a better estimate of where the av plate should be in relationship to the bottom. Here's mine. Yardstick is sitting on top of av plate.
                  Just curious, have you looked at which lower leg height is water flow passing at speed once combo is running on plane parallel to water level ? To check that a visual inspection on what's going on at back of engine is needed. A dry installation means nothing as each combo is unique, can float more or less and once on plane it's a different story..

                  Happy Boating


                  Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

                  Comment


                  • #10
                    Wouldn't do any good to check as it can't be mounted any higher and I sure don't want it mounted any lower. It was originally mounted 1.5" lower. Gained smoother ride and 10% speed and fuel economy after raising it. I'd bet it's running the av plate just above or at water at wot.

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                    • #11
                      Finally got a chance to take some measurements. One is with motor mounted as low as possible, the same position it was mounted for the aforementioned speed tests. I remember looking out the back at wot once and the water was flowing about 1-1.5” above the cav plate. Another pic is with a 3/4” lift on the motor. I feel like this might be a good spot for it, will have to test once the lakes melt again in 4-5 months.
                      Last edited by shuether; December 7th, 2018, 05:30 PM.

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                      • #12
                        If, at wot the water flow was 1-1.5" above the av plate, then you need to raise the engine 1-1.5" for max speed.

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                        • #13
                          Before you go jacking the engine up. On the Mirrocraft, being Aluminium, there is a keel rib running down the centerline of the hull. How far forward of the transom does it it taper or does it run right back to the transom? If it runs back to the transom of less than 18 inches from it, then the keel becomes the bottom of the hull and mounting the engine higher than that will induce ventilation, especially on turns.

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                          • #14
                            Originally posted by jimmbo View Post
                            Before you go jacking the engine up. On the Mirrocraft, being Aluminium, there is a keel rib running down the centerline of the hull. How far forward of the transom does it it taper or does it run right back to the transom? If it runs back to the transom of less than 18 inches from it, then the keel becomes the bottom of the hull and mounting the engine higher than that will induce ventilation, especially on turns.
                            The keel rib tapers off a few inches forward of the transom, I'd say the taper starts at about the 3" mark. So if I understand right, the main thing to look for when raising the motor to find the optimal spot is avoiding ventilation in general, especially when preforming hard turns under acceleration?

                            One other question. On the last boat I had, the trim position of the motor made a huge difference in getting the boat to plane and how high the nose was out of the water when on plane as well as top speed. With this boat it doesn't seem to make a lot of difference. The boat planes well in almost any trim position, and other than the highest trim position it doesn't seem to affect speed noticeably.

                            Is there an advantage to running in a certain trim position in general? I know the lowest will help keep the nose from proposing when riding alone, but I usually adjust the weight of cargo forward to get a fairly balanced load anyway when riding solo.

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                            • #15
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                              In theory the centerline of the propshaft should be perfectly perpendicular to vertical when boat is moving. In reality that doesn't always provide highest top speed. Experimentation will reveal that. Higher trim settings may result in ventilation when engine id turned to steer boat.
                              Higher engine heights will also increase risk of Ventilation, especially when there is a pronounce Keel, like the one on your boat. Higher engine heights also reduce the ability to raise the bow at any give trim.

                              40 yrs ago we had a couple of Mirrocraft Deep Fishermen 14 footer(I think yours is a Resorter). The engines(25 on one, 35 on the other) were mounted right on the transoms. When running solo I had to keep the engine trimmed all the way to reduce porpoising. When loaded, we would reset the tilt pin to 2 or 3 away from the transom
                              Click image for larger version

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