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Prop for 50HP Mercury 4 stroke

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  • Prop for 50HP Mercury 4 stroke

    Hello,
    I recently purchased a 2000 Sylvan aluminum fishing boat with a 50 HP Mercury 4 stroke engine, model 50 ELPT 4 S. According to the sticker on the lower unit the engine is supposed to use a 12X22 prop. I can't find any markings whatsoever on the prop so I'm not sure if it's the original or not.
    Anyway, the boat is extremely slow out of the hole and takes forever to get up to speed. Engine runs smooth, but no umph. Top speed according to boat speedo is 27 MPH trimmed out. Any suggestions on a prop change/size that will get me out of the hole quicker. I'm not really concerned about top speed or trolling speed.

    Thanks
    Brad


  • #2
    Re: Prop for 50HP Mercury 4 stroke

    You are grossly mistaken about what that sticker means. 12:22 is the lower unit gear ratio which if you run the math comes out to 1.83:1. This merely means the engine turns 1.83 revolutions to make the prop turn one revolution. That engine is typically fitted with a 13P prop on most 15 - 17 foot aluminum fishing boats. If you check the hub of the prop very carefully between the blades you will find a Mercury part number and it will be followed by a xxP number. Those numbers are the pitch. A 12 inch diameter 22 inch pitch prop would be waayyyyy to much prop for that motor.

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    • #3
      Re: Prop for 50HP Mercury 4 stroke

      Thank you. That makes a lot more sense.

      Brad

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      • #4
        Re: Prop for 50HP Mercury 4 stroke

        your ideal prop would be a 16p vengeance merc prop .....
        __________________________
        ... THE MAXIMUM HORSEPOWER IS NEVER ENOUGH ...

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        • #5
          Re: Prop for 50HP Mercury 4 stroke

          16P might be a little much but that all depends on which "Sylvan" Aluminum fishing boat he has. If this is a standard row boat style then yes, that might work. If its a heavy, wide beam, side console then a 16P is definitely too much bite. 50 HP two-strokes with the same gearcase on Lund Explorer 1650's, Alumacraft Navigator 165's, etc., all are generally fitted with 13P props.

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          • #6
            Re: Prop for 50HP Mercury 4 stroke

            Its a Sylvan Shadow which has a full walk thru windshield and a fairly wide beam.

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            • #7
              Re: Prop for 50HP Mercury 4 stroke

              Forgot to mention that it's 16 feet. The open bow deck is raised about 8 inches above the rear deck.

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              • #8
                Re: Prop for 50HP Mercury 4 stroke

                Okay, I took the prop off the motor and found the pitch inside the hub. It's a 13P. I did some research and the boat weighs in at about 1000 lbs, the motor is 224, full tank of gas about 186, 500 for anglers and equipment, etc for a grand total of 1910 lbs. So with that weight and a 50 HP four stroke, should I go to a 11P prop to get on plane sooner? I did notice two things, the fuel filter appears to be the original. Even though the engine was not missing or didn't seem to be starving for gas, I will replace it before the next trip. Also, the stops on the transom bracket where in the holes closest to the transom which put the motor lower unit at a slight angles towards the transom instead of straight down or vertical. I moved the stops to the next hole which put the lower unit at just slightly outward of vertical. Although I'm not sure that would make a big difference since I have electric trim and can adjust the trim angle as needed. Am I just expecting two much from a 50HP engine? This is my 5th boat and I've had a 15 HP and 85HP outboard, and two V-8 outboards so I don't really know what to expect from a 50HP, but did expect more power. Silvertip you seem to be very knowledgeable about these motors. Do you have any idea what the compression is supposed to be? Mine in 110/115 on all four cylinders, cold. Seems sort of low to me, but I'm used to car engines. Never done a compression check on a boat before.
                Any ideas, comments, suggestions are welcome.
                Thanks
                Brad

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                • #9
                  Re: Prop for 50HP Mercury 4 stroke

                  First of all, put the motor position back where it was. If you have a power tilt and trim, you do not need to be limiting how much you can tuck the motor in. That's what helps get you out of the hole quickly. Which leads me to another question.

                  Do you have the motor trimmed all the way down (tucked inward) when you throttle out of the hole? Or are you starting off with the motor trimmed up a little?

                  I can't find any information on line about a Sylvan Shadow. Is there a Coast Guard plate that lists it's Maximum Horse Power rating?

                  With all the detailed info you are providing, you have not provided anything about RPM's. What RPM's are you getting at wide open throttle?

                  From your description, should we assume you are sluggish out of the hole with with the full load you describe? To select the best prop, you should see what maximum RPM's you are achieving with just yourself, the boat rigged as you intend (trolling motor, batteries, anchor, etc) and maybe a 1/4 tank of fuel. You want a prop that will turn that motor at the peak of it's RPM range which I believe is close to 6000 but I don't know for sure.

                  Is the main motor support bracket mounted all the way down on the top of the transome? You can usually tell by looking at the top pair of bolts and what holes they are in. Top hole, 2nd from the top, third from the top, etc. With your boat level on the trailer and your motor trimmed straight vertical, measure in inches where the cavitation plate is in relation to the bottom V of the boat. How many inches is the cavitation plate above or below? Sometimes performance problems can be cured just by raising the motor a couple of holes if the motor is currently mounted all the way down.

                  Silvertip is probably right on with the 13 pitch suggestion but after providing the answers to these questions, you may end up needing to go to a 12 or 11 pitch. Don't take the advice of the poster who suggested a 16 pitch prop until you provide all the other information.
                  KP

                  Keep our waters clean! Keep your garbage in your boat and keep your boat maintained.

                  Comment



                  • #10
                    Re: Prop for 50HP Mercury 4 stroke

                    Compression numbers are ok. However, four-strokes vs two-strokes of the same HP on identical boats will suffer defeat on hole shot. To make matters worse, your boat has an engine that is at the very bottom of the HP range so the all up weight makes it struggle. Normally, a boat has a Max HP tag. If you install motor of lesser HP, it should not be less than 75% of maximum in order to have reasonable performance. In other words, if the boat can have 100 HP max, the smalles that should be installed is about 75 HP. The key to performance is to prop the engine so it runs at its manufacturers recommended WOT rpm with an average load. In your case, the motor should run 6000 RPM with an average load at wide open throttle. If it cannot, you step down in pitch. If it runs at or very near 6000 thats perfect and speed will be what that prop provides. If the engine over revs, you step up in pitch. Increasing pitch 1 inch decreases engine rpm about 150 -200 RPM. Decreasing pitch 1 inch increases engine RPM about 150 - 200 RPM. Don't overlook engine height. Raise the engine on its mounting so the antiventilation plate is about flush to and inch or so above the bottom of the boat. Less drag = better performance. Just don't go so high that you lose water pressure. The long and short of this is that not knowing what your current wide open throttle rpm is -- we can't tell you whether an 11P would help or hurt. I suspect the 13P is right however.

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                    • #11
                      Re: Prop for 50HP Mercury 4 stroke

                      K.P. and Silvertip, thank you for your suggestions and very informative posts.
                      I will check everything you said and get back to you. I'm taking the boat out this Thursday and will hook up a Tach to check RPM. I'll get back to you then.
                      Thanks again,

                      Brad

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                      • #12
                        Re: Prop for 50HP Mercury 4 stroke

                        K.P. and Silvertip
                        Took the boat out Thursday with my son for a day of fishing. K.P., I put the engine back down to the first stop on the transom plate. Yes, when I take off I always have the motor trimmed/tucked all the way down/in. The maximum HP rating on the boat tag is 65 HP. So Silvertip you are right on, it's just shy of being less than 75% of the recommended HP at 50 HP. H.P yes the main motor bracket is all the way down in the top hole. With the engine in the vertical position the cavitation plate is 18 1/2 inches from the ground and the bottom of the boat, deep vee, is also 18 1/2 inches from the ground.
                        Hooked up a tach and the engine is only turning 4800 RPM at WOT, well below the Motor manual's recommended 5500-6000RPM???? Checked the top speed with a GPS and fully trimmed out, calm water, with WOT it was 27.5 MPH.
                        The engine starts easy and runs smooth. No issues with how it runs at all. Little surprised at the low RPM at WOT. Is that just because the boat is too heavy for that HP rating? I would think that even if the boat was too heavy that once it got up on plane that the engine would operate at it's expected WOT RPM range or at least much closer to it. BTW, I did check the linkage at the engine and verified that the engine/carb linkage is at the WOT position when the hand throttle is also.
                        So unless you have any other suggestions, I guess I'll drop down to a 11P prop. What's your opinion on a 4 blade vs a 3 blade? Also, what's your opinion on those "whale tail" things that attach to the cavitation plate?
                        PS. One good positive is that after two full days of fishing the gas guage is still on full. That's a good thing considering the price of gas!
                        Thanks again,
                        Brad

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                        • #13
                          Re: Prop for 50HP Mercury 4 stroke

                          I have a walley boat that I put a bigger engine on for the very same problem you are having. It had a 50 HP and was rated for 75. Although it performed ok, the 50 would not live long as it needed to run wide open or near wide open to maintain any semblance of performance. Speed was about the same as you are experiencing. Yes, I tried the foil and it made for handling issues. The boat was rated for a 75 which I installed and it went from a slug to a very fine performing boat and much more fun and versatile.

                          Four blade vs three blade is another issue. Generally, four blades help hole shot but cost you a little on the top end. Unfortunately, I think you boat is underrated at 65 HP and that weight. Are you sure the boat is not holding water soaked foam in those compartments? A boat rated for 65 HP should perform ok with a 50. While you might try a foil, I've tried them twice and removed them twice. The 11P prop is worth a try. I just ran the numbers through a prop calculator and at 6000 RPM with an 11P prop, 1.83:1 gear ratio and 12% slip the boat should see about 30 MPH which is typical for that size and power. Also, if you are trimmed all the way down during hole shot, you are actually forcing the bow to plow rather than raise up. Don't trim up high but definitely trim up a little.

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                          • #14
                            Re: Prop for 50HP Mercury 4 stroke

                            Silvertip,
                            Thank you for your input and suggestions. Don't know about the water soaked foam, but I'm going to install "bass" type seats forward and aft which will require removing the floor. I'll check foam them.

                            I think I'll try a 4 blade 11P prop. Not really concerned with top end that much, just a better hole shot. Good advise about not trimming down all the way. I'll try that.

                            Once again thank you for your expert advise.

                            Brad

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                            • #15
                              Re: Prop for 50HP Mercury 4 stroke

                              Silver tip,
                              Forgot to ask, any recommendations on make and model/type of prop?

                              Thanks
                              Brad

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