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Is a 55HP Jet too much for a 16ft Flatbottom

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  • Is a 55HP Jet too much for a 16ft Flatbottom

    Im having some issues with a boat I bought a month or two ago. The boat is a 16ft Grumman Flatbottom with a 55hp Evinrude Worktwin.
    With 2 people and some gear (no more then 700lbs people and gear) it will barely push the boat at 5mph (guestimation). However with 500lbs in it, it screams. But that is with all but 200lbs (me) at the front of the boat.
    The boat with no one in it sits about 1ft into the water in the back. Could the outboard be too heavy for this boat? Would that explain why it runs faster with more weight in the front?
    Thanks for the help

  • #2
    A 55 HP jet, that sounds like a very odd # for an outboard. Typically a 50 hp motor with a jet will put out 35 hp at the jet. This is more than enough to push a 16' boat with a couple people in it. It won't go to fast but it will get on step and run the rivers. If this is a used boat I would suspect that you have an issue with your jet, something plugged, sleeve and impeller wore out or someting of the sort. You shouldn't have a problem with what you described.
    On another note are you sure the motor is trimmed correctly?

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    • #3
      Not sure its trimmed right. I did a compression test, 74psi on one and 80psi on the other. From what I understand the 55hp worktwin will have lower compression, and is the commersial variant.

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      • #4
        What is the powerhead rated at?

        When you say 55hp do you mean out the pump or the motor itself? The reason I ask is a jet unit will reduce the amount of hp by about 1/3 which means a 55hp will push around 35ish hp. I have a 50/35 jet on my 16ft lund and it will push 2 guys and gear no problem. Make sure the impeller is poperly shimmed, a google search on the subject should yield some good info. take a look at the jet and make sure it is sharp, it may need to be rebuilt or even replaced. Jets are very sensitive to the way they are set up and if they are not dialed in just right you can have problems. There is a key on shaft that will shear in the event that something gets lodged in there, that may be worn. If you are good with tools and can follow a diagram (it took me awhile to figure mine out) than you can fix it in the driveway. If not take it in and have the jet unit serviced.

        PS - grease the jet everytime you look at it (at least after every day you use it)

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        • #5
          Its a 55hp outboard. So like you said around 35 hp at the jet. I have tried to find info on jets out there but havent run accross much info. Any site in particular you can recomend?

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          • #6
            Something is wrong....

            I am running a jetted Johnson 40hp (powerhead) on my 16' Lowe and ran the Little Su this summer (on step) with 4 adults and 5 small kids. You should be cooking right along with that 55 on it.

            www.outboardjets.com

            A good site with instructions on how to sharpen your impeller.
            AKmud
            sigpic


            The porcupine is a peaceful animal yet God still thought it necessary to give him quills....

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            • #7
              Another good site

              http://www.scottandjenn.com/etomite/index.php?id=22

              Some good maintenance tips there.
              AKmud
              sigpic


              The porcupine is a peaceful animal yet God still thought it necessary to give him quills....

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              • #8
                When you did the compression check, was that with an engine that was warmed up, and with the throttle blocked wide open? If you, your compression sounds low. Much under 120 psi and an engine really doesn't want to fire.

                Something definately sounds wrong, as I don't think you have a power problem. Either the pump is hosed up and you just aren't moving any water through it (but that doesn't seem likely from your previous performance) or the way your boat was loaded the engine could never get the hull on step, and hence you were stuck at displacement speeds.

                Properly trimmed, and assuming 35hp at the pump, you should have enough power to get 1600#'s of hull, engine, fuel, people and gear on step. It's possible you underestimated your load and were trying to push closer to 2000# gross weight.
                Those that are successful in Alaska are those who are flexible, and allow the reality of life in Alaska to shape their dreams, vs. trying to force their dreams on the reality of Alaska.

                If you have a tenuous grasp of reality, Alaska is not for you.

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                • #9
                  Hopefully this may help. When no one is in the boat the water line across the outboard comes up to about 6" from the original top of the boat in the back. Or about 7-8" above the top of the jet.
                  The compression test was done about 1 hr after the motor was run for an hour or so.
                  Last edited by 79FSJ; 01-08-2007, 20:23.

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                  • #10
                    The most important thing to check is the condition of the impeller and the adjustment (shimming) of the impeller to the shoe. According to Specialty Mfg. Co., which is the company that builds virtually all outboard jet units, once the clearance between the impeller and liner exceed 1/32", it needs to be re-shimmed to reduce the clearance. This specification is for a 4 bladed stainless steel impeller, so you may want to call them and check if it's the same for a 3 bladed aluminum impeller. Also, the impeller should have sharp leading edges and be free from dings and such.

                    It's hard to tell, but from looking at your 2nd picture, it looks like your motor may be mounted too low. Ideal mounting position is to have the forward edge of the jet shoe flush with the bottom of the boat. If it's too low, you get too much drag. Also, the forward edge of the jet shoe should be as close as possible to the transom. This enables the cleanest (non aerated) water possible being fed to the impellar.

                    These are the easiest and cheapest things to correct, so I'd start there. I agree with Paul H...Your compression numbers seem low to me. I'd call Ak. Mining and Diving or the like and see what the actual spec. for that motor is. If your compression is low, then the bigger load you put on the engine, the easier it is to notice.

                    Here's their address:

                    Specialy Mfg. Co.
                    2035 Edison Ave.
                    San Leandro, CA 94577
                    (510)562-6049
                    (510)562-0309 FAX

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                    • #11
                      engine too low and....

                      It should be tipped a little more. It is too straight up and down. Adjust the bolt that controls the angle of the motor so the jet unit is closer to the boat. This will allow the water to be "pushed" into the shoe as you are moving as well. The leading edge of the shoe should be almost exactly at the same level as the bottom of the boat with the rear of the shoe about 1-2" lower than the bottom of the boat. (hope this makes sense) The way it appears, is your shoe rides almost flat on the water when the boat is on plane. This setup will cause cavitation problems more often than not.

                      Hope that helps.
                      AKmud
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                      The porcupine is a peaceful animal yet God still thought it necessary to give him quills....

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                      • #12
                        Woth the engine straight down along the lines of the transom the front of it drops about 3-4" below the bottom of the boat? Is this right or does it need to be raised that 3-4" to ride even

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                        • #13
                          too low

                          you need the leading edge of the intake shoe flush (even) with the bottom of the boat. On my 16' I have an aluminum riser which is about 6" higher than the stock transom in order to bring the motor height up to match the jet shoe to the boat bottom. You may need to do something similar to get everything to line up.
                          AKmud
                          sigpic


                          The porcupine is a peaceful animal yet God still thought it necessary to give him quills....

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                          • #14
                            After I saw this pic, I relized what you were talking about

                            I believe mine sits a few inches lower then this.

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                            • #15
                              Yep!

                              If you can get yours set up like that it will feel like an entirely new boat.

                              Good luck.
                              AKmud
                              sigpic


                              The porcupine is a peaceful animal yet God still thought it necessary to give him quills....

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